Homebrew Dummy Load Project
Charles (KC8VWM)
on
June 18, 2009
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I have always enjoyed reading about homebrew projects submitted to the site so I though I would submit one of my own.
This submission is a quick "how to" article describing the idea of constructing a high powered homebrew dummy load on wheels for your ham shack.
The costs associated with this project is as economical as acquiring someone else's thrown away trash.
This high powered shack dummy load design uses a "dumpster special" electric oil filled room radiator like this one that no longer functions:
http://ep.yimg.com/ip/I/yhst-91657104930118_2054_32722836
Start by stripping off all the "show plastic" and front control panel so basically all you have left is the heater fin tubes resting on the wheels.
Next you rest the oil filled heater on it's end with the heating element up so the oil remains inside while you unscrew the "hot water" heating element from the heater.
Once removed, the heating element should look like this:
Cut the heating element off close to the bottom but leave enough of a stub on it to install the desired dummy load resistors on it.
Attach the feedline of your choice where the electrical cord used to be connected to the heating element using the screws.
Screw the "dummy load" back inside the oil filled heater.
I don't exactly know how much RF it will handle exactly. It's mostly dependent on the type and specifications involved with the resistors used.
I suspect the oil filled heater fins are an excellent heat sink and they shouldn't have any trouble dissipating a LOT of RF generated heat for a VERY long period of time.
Your home brewed high powered shack dummy load on wheels should be a very cost effective homebrew project anyone can enjoy.:)
Improvements... suggestions and comments are always welcome.
My Best,
73 de Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by K1CJS on June 18, 2009
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Only one drawback with this arrangement--you've got to keep on checking your match to heat your shack! ;-)
Seriously though this is a nice thought for reuse of those old radiators.
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Homebrew Dummy Load Project
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by K1CJS on June 18, 2009
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One other thing--you may have to redefine 'radiator' after this project!
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RE: Homebrew Dummy Load Project
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by WX1F on June 18, 2009
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Your article lacks any mention of resistor values, mounting methods or parts sources other than your local landfill. Not very helpful to newbies who haven't built up a deep junk box.
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RE: Homebrew Dummy Load Project
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by K5END on June 18, 2009
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My water heater draws roughly 22 amps at 240 volts.
Considering the error of my clamp on meter (not so much designed for current this low) I'd say the element was around 10 ohms, give or take a few.
But that curve in the folded element may have some reactance.
Best Ham gear investment I made so far: MFJ-259B
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RE: Homebrew Dummy Load Project
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by K5END on June 18, 2009
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Meant to add, good article. Very resourceful.
I used something like this for tube guitar amps, sharing the load with the speaker.
I can drive the amp to get the tone I want, but this way the neighbors don't call the cops when I like to practice late at night.
:)
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RE: Homebrew Dummy Load Project
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by N5TGL on June 18, 2009
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"But that curve in the folded element may have some reactance. "
The article doesn't state that you should use the factory installed element as a load. Maybe you missed this part of the article?
"Cut the heating element off close to the bottom but leave enough of a stub on it to install the desired dummy load resistors on it."
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RE: Homebrew Dummy Load Project
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by K0BG on June 18, 2009
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Bobbing along with the jetsam and flotsam on the Internet, is a almost identical article. In that one, a salvaged Heathkit Cantenna resistor was used. It's a lot of work, for not much gain in my opinion, and here's why.
The resistor in this case, is the limiting factor. No matter how much oil surrounds the resistor, there is only so much surface area to dissipate the heat. Engineers call this Energy Density, and it's usually rated in Joules. At some given level of RF, the maximum power level will be reached, regardless of the amount of oil surrounding the resistor, and the resistor will fail. The increase in oil volume will allow a lower power input to be sustained for a longer period, but how many amateurs need to transmit a 100 watt carrier for say an hour or more?
There is another issue, and that's insertion loss. For example, the Heathkit Cantenna was well designed, in that the resistor was surrounded by an aluminum, coaxial sleeve. This kept the SWR relatively low into the lower VHF region. If you carefully assembled it, the SWR at 440 MHz, was still under 2:1. If you look at the current dummy load offerings from MFJ, most of them aren't rated past 30 MHz, and those that are have very low max power ratings. Low Energy Densities, in other words. Only Alpha Radio Products makes a decently rated one (1,500 watts continuous at 30 MHz max), all for just $2,000!
In all fairness, if you have a burned out heater, and you don't want to replace the element, I suspect making a dummy load out of it is a worthy project. However, if you're going to use it upwards of 30 MHz, you'd best do a little research first.
Alan, KØBG
www.k0bg.com
PS: You can buy the necessary resistor, and other parts directly from MFJ, if you're so inclined to repeat Charlie's project.
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RE: Homebrew Dummy Load Project
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by W6RMK on June 18, 2009
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Caddock non-inductive resistors are available from Mouser (and others), and would be very suitable for this kind of project. They're inexpensive and available in various power ratings and resistances, and pretty cheap.
Data on thermal resistance immersed in oil is hard to come by, but you could probably guestimate and go from there.
What you might want to do is find a plumbing fitting with the same thread as the heater element (it's a standard pipe thread, usually), and drill it for a suitable hermetic coax connector. Why hermetic? Because you don't want oil on your floor, and the connector has to be at the bottom (you don't want your resistor surrounded by an air bubble, right?).
For what it's worth, silicone sealer does not seal oil tight. You get a slow seepage.
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RE: Homebrew Dummy Load Project
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by K0BG on June 18, 2009
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Jim, non-inductive is a bit of a misnomer, as all objects used at any reasonable radio frequency will have some inductance; even a straight piece of wire! Knowing that, it's easy to design a coaxial holder which essentially forms a transmission line. This is what I alluded to above with respect to the Heathkit Cantenna.
While a simple carbon resistor soldered on the end of an SO239 will act as a dummy load at low frequencies, as the frequency increases, so does the reactance. You have to cancel it out in some fashion.
If you go to the Alpha site (http://www.alpharadioproducts.com/2100.asp) you'll see a photo of their compensating network. Their overall design limits the dummy load to HF.
An HF-only dummy load is fine, but I use mine clear into the SHF range (1,296 MHz). My thought is (opinion really), if you're going to have one, don't limit yourself to just HF.
Alan, KØBG
www.k0bg.com
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RE: Homebrew Dummy Load Project
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by KR4WM on June 18, 2009
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K5END: Considering the error of my clamp on meter (not so much designed for current this low) I'd say the element was around 10 ohms, give or take a few.
I found an article on the internet a few months ago that detailed how to determine if a heating element in an electric water heater was bad. The figure given for a good element (if I remember correctly) was 7 ohms.
But- this article mentions cutting the element off, and then does not describe what is meant by "close to the bottom". I think what is a more accurate description would be "close to the threaded ring".
One object to overcome is that I believe the internals of the heating element will be made of tungsten. I have no idea how one would solder to that! So I must assume that the author means for the builder to attach the resistors with hardware (nuts and screws). Either that, or remove the hardened goop that will probably be found at the interface between where the element is cut off and the attachment screws, and figure out a way to connect directly to the screw/connector assembly.
This article leaves a lot to the imagination. This is not a problem for guys who have been around plumbing and electronics for awhile, but for a newcomer to the hobby who knows zilch about electronics and is just getting his/her feet wet in the hobby, the author might want to fill in the finer details of the project so these folks might gain realistic expectations of what they are getting themselves into if they choose to build one of these "contraptions". Details such as the correct value of resistors they need to obtain, where to obtain them, and exactly how to attach them would be helpful. Details of installing an SO-239 might also help a newcomer. (I assume a coax pigtail was used?)
One final consideration- most of these things are manufactured in China (probably). Has anyone thought about the possibility of PCB contaminated oil perhaps sneaking by U.S. inspectors maybe being inside these oil-filled heaters? Getting yourself all covered in this type of oil would be a major health and safety concern.
My overall feeling on this is that instead of trying to make treasure out of trash, if someone can afford a $1000.00 HF radio and $1500.00 kilowatt-class HF amp, then a $69.95 MFJ-250 isn't going to break the bank!
73, -KR4WM
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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Yes, it is true some details were "intentionally" left out of this article.
In particular, I was sure to avoid mentioning any particulars involving the "type of resistor" or "technical specification" information.
The reason why this was done was to provide readers with something to think about. This serves to provide the foundation for a "group discussion" in this message forum.
The hope is that "newcomers" will not only read the posted article itself, but perhaps they will also extrapolate information posted from the pool of hams posting their own design criteria and related technical information involving such a project.
Basically, the objective behind this posted article is to involve everyone in the forum in the actual construction and design of this project in an online sense.
So far after reading some of the responses, it seems the main objective of this article has been met.
Therefore, any criticisms, redesign construction information and/or other suggestions are most welcomed. I am sincerely enjoying the open technical discussion.
My Best.
73 de Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by WA7NCL on June 18, 2009
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Its no where near 50 ohms. You might just as well use a light bulb like the oldtimers.
You can make a 20W dummy load easily with 10 2watt composition resistors (470 or 510 ohm). With a reasonalble duty cycle and cooling you can push them much higher.
Another technique I've used that useful if you do not need a non-reactive load is to find large wire wound power resistors that are between 10 and several hundred ohms (with 50 to 100 being very good) and put them on the output of an antenna tuner. Match them up at low power and then you have 50 ohms at the match frequency. You can find some pretty big resistors surplus and in junk boxes. An auto tuner makes this sort of set up even easier.
I don't see any reason to mess with oil and its attendant mess and fire hazards. You certainly don't want to mess with the old stuff with PCBs in it.
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RE: Homebrew Dummy Load Project
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by K5END on June 18, 2009
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quote
"The article doesn't state that you should use the factory installed element as a load. Maybe you missed this part of the article?"
No, I read that part.
In any case it's still going to have a hairpin, regardless of which end is cut.
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RE: Homebrew Dummy Load Project
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by K5DVW on June 18, 2009
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So, have you ACTUALLY built this, or are you just daydreaming?
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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So, have you ACTUALLY built this, or are you just daydreaming?
-----
Both... :)
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RE: Homebrew Dummy Load Project
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by WX7G on June 18, 2009
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The heating elements in the heater might be used. On the 750 W setting the heater resistance is about 18 ohms. Three of these elements and we have a high power 54 ohm load.
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RE: Homebrew Dummy Load Project
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by N0YXB on June 18, 2009
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Oh stop being whiney babies about a dummy load! If you are like the author and enjoy experimenting, kudos! A lot of hams like to do things like this, and it's part of our ham heritage. On the other hand, if you like to buy everything prebuilt and you don't like to tinker, then go to a commercial site and buy a dummy load. There's no need to belittle the author because you don't like to experiment, or because you need someone to hold your hand and tell you every step. Thanks for the article, for some of us it was thought provoking.
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RE: Homebrew Dummy Load Project
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by N3OX on June 18, 2009
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"all objects used at any reasonable radio frequency will have some inductance"
The thick film resistors in TO-247 packages have little enough inductance that if you solder them close to a BNC, you can get reasonable SWR through UHF. It's a pretty cheap way to make a 100W-for-several-minutes dummy load with a scrap piece of heatsink, and with a bigger chunk (or maybe an oil cooled heatsink) you could run 100W forever.
I'm sure the return loss isn't -40dB or anything, but for a ham dummy load it probably doesn't have to be :-)
It's better than my antennas.
73
Dan
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RE: Homebrew Dummy Load Project
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by KB1LKR on June 18, 2009
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No, no, NO...
You're NOT using the 120 volt heating element as a resistor!
You're just using its MOUNTING BASE to mount parallelled (non inductive) resistors to get ~50 ohms, e.g. ten 499 ohm or twenty 1k ohm, and its fluid and housing to absorb/dissipate heat.
Since the resistors are immersed in cooling fluid they'll safely handle much higher power than they would in still air.
Similar can be/is done on smaller scale, lower power version w/ a gallon (or quart -- lower power still) paint can (from any paint store or McMaster, or wash out an empty can even) filled w/ mineral oil, (or transformer oil) and paralleled resistors on the lid -- basically a home built copy of the Heathkit Cantenna.
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RE: Homebrew Dummy Load Project
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by G3LBS on June 18, 2009
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Aren't some oils and their vapors carcinogenic?
I built a salt water dummy load but was advised against using it because critics said I would blow myself up when hydrogen and oxygen were produced.
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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Well by popular demand here's the MSDS sheet...
http://www.paratherm.com/_engineering/OR_MSDS.pdf
The heat transfer fluid used inside these heaters are known as Thermic fluid. Thermic fluids are a hydro treated 99% mineral based, non hazardous, non controlled substance.
Potential Health Effects:
Eye: Non-irritating to eyes on direct contact before use. Mildly irritating to eyes after product is used.
Skin: Non-irritating on direct single or repeated and prolonged contact when new. Repeated or prolonged contact of used product may cause minor skin irritation.
Ingestion: No harmful effects are expected from ingesting new product or small amounts of used
product.
Inhalation: Exposure to smoke or mist while product is in use may cause irritation to upper
respiratory tract and lungs.
Recommendations:
Please do not eat, drink, inhale or bathe yourself in these heat transfer fluids because minor irritations may occur.
In addition, there are no harmful or EPA regulated transformer oil containing any PCB's inside these everyday consumer products... lol
My Best,
Charles - KC8VWM
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Homebrew Dummy Load Project
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by KC8RWR on June 18, 2009
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My first thought is to use multiple resistors, maybe 10 500 ohm or 20 1k ohm in parallel. Someone mentioned doing this and skipping the oil but why not do both? Not only do you get the higher power rating from the combined resistors but you also get the higher surface are making contact with more oil.
I was imagining only cutting the end off of the old element instead of cutting close to the base. Then you could solder the resistors across the elements evenly spaced so that there is lots of room for oil to flow between them. The problem with that is that then you would have to take into account the resistance of the remaining heating element.
Someone mentioned finding a plumbing fitting that would fit on in place of the heating element. If you could find two long copper bolts maybe they could be ran through the plumbing fitting with the resistors soldered across them. There would be some reactance but hopefully not too much.
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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Nice Job....Chuckie!!!!
I getting transformer oil spilled on me, when the station I worked for had to meet OSHA standards.
Same Carcinogenic/Mutagenic crap is used ib some older oil filled caps.
I am going to give your gizmo a try. Bet your device radiates no worse than a cantenna.
I have a mil spec bird dry load. It is only good up to 100W for 30 secs or less.
Nice job of Recycling! I have a feeling finding the device free in NYC, will take some time:(
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Homebrew Dummy Load Project
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by KC8RWR on June 18, 2009
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I think the people mentioning PCBs and the potential for dangerous compounds in the oil have an excellent point for why one should probably think twice before doing this with a heater which appears to be 30 or more years old. I really doubt you are likely to run into much worse than mineral oil with a bit of rust in it inside anything newer than that. Such things have been out of use plenty long enough for safe broken heaters to be available.
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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<<<<<<RE: Homebrew Dummy Load Project Reply
by N0YXB on June 18, 2009 Mail this to a friend!
Oh stop being whiney babies about a dummy load! If you are like the author and enjoy experimenting, kudos! A lot of hams like to do things like this, and it's part of our ham heritage. On the other hand, if you like to buy everything prebuilt and you don't like to tinker, then go to a commercial site and buy a dummy load. There's no need to belittle the author because you don't like to experiment, or because you need someone to hold your hand and tell you every step. Thanks for the article, for some of us it was thought provoking.>>>>>>>>>
THIS IS WORTH REPEATING!!!! TNX N0YXB de WA2JJH
I love the nit pickers! They act like they were banned from Pattaya Beach!!!
TNX AGN TO CHUCK FER HIS TIME!!!!!!!! People biotch about not enough tech on eham. GEESHH!!!!!
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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Mineral oil is very safe. It has no carcinogenic/mutagenic properties. Only problem I had with mineral oil was tripping on a Cantenna. An entire gallon of mineral oil is ultra difficult to get off the floor!!!
The longer it sits on the floor, the harder it is to remove.
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RE: Homebrew Dummy Load Project
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by AB7E on June 18, 2009
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N0YXB: "Oh stop being whiney babies about a dummy load! If you are like the author and enjoy experimenting, kudos! A lot of hams like to do things like this, and it's part of our ham heritage. On the other hand, if you like to buy everything prebuilt and you don't like to tinker, then go to a commercial site and buy a dummy load. There's no need to belittle the author because you don't like to experiment, or because you need someone to hold your hand and tell you every step. Thanks for the article, for some of us it was thought provoking."
Sorry, but I don't for a moment think that KC8VWM actually built one of these. He shows a catalog picture of a water heater element but none of his modified unit. He speaks in vague generalities about the choice of resistor without mentioning what he used. And he offers no information on the finished unit's performance (temperature rise for x watts of power over y minutes of time, SWR as a function of frequency, etc).
I'm not even sure heating elements can be modified as he describes. If I remember correctly from the last time I removed a burned out water heater element, the casing merely encloses the actual wire element which is surrounded by a ceramic medium. I don't think the casing (the tubing you see) is electrically connected to the external terminals. It would probably be possible to safely do so for an element immersed in oil, but not for one in water, and I seriously doubt that space heater manufacturers are going to use a different element design when the manufacturing volumes would be so much lower.
In my opinion, whatever experimenting KC8VWM may have done was performed in some sort of virtual reality. I'm not knocking that in general because ideas are ideas and can spawn other ideas, but I think you're giving the author way too much credit in this case, and I think your indignant reaction to the comments here is overblown. I think this is a musing posing as a construction article.
If it turns out I am wrong, I'll apologize.
73,
Dave AB7E
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RE: Homebrew Dummy Load Project
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by KB1LKR on June 18, 2009
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"Aren't some oils and their vapors carcinogenic?"
Yes, (e.g. old polychlorinated biphenyl (PCB) containing transformer oils) but USP grade mineral oil (available at any pharmacy) isn't, nor are modern transformer dielectric oils (which are even better than USP mineral oil for cooling) e.g. Shell Diala®, Exxon Univolt, etc. though these may be hard to source in small volumes (5 gallons or less).
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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Sorry, but I don't for a moment think that KC8VWM actually built one of these. He shows a catalog picture of a water heater element but none of his modified unit.
----
Hard to take pictures of a project when you write and submit articles to the site when you are working on the night shift at a completely different location.
Dave, can I just simply state that the project started out as a broken heater that I used to heat my hamshack?
It's funny how if you stare at something broken and sitting in the corner like this long enough, how it starts to resemble a large heatsink.
Of course, like most hams, I too might like to take things apart before throwing them out. Strange what you can "discover" in the process. Sometimes they turn into an article like this one.
Yes it's a very real project...
Do I really have to go into "explanations" with you Dave? I would rather not, but I can.
My Best.
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RE: Homebrew Dummy Load Project
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by AB7E on June 18, 2009
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Well, I'm honestly curious what you found when you cut the heater element, and how did you connect the resistors to whatever you found there?
73,
Dave AB7E
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RE: Homebrew Dummy Load Project
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by N3JBH on June 18, 2009
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Charles they unit you show is the Chromalox SG1453-430334. It is for a water heater with some mineral hardness. Is this the same as found in your average radiant oil filled space heater?
I was under the impression oil immersion elements used in these type heaters had a nickle coating on them. Was some what wondering how you attached any resistors. Jeff
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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We should be more tolerant with each other.
Having Cliques of Hams, gather on one dude is stupid.
I have known Charles on Ebay for many years. He is aways on the money!
Do some Data mining. You will see patterns of who cliques up with who!!!!
I think Charles is well due for an apology. He said right up front his gizmo was less than perfect device.
Some dude even accused him of total fabrication!!!!
Charles IS A PATENT ENGINNER. HE GETS PAID THE BIG BUX.
HIS UNIQUE design is "pearles before the swine!
Sure, he could have thown in better pics. However,he just put too much faith in his fellow hams.
Some of the more rude comments were.....I would rather buy a commercial mil spec job!
Being accused of making it all up was another classic!
Ugly fact is that eHam has some technocratic cliques.
Charles and a few others of us, have our own clique too! Do not worry, We do not do a hot prison shower scene on you. We just email what a bunch of OCD-AR, PSUEDOINTELLECTUAL POSERS, the other clique are!!!!
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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In my experiment, the "long" resistor leads were wound around and soldered to a copper "clamp" in a parallel configuration. These clamps look like this:
http://www.lakewoodconferences.com/direct/dbimage/50277973/Copper_Wire_Clamp.jpg
Once the resistors are soldered to the copper clamps, the clamp is secured using pressure around the heater "stubs" I am suspecting the oil is going to prevent corrosion and electrical contact issues occurring between the clamp and the stub. Feel free to experiment with other ways. I thought about an actual PC board with a string of resistors on it. But I was experimenting with the "quick and dirty" design at the time.
I suppose there may be other ways to achieve this overall contraction to achieve the desired results and I purposely left this open for further thought, discussion and consideration. (This is the part that may be misinterpreted as coming across as vague to some readers.)
The resistors I was using were what I happened to have in the shack at the time. I had a few cards of 1k - 1 watt carbon resistors on hand. I calculated I needed 20 of these wired in parallel to achieve a range of approx. 50 ohms or at least it "should be" close enough when attached to the heater stubs etc..(1000/20 = 50 etc..) These resistors provide 21 watts continuous "dry" output power. (21 x 1 watt = 21 etc..) However it's possible to achieve higher "intermittent" duty when submersed in oil.
Looking back I was thinking 3 watt metal film resistors would have been a better "temporary" choice for my experiment however the main objective is to finalize the design using an even greater power handling capability resistor arrangement. Ideally a small billy club of a resistor (like Cantenna uses or better) would fit nicely inside the heater element space.
No I did not originally create a particularly high powered dummy load in reality (yet) but keep in mind I simply indicated in the article the project is "capable." That's why I wasn't too specific about the details in this area.
However, using this design it's easy to state the possibility definitely exists.
For example it's possible to install 2 x 200 ohm x 250 watt non inductive resistors into this project configuration. I have yet to acquire them for my own project so no I suppose there is no point in conducting voltage measurement curves in the article until I am in a position to test and finalize the design.
However, the project is promising and functions as expected. Seems I am able to use the dummy load at 50 -75 watts output for intermittent testing purposes. However I have not "pushed" it to the limits of a 100 watts to see what it can do at this point in time because the current string of resistors I am using are the limiting factor in the equation.
However, I feel it's a fair assessment and I cannot find any other reason why other non inductive resistors could not be used in a similar fashion.
It's worth mentioning in lieu of resistors, I have been thinking and researching ways to construct a high powered resistive non conductive load using nichrome wire in combination with some sort of homemade form attached to the heater "stubs"
Nichrome (NiCr 60)wire exhibits resistance in the order of .831Ohms/ft.
This approach would appear similar in construction to how parasitic suppressors are used on the grid caps in a typical linear amplifier design.
One concern is if the heated oil would change the physical resistive properties involved or if the oil will react with the Nichrome wire in some unexpected way. I am also concerned about the "balancing act" involved with how DC current would behave or how capacitance might fit into this equation across a swath of various frequencies with this type of construction however I suppose it's worth investigating.
My Best.
Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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I remember building my Heath kit cantenna 30 years ago.
I paid 39 bux for mine. Frankly, nobody had a cheap 1KW for sale.
It is a real 5 gallon paint can. One had to solder the DC coupler/sampler to the top of the can. It was not really needed.
The idea was to have an S0-239 connector. The back of the sampler had an RCA connector. The RCA connector was handy for getting less than a volt out. So you could hear your AM audio, or make a poor wattmeter.
I remember soldering the long carbon 2 W RESISTOR lattice of resistors. The resisitors were in a small aluminum shell. I am not sure if that was for less RF leakage, or a capillary flow action would move the hot oil between the resistors in the tube and the 5 gallon resouvoir. My money is on the capillary action.
Many hams reported having QSO's using the Cantennas as an unintended antenna!!!!!!
It did handle a KW very well. Freq responce was very good too. Non inducting resistors all in parallel made any resistor lead length moot! One did have to make sure the resistor ladder was tight and symetrical pickled herring.
For under $30, it was Heathkits big bargain. The one I build said MINERAL OIL ONLY. If you suspect you purchased a used one from "Love Canal"......BURN IT!!!!!
What drove me to DRY LOADS were 2 incidents.
1)I tripped on my mineral oil cantenna. It took weeks to get the floor safe.
2)Many years ago working at the Empire STATE buiding.
CH-11s back up tX was located there. A co-worker spilled a can of transmitter coolant!!!!
WE paniced. Got Lawyers and called the Union. Next day a rep. from the oil, told us it was safe. Quoat......Ya can fry your eggs in it.
To this day I feel the dude lied!!!!!!!
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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I remember building my Heath kit cantenna 30 years ago.
I paid 39 bux for mine. Frankly, nobody had a cheap 1KW for sale.
It is a real 5 gallon paint can. One had to solder the DC coupler/sampler to the top of the can. It was not really needed.
The idea was to have an S0-239 connector. The back of the sampler had an RCA connector. The RCA connector was handy for getting less than a volt out. So you could hear your AM audio, or make a poor wattmeter.
I remember soldering the long carbon 2 W RESISTOR lattice of resistors. The resisitors were in a small aluminum shell. I am not sure if that was for less RF leakage, or a capillary flow action would move the hot oil between the resistors in the tube and the 5 gallon resouvoir. My money is on the capillary action.
Many hams reported having QSO's using the Cantennas as an unintended antenna!!!!!!
It did handle a KW very well. Freq responce was very good too. Non inducting resistors all in parallel made any resistor lead length moot! One did have to make sure the resistor ladder was tight and symetrical pickled herring.
For under $30, it was Heathkits big bargain. The one I build said MINERAL OIL ONLY. If you suspect you purchased a used one from "Love Canal"......BURN IT!!!!!
What drove me to DRY LOADS were 2 incidents.
1)I tripped on my mineral oil cantenna. It took weeks to get the floor safe.
2)Many years ago working at the Empire STATE buiding.
CH-11s back up tX was located there. A co-worker spilled a can of transmitter coolant!!!!
WE paniced. Got Lawyers and called the Union. Next day a rep. from the oil, told us it was safe. Quoat......Ya can fry your eggs in it.
To this day I feel the dude lied!!!!!!!
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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WE paniced. Got Lawyers and called the Union. Next day a rep. from the oil, told us it was safe. Quoat......Ya can fry your eggs in it.
-----
Mikee,
So instead of a qualified individual from the company responding to contain the hazardous situation, apparently you had some sort of New York city short order sous chef arriving on the scene?
:)
Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 18, 2009
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SRI FOR HOGGING THREAD.
How about 300W worth of 2W carbon resistors....Chilled with a T.E.C!!
One of the TECS can remove over 500W of waste heat when used with a chain of 12V ULTRA large computer fans?
Cost. 150 2W resistors.....$35-50
2 150W TECs...ebay $50each
6 12V 700ma computer fans.....$25
Used computer PS 450W+ Free-$45
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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Jeff says:
I was under the impression oil immersion elements used in these type heaters had a nickle coating on them. Was some what wondering how you attached any resistors.
----
Well you need a method to achieve a mechanical bond with the heater element stubs if that makes sense.
I suppose you could try to solder the resistors on but good luck with that.. been there tried that.. failed the solder test.. :)
When considering this project I used the heating element stubs and figured, "Why reinvent the heating element?"
It already provided an adequate seal to prevent the heat transfer fluid from leaking out so I stayed with the original heating element threaded design and seal arrangement but in a modified sort of way.
Too bad they don't manufacture 50 ohm heating elements huh? .... or do they? :)
My Best,
Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by KE6RAD on June 18, 2009
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I want to build a home brew QR0 dummy load
I want it to sink 1500 watts, with 50% duty cycle, at 40 deg ambient air temp, with no time limit other than the duty cycle.
This is just so I don't have to worry about waiting for it to cool off, ever.
I'd like temp sensors back to my PC with a COM port, so I can track the temps.
Just for 80M-10M. Fine if SWR is <1.3:1 (but even after heating)
For a high duty cycle, I don't see why a lot of oil is desired as a thermal mass to absorb heat Water actually has a higher specific heat than oil, anyhow, right?. So water would absorb more heat per lb per allowed deg F temp rise. It also has 3x better thermal conductivity than oil.
Unless you're worried about steam? Or you want to allow a higher max temp with oil? (I guess you have the electrical conductivity issue, so you don't want direct contact with water?)
The thermal conductivity of oil is a lot worse than copper or aluminum. So it's not clear why oil being the first thing contacting the resistor is best? (I can see it's better than still air by a factor of six or so, though).
So these new heatsinkable planar resistors (there are many vendors other than Caddock) seem interesting (TO-224 packages or other flat). Cause you can bolt them to Copper or Aluminum. And the resistors have a nice thermal conductivity number.
It would seem it's all about dissipating from the resistor to heat sink to air, quickly. Lots of surface area of Copper would be best (best thermal conductivity to the resistor). Aluminum next best. Can't go to air directly because air has such low thermal conductivity and the resistor has small surface area.
Bent sheet could be used to create a lot of U shaped fins, that could be riveted together, and air flow is cheap (muffin fans).
Assuming a 80 deg C temp rise for the resistor is okay, then 200 watt dissipation from resistor to heatsink + fan(s) should be doable? So with some extra thermal mass, and maybe 4 to 8 resistors, a 50% duty cycle of 1500 watts should be possible?
I was also thinking of home brew possiblities from lots of copper pipe. Strapping a lot of pipe sections together with air flowing, is like a finned heat sink.
Or potentially with aluminum fins like a baseboard heater.
Also was looking at all the nice PC CPU heatsinks now with copper heat pipes and big fin towers and fans. Dissipate 150W easy. But too expensive to parallel a bunch of them.
For this article:
I don't see what problem the oil solves, except for low duty cycles? I don't see why this approach is a good one for a home brew QRO dummy load? Unless the surface area of the radiator is enough to dissipate 1500 watts at 100% duty cycle? is that it?
Can someone due the math on thermal conductivity, specific heat, and heat rise of the various pieces, and describe the duty cycle supported here?
Most commercial stuff seems to talk about 10% duty cycles?
In short, independent of the lack of construction details, I can't see what requirements this project meets. The Cantenna doesn't meet my requirements. Many commercial don't. My rough guess is that $.10/watt is needed for construction. I'm interested in solutions that get below $0.10 watt for the goals I described.
I was surprised by the article, since I've been musing on this for the last couple days, since I burned out my lightweight dummy load (well it's a nice 75 ohm dummy load now :). But this article doesn't answer any of the questions I have about what would be good for me to do..
I ended up telling myself the Vectronics DL-2500 with more air flow through added fans, was the most cost effective solution. (<$200. You get 4 nice resistors). Good to 54 Mhz.
http://www.vectronics.com/man/pdf/DL-2500.pdf
MFJ and Ameritron seem to be selling the same thing. I suspect Vectronics is the OEM?
Probably won't quite meet the duty cycle requirement. Easy to add the temp sensors to a serial port with some kit stuff.
Thoughts?
-kevin
ke6rad
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RE: Homebrew Dummy Load Project
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by AB7E on June 18, 2009
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KC8VWM: "Once the resistors are soldered to the copper clamps, the clamp is secured using pressure around the heater "stubs" I am suspecting the oil is going to prevent corrosion and electrical contact issues occurring between the clamp and the stub."
Uhh, that isn't a picture of the clamps ... that's a drawing. What on earth is so difficult about providing a picture to show how you did it?
And let me get this straight --- you're saying that the tubing stubs ... the portion you are clamping to ... is directly connected to the external terminals? Sorry, I don't believe that. That tubing is NOT the resistive element in one of those heaters. It's merely a protective shield for the element inside.
WA2JJH: "Some dude even accused him of total fabrication!!!! "
Yup ... that would be me. Still is. And my offer to apologize also still stands if I see even the slightest evidence that I am wrong.
73,
Dave AB7E
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RE: Homebrew Dummy Load Project
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by AB7E on June 18, 2009
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p.s. Metal film resistors are a really bad idea for a dummy load ... they are pretty fragile and won't take excessive high current.
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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No they don't take excessive high current but they are pretty darn cheap in comparison Dave..
:)
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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Uhh, that isn't a picture of the clamps ... that's a drawing. What on earth is so difficult about providing a picture to show how you did it?
----
Who do you work for the NSA? FBI?
Give it a rest already...
Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by KE6RAD on June 18, 2009
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In sanity checking my Vectronics DL-2500 plan, I looked at the internal pictures of the Palstar DL5K and DL2k
http://www.palstar.com/dl5k.php
http://www.palstar.com/dl2k.php
4 resistors in the dl2k but only 2 (bigger) in the dl5k
They say the dl5k can handle 1000W continuous.
Two 160 CFM fans seem to be the big difference compared to the DL2K.
Also, they keep nice loose specs which are fine:
50 ohms +0/-10%
DC to 300 Mhz.
So I'm thinking it should be pretty easy to add two ~160 CFM fans to the Vectronics DL-2500. The four resistors are nice for dissipation surface area.
a 120mmx120mm 38mm dc axial fan at 3900 rpm can do 130 cfm, from 12v 1A (each) So two of them should fit. Don't care about the loudness. (50+dbA)
Even 4400 rpm and 200 CFM seems doable, at 12V 2.5A each. (60+dBA)
fan data (example)
http://www.ystechusa.com/dc120series.html
I'm still thinking that this kind of setup, at $190 or so, is the best solution. Homebrew or otherwise. Unless you like low duty cycles.
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RE: Homebrew Dummy Load Project
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by KC8VWM on June 18, 2009
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the portion you are clamping to ... is directly connected to the external terminals? Sorry, I don't believe that. That tubing is NOT the resistive element in one of those heaters. It's merely a protective shield for the element inside.
----------
You are completly lost Dave... I am assuming when constructing this project you are making an appropriate electrical connection between the screw terminals on the outside and the copper clamp silly.
I "assume" (perhaps I am wrong) most hams are smart enough to figure out they should conduct a continuity check of the circuit when connecting such an arrangement.
Do I also need to specifically indicate the direction the solder should flow when connecting the PL 259 connector to the coaxial cable? Sheesh..
.. Really now. Your grasping at straws and pointing out the obvious for some apparent reason.
No need to continue flapping your wings here Dave.
...Really.
Charles - KC8VWM
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RE: Homebrew Dummy Load Project
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by AB7E on June 19, 2009
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KC8VWM: "I am assuming when constructing this project you are making an appropriate electrical connection between the screw terminals on the outside and the copper clamp silly. "
That isn't feasible, as you would know if you had ever actually held one of those heating elements in your hand. The more you try to bluff your way through this the more obvious it is that your "project" is a myth.
KC8VWM: "Really now. Your grasping at straws and pointing out the obvious for some apparent reason. "
What I've been pointing out all along is simply that you didn't build this project ... it's just an idea you had. There would have been nothing wrong with that except that you now keep claiming you did build one. Oh ... that and the fact that it is not possible to build one in the manner you've stated. As soon as you admit to that I'll give it a rest.
By the way, I'm pretty sure that my inlaws have a defunct oil-filled radiating heater almost exactly like the one you referenced. I plan to check tomorrow, and if they do, I'll take it apart and take pictures as I go to prove my point ... or yours, whichever turns out to be the case. If the latter, I'll apologize. If the former, I'll expect you and WA6JJH to do the same.
73,
Dave AB7E
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RE: Homebrew Dummy Load Project
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by AB7E on June 19, 2009
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AB7E: "If the former, I'll expect you and WA6JJH to do the same. "
My sincere apologies to WA6JJH ... that should have been WA2JJH.
Dave AB7E
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Homebrew Dummy Load Project
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by KB4QAA on June 19, 2009
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As I recall the 1977 ARRL Handbook had plans for an oil filled paint can dummy load. The resistor element could be made from copper sheet soldered together.
The resistor was a bi-conical shape with holes and exterior fins. The bi-conical shape was supposed to maintain impedance across the frequency range.
I don't know if the design had previously been published in QST. That is just before my time and I haven't used the online QST archives yet.
The 1988 and 2005 manuals don't have those plans.
73, Bill
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RE: Homebrew Dummy Load Project
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by N2EY on June 19, 2009
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M3OX writes: "The thick film resistors in TO-247 packages have little enough inductance that if you solder them close to a BNC, you can get reasonable SWR through UHF. It's a pretty cheap way to make a 100W-for-several-minutes dummy load with a scrap piece of heatsink, and with a bigger chunk (or maybe an oil cooled heatsink) you could run 100W forever.
I'm sure the return loss isn't -40dB or anything, but for a ham dummy load it probably doesn't have to be :-)"
Here's how one ham did it:
http://www.n0ss.net/dl_30w_hf-uhf.pdf
73 de Jim, N2EY
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RE: Homebrew Dummy Load Project
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by WX1F on June 19, 2009
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To CHUCK(KC8VWM)
Betcha you went fishing with the can of worms YOU opened,huh?
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Homebrew Dummy Load Project
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by N3QE on June 19, 2009
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I have my 100W light bulb, I've been using it as a dummy load since the 1960's :-).
If you want to do some math: Power = Volts*Volts/R, so R = Volts*Volts/Power. So to find a random 120VAC light bulb or heater rated in watts and turn it into equivalent ohms, here's a little table:
100W = 144 ohms
200W = 72 ohms
300W = 48 ohms
Note that light bulbs and many heaters are NOT simple resistors. Their resistance depends very strongly on their temperature. When cold, their resistance is lower than the above formula will give you. So a 100W light bulb is actually not the worst load in the world to a 50 ohm or 75 ohm boatanchor transmitter that only puts 60W out.
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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KE6RAD,
I was also thinking of home brew possiblities from lots of copper pipe. Strapping a lot of pipe sections together with air flowing, is like a finned heat sink.
Or potentially with aluminum fins like a baseboard heater.
------
Another approach similar to the fans you mentioned I was thinking about is to install a small RV style (Shurflo) circulation pump to move the heat transfer fluid around inside the oil filled dummy load.
I also figure that resistive nichrome wire wound around a large form would dissipate heat more effectively if installed over a larger surface area. It would seem physical size of the actual load matters when it comes to efficient heat tranfer dissipation.
73 de Charles - KC8VWM
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RE: Home brew Dummy Load Project
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by K9FV on June 19, 2009
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"banned from Pattaya Beach" Now that would be terrible!!! "IF" I were just back in Thailand {smiling dreamily}
Ken
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RE: Home brew Dummy Load Project
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by KE6RAD on June 19, 2009
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Charles replied
"Another approach similar to the fans you mentioned I was thinking about is to install a small RV style (Shurflo) circulation pump to move the heat transfer fluid around inside the oil filled dummy load."
No, if you're going to pump, you don't want to pump oil. Water is better (go back to what I said about water's thermal conductivity and specific heat, compared to oil). But in a home brew dummy load, you don't want to pump? Because up to 1500 watts, air flow is sufficient. Unless you're worried about noise.
Here's a picture of a 5kw dummy load, with water recirculation. You can see the use of copper slabs. The copper has the best thermal conductivity (other than silver, say).
http://www.qsl.net/f5vby/6kwdummyload-large.jpg
The key issue is the resistor(s). The caddock-style power resistors are nice for mounting to a metal slab. There are other flat packages besides TO-247 also.
In looking for "cheap" I checked out http://www.token.com.tw/power-resistor/power-resistor-rmg100.htm
quoted 50 pieces of
RMG-100-F(+/-1%)-P-E(+/-100ppm)-1001(1000 ohm) at $4.2 apiece. So that would be a low-ball kind of price.
example of another: Ohmite has this TO-247 70 watt resistor
http://www.alliedelec.com/Images/Products/DataSheets/BM/OHMITE/Ohmite_Actives-and-Passives_2962570.pdf
etc.
Seems like you'd want to budget 1.5 deg C/W temp rise at the resistor, based on their spec sheets. Plus best case 0.5 deg C/W temp rise for a great heatsink fan setup.
They spec a max resistor temp of maybe 150 deg C (although I'd be worried about heat going back to the SO-239 also), and ambient of maybe 40 deg C worst case..110 deg C rise at the resistor allowable.
2 total deg C/W ..so 55 Watts per resistor (100%) seems like a max? maybe 100W at 50% duty cycle.
So you can see how I started thinking that even 8 of these TO-247's wouldn't be enough.
So you can see how I'm stuck. I can't see how the math adds up to do a QRO dummy load with a bunch of TO-247 power resistors, unless 16 or more are used? And then that's a lot of heat sink. I suppose using 8 would be tolerable. But depending on the vendor, you might spend $6-7 apiece. Don't need 1% tolerance. Was looking for 5% solutions also.
I can't find anyone who's done a home brew QRO dummy load, except at low duty cycles over time.
The Vishay NH series aluminum package low inductance wound power resistors seem usable (others have these also). But they don't seem to save cost.. $11 for 50 watts? still would need 8 or so of them?
http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMuQ5%2fsBR7SoAhX3PKBRPX6uEbhgb8j7QqA%3d
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RE: Home brew Dummy Load Project
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by KE6RAD on June 19, 2009
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okay, this $32 resistor from MFJ seems like a good deal to me. I guess I'm suggesting that no one can propose a better low-cost solution for 1500W high duty cycle, than as follows. Interested in feedback. Note I'm also saying no oil-in-radiator-with-unspecified-resistor(s) project makes sense.
Details:
MFJ, Ameritron, Vectronics all seem to sell the same DL-2500.
It has 4 ceramic metal resistors, that are 6 in x 1 in.
50 ohm +-10%.
I guess that means two resistors are in series, and the pairs are paralleled, so four give 50 ohms also.
They looks similar to the size resistor used in the Palstar DL1500 lightweight dummy load.
(they have single resistor, no fan, on that one).
They say resistors have energy density 7000 J/inch**3/sec. Dispersed carbon particles in a ceramic matrix
Voltage gradient 10kv per inch.
I measure 3.25" circumference, 4.75" of resistor area (the rest is contact area on the ends).
so that's 15.4 sq inches for heat dissipation. 61.6 sq in total for 4 resistors.
MFJ sells the resistor as a replacement item, for $32.31. Part # 119-1500.
MFJ says 200 Watts for the resistor. That's 13W per sq. inch. These resistors can tolerate a larger temp rise. In any case, 13W per sq in might be reasonable for still air.
Interestingly the DL2500 user manual says 50% duty cycle with the fan. With 70 seconds max at 1500 watts.
The fan is pretty cheap. 3110KL-04W-B40. 12v 80x80x25 3250 rpm 38.8 CFM.
An easy replacement to improve cooling is the Sunon 80x80x38 PMD1208PMMB1-A, ball bearing fan which gives 84.1 CFM at 55.2 dBA with 5700 rpm. Double the air flow. 0.76A at 12V.
A parting link. Altronic has some cool dummy loads for very high power. Nice looking at their product manuals to see how they arrange the cermet resistors for convection cooling. They have details on their cermet resistors (like the MFJ resistor) here:
http://www.altronic.com/catalog/resistors.htm
Altronic says 17 Watts/sq in for forced air, and 8.5 watts/sq in for still air, for their resistors. (450 deg F max temp rise above ambient of the resistor? Not sure if 17 watts/sq in is at that max resistor temp)
Altronic say 25 Watts/sq in for immersed mineral oil.
So, seems to me, that 4 of these resistors, like in the DL-2500, with enough forced air to support the 17 watts/sq in altronic talks about, would give 17 * 61.6 = 1047W continuous. So 50% at 1500W seems pretty good.
Note immersed mineral oil doesn't even double the watts/sq in. that Altronic talks about, and they know what they're talking about (look at all their products and information. You can see how they arrange their cermets here, page 14. They use 4 longer ones. Convection only. 6000 watts. 110F ambient tolerated.
http://www.altronic.com/techman/convcooled/6606.pdf
some data here:
http://www.altronic.com/images/catalog/larger/resistors.gif
So for my 1500W application, airflow over 4 of these resistors seems like the best cost solution. And the MFJ price seems reasonable, compared to other solutions.
Comments?
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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So for my 1500W application, airflow over 4 of these resistors seems like the best cost solution. And the MFJ price seems reasonable, compared to other solutions.
Comments?
-----------
I was just researching AM broadcast transmitter dummy loads and came across this.
There is one that can handle 20 KW and it's quite simple to make.
Basically the "ham" version of this broadcast transmitter dummy load uses a plastic garbage can with a SO 239 connector installed on top. There are two "plates" which are installed on each side of the garbage can similar to how a capacitor is constructed. The plates connect to the SO 239 connector.
Next you fill the garbage can with clear water. Connect the SWR meter from the transmitter to the dummy load and set the transmitter at minimum power. Start adding salt to the water until the SWR reaches 1.1:1
That's it! .. It will handle 20 KW for a short period of time before the water starts boiling. It will handle 1500 watts continuous duty all day long.
More information (on a smaller scale) about this type of dummy load construction here:
http://www.qsl.net/k5lxp/projects/SaltLoad/SaltLoad.html
I reading how this version had a very acceptable VSWR ranging from 160 - 2m VHF.
Seems to be a pretty economical way to go and no expensive resistors needed.
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RE: Homebrew Dummy Load Project
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by W4VR on June 19, 2009
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And to think that all these years I've been using a 100 watt light bulb for a dummy load!
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RE: Home brew Dummy Load Project
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by KE6RAD on June 19, 2009
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I had looked at salt water stuff.
But think about some engineering issues.
It has to be a closed system. If you get evaporation, the load changes.
A garbage can doesn't work. Water will evaporate over time. At the very least you wouldn't know. You'd have to test before use every time.
Also, there are some safety issue with water. Turns to steam at 212F, increasing pressure. You don't want something to explode with steam? Sure you can have an open system, but then you have the evaporation problem.
I can see a closed system, with maybe a relief valve or minimally a temperature monitor, so you know what's going on. Some calcs showing the surface area and heat dissipation behaviors of the container, over various ambients would be nice also.
But you can't just take random potshots at it right? Have to design something.
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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Also, there are some safety issue with water. Turns to steam at 212F, increasing pressure. You don't want something to explode with steam? Sure you can have an open system, but then you have the evaporation problem.
--------
That's where a hot water thermocoupler would come in.
When water reaches a pre defined temperature, it would open the circuit to the PTT switch. Or similarly, the thermocoupler could be configured to prevent the amplifier relay from closing. Effectively disabling TX ability until the thermocoupler resets.
This would allow you to construct a closed system and add the safety aspect to the system.
Also I think it could be rescaled to operate using a sealed 5 gallon pail. The plate electrodes could be constructed using stainless steel to prevent corrosion. I thought about adding a ratio percentage of "glycol" to the water as this may effectively help to increase the boiling point temperature.
73 de Charles - KC8VWM
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RE: Home brew Dummy Load Project
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by KE6RAD on June 19, 2009
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Hmm. antifreeze? ethylene glycol? seems like a good way to poison the neighborhood cats and dogs. (it tastes sweet to dogs)
The PC world has created an infinite selection of fans of all shapes, airflow, and dbA. Running off 12v. I always think we're behind the times in terms of thermal management with ham gear.
There's no reason not to use the PC fans. Some come with the thermal sensors already there, so they slow down at lower temps.
I just ordered a Thermaltake Smart Case Fan2 from Newegg.com
80mmx80mmx25mm so it's a drop in upgrade of the dl2500 fan. (there are better selections, but examining the dl2500 made me realize that I couldn't go thicker or wider, or even double the fan without cutting the chassis or making a new cover (hmm perforated sheet metal?).
$8.49 + $2.99 + $0.70 tax
it has a mode that uses a temp sensor that comes with it:
Fan speed : 1300 rpm at 20°C , 4800 rpm at 55°C
Noise : 17 dB at 1300 rpm, 48 dB at 4800 rpm
Bearing Type: Two Ball Bearing
Max. Air Flow : 20.55 CFM at 1300 rpm, 75.7 CFM at 4800 rpm
Air Pressure : 1.45mm H2O at 1300 rpm, 8.43 H2O at 4800 rpm
75.7 CFM is quite an improvement over the 38.8 cfm fan that comes with the DL-2500. Plus I might be able to just leave it plugged in all the time. Good for making sure the fan stays on high long enough to cool down.
I may have to nibble out the chassis cutouts to make sure they don't impede airflow.
What's wrong with airflow? Why invent other solutions?
(I've still not seen a workable solution, in any case)
I think the right construction article would be to describe how to get the right resistors, and the right temp controlled airflow, with measured data on the resistor temps at various duty cycles and fan cfm.
The DL-2500 has nice copper clips for the resistors. So there's a number of minor issues to deal with too. You don't want to solder because of the allowed resistor temps.
All in all, I think a homebrew project (with my goals) has got to be around $150. I'd be interested in other sources other than MFJ for the $32 resistor I mentioned.
I actually need a QRO dummy load that's reliable. Don't want a random experiment. I hate reading these oil-filled articles that have no data on the thermal behavior and try to pass on anecdotal info. Shouldn't we be past that?
Sure light bulbs are great, but as stated, variable resistance. If you change the goals to include variable resistance with temperature, than there are lots of solutions.
-kevin
ke6rad
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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The question is 75.7 CFM airflow fan capable of cooling 5120 BTU's heat energy. (1500 watts)
CFM = btu / (1.08 * tRise)
Depending on humidity / altutude etc. but 1.08 is good enough figure for general purposes.
Suppose the room temperature is 65 degrees.
We have a 75.7 CFM blower. We measure an output temperature of 100.
100 - 65 = 35 tRise.
Now we can calculate our BTU/hr.
75.7 * 1.08 * 35 = 2861.46 btu/hr
For reference: 1 kw/hr = 3412.14 btu.
Assume we have a 1.5kw amplifier.
1.5kw * 3412.4 = 5118.6 btu
Now we can solve BTU for CFM
5118.6 / (1.08 * 22) = 215.42 CFM is required to cool 5118.6 BTU
215.42 - 75.7 = 139.72 CFM
73 de Charles - KC8VWM
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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Also, there are some safety issue with water. Turns to steam at 212F, increasing pressure.
--------
Salt water has a higher boiling temperature because of increased density requiring more kinetic energy to excite the molecules.
73 de Charles - KC8VWM
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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Some calcs showing the surface area and heat dissipation behaviors of the container, over various ambients would be nice also.
But you can't just take random potshots at it right? Have to design something.
-----------
1. Dummy load liquid capacity is 5 gallons
2. Maximum temperature rise peak assumes 212 degrees
1 gallon of plain water = 8.33 lbs.
5 gallons of water weigh 41.65 pounds
1 gallon of H2O = 8.33 lbs. * 1 BTU = 8.33 BTU’s to heat 1 degree in 1 hour
5 * 41.65 lbs. * 1 BTU = 208.25 BTU’s (to raise the water 1 degree in 1 hour)
Conclusion:
It takes 208.25 BTU’s to raise 5 gallons one degree in one hour
208.25 BTU’s * 212 degrees = 44,149 BTU’s are required to raise the water to maximum peak of 212 degrees in one hour.
1500 watts is 5118.6 btu's
73 de Charles - KC8VWM
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RE: Home brew Dummy Load Project
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by KE6RAD on June 19, 2009
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I want to stop posting because there's no "thing" we're talking about. We're just rambling about possibilities.
But here's a correction:
In your calcs, you have to do the temp rise from the ambient temp..i.e. 212- maybe 90F? so 122F rise, not 212F rise.
You can see why it's good to do numbers. 5 gallons of water is more than 10x better for storing heat, than 1 gallon of oil, for a given max temperature (below 212F).
Oil has the benefit of higher temp tolerance.
I don't know what water solution you're investigating, maybe the salt water thing.
Analyzing one gallon of oil in a cantenna, assuming no dissipation from the can, you can see why it can get hot.
Analyzing the resistor temps is the next step. Have to deal with thermal conductivity instead of specific heat. It's two steps if it's resistor/heatsink/oil or water
But I have my solution. air-cooled with 4 cermat resistors, roughly 60 sq in of surface area on the resistors.
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RE: Homebrew Dummy Load Project
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by W6EM on June 19, 2009
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KB1LKR:""Aren't some oils and their vapors carcinogenic?"
Yes, (e.g. old polychlorinated biphenyl (PCB) containing transformer oils) but USP grade mineral oil (available at any pharmacy) isn't, nor are modern transformer dielectric oils (which are even better than USP mineral oil for cooling) e.g. Shell Diala®, Exxon Univolt, etc. though these may be hard to source in small volumes (5 gallons or less)."
Sorry, but PCBs are not carcinogenic in humans. Probably the most famous mortality study of capacitor plant workers who had their hands in it every day for years had no higher incidence of cancer than unexposed folks.
True, PCBs can break down in arcing scenarios into some rather nasty constituents, but Askarel, Pyranol, Flamtrol, Inerteen, etc., are not carcinogens. Nor, for that matter, would be mineral oil contaminated by transfer pumps and such.
Now, arcs in oil from such things as opens and to ground can break down oil into hydrocarbon gases that are quite explosive. Utilities, in fact, monitor gas content in transformers to determine if arcing is taking place to avoid catastrophic explosions.
Make sure that the resistors you chose are of the material type that can be immersed continuously in mineral oil without decomposing.
Also, as Alan alluded to earlier, the energy density, or ability of the resistor to transfer heat by conduction to the oil, and the oil to the fins is important. The temperature of the original heater metallic loop probably was up there. Several hundred centigrade in order to cause enough convective oil flow and radiation from the fins to raise ambient room temperature. Metal, and perhaps calrod material, is a lot heftier than a square inch of so of cheap carbon resistors.
If you don't think power resistors have problems radiating heat, take a look at some premium Dale resistors and what do you see? A metal outer radiator to allow the surface area to be increased to aid radiation of heat. Why are power resistors now offered in such things as a TO-220 case? Hint: So they can be attached to a large heatsink to radiate away the I squared R times T. (Energy)
Suggestion: If you can find TO-220, non inductive resistors in moderate power ratings, you can also probably find some decent, elongated aluminum heatsinks to increase surface area. Insert the menagerie into the oil filled heater and, well, it might just work...... No, I haven't tried it. Just a dream.
73.
Lee
W6EM/4
Former PCB mouthpiece for a major utility.
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RE: Home brew Dummy Load Project
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by KE6RAD on June 19, 2009
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on your airflow calcs, I don't know why you constrained the outlet temp to 100 deg F. That's silly. The resistors can run hot without damage way above that. The limit will be more due to things like overheating the cable at the so-239. But peak outlet temp will be above 100F.
and you calculated for 100% duty cycle, which your amp won't tolerate for long. 50% is a reasonable target.
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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on your airflow calcs, I don't know why you constrained the outlet temp to 100 deg F. That's silly.
The resistors can run hot without damage way above that. The limit will be more due to things like overheating the cable at the so-239. But peak outlet temp will be above 100F.
>>> The airflow calculations account for air flow temperature on the output of the fan, not from the amplifier transistors, resistors or other components otherwise. It accounts for "distance" the fan is located away from the internal heating source. Of course "100" is a nice baseline number to work from and you could always modify the calculation based on real world measurements.
and you calculated for 100% duty cycle, which your amp won't tolerate for long. 50% is a reasonable target.
>>> Again 100% duty cycle establishes the "baseline" to make additional calculations as needed.
ie. for 50% duty cycle operation we would adjust the calculation as follows:
5118.6 / (1.08 * 22) = 215.42 CFM for 100% duty cycle
215.42 / 2 = 107.71 CFM for 50% duty cycle.
My Best.
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RE: Home brew Dummy Load Project
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by AB7E on June 19, 2009
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OK ... I did indeed find an old non-functional DeLonghi oil-filled heater in my inlaws' garage and I got their permission to take it apart. Here's a picture of the unit (you might have to stitch the URL together):
http://i228.photobucket.com/albums/ee141/xazdave/OldDeLonghiOilFilledHeater.jpg
Here's what the heating element looks like while it's still in the unit:
http://i228.photobucket.com/albums/ee141/xazdave/OilFilledHeaterElementConnections1.jpg
Here's what the element looks like after being removed:
http://i228.photobucket.com/albums/ee141/xazdave/OilFilledHeaterElementafterremoval.jpg
Here's a closeup of the element connections, clearly showing the ceramic insulator surrounding the heating wire inside the element tubing.
http://i228.photobucket.com/albums/ee141/xazdave/OilFilledHeaterElementConnections2.jpg
This particular element had two sections, one with a resistance of about 15 ohms and the other with a resistance of 23 ohms. The control switch was designed to switch in various combinations of those two sections for different amounts of heat, and in this case it appears it was the switch that was defective since the elements themselves at least had continuity. The resistance of the tubing, no matter where I connected the probes, was 0.1 ohm on my VOM.
It's pretty obvious that the tubing isn't the heating element based upon those readings, right? Well, here's the kicker:
http://i228.photobucket.com/albums/ee141/xazdave/OilFilledHeaterElementbrazedtubing.jpg
Each end of each tube is fully brazed to the inside surface of the threaded metallic plug! As I stated before, the tubes are merely protective shields for the actual heating elements, and the tubes are SHORTED TOGETHER right at the plug. There is no way KC8VWM ever built a dummy load by cutting off the tubes and clamping resistor leads to the stubs as he has repeatedly claimed. It simply cannot work. Not unless he has a need for a 0.1 ohm dummy load.
On the off chance that more modern units might be constructed differently, I did some Google searching on oil-filled space heaters and discovered that current DeLonghi models are specified to be permanently sealed ... the elements are not replaceable. In fact, I couldn't find ANY current brand or model unit that wasn't permanently sealed. At an average price for a totally new unit around $65 that's not really surprising.
And look what I stumbled upon at Amazon.com while I was doing that searching:
http://tinyurl.com/nhve5w (click to zoom in)
and
http://tinyurl.com/m8xx9j
Either of those pictures looks familiar? They are exactly the same as the ones KC8VWM used in his article.
Finally, here are some pictures of various water heater elements that I quickly found online. The pictures aren't very large, but when expanded it is easy to see that they are constructed the same way ... the tubing is brazed to the inner metallic surface of the element so that the water cannot leak out and cause damage.
http://tinyurl.com/m8xx9j
http://tinyurl.com/mpw8jg
http://preview.tinyurl.com/lxt5cn
It took me longer to write this comment than it did to disassemble the heater and find the various online pictures. It appears that KC8VWM spent even less time on his "project" than he did on his article, none of which appears to be based on reality.
73,
Dave AB7E
p.s. Anybody see WA2JJH lately?
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RE: Home brew Dummy Load Project
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by KC8VWM on June 19, 2009
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Dave says and I quote:
"This particular element had two sections, one with a resistance of about 15 ohms and the other with a resistance of 23 ohms"
Now ask yourself....
Does something that has no continuity exhibit any resistance readings? Can you measure empty air space or something NOT making any contact with your VTM probes?
Oh and yes, as I said earlier (no secret really) the article was written while I was at work on the night shift (AKA not at home) and so I used "photos" from the internet to demonstate the concept behind my article. ..Uh..so what?
BTW.. there are only 2 photo's used in this article.
1. One of a generic consumer model space heater (similar to the one I used.)
2. A photo demostrating what the actual heater element looks like which is inside.
So what's the BIG deal with that? lol...
Get a life Dave.. Your too funny. Curious, do you make it a personal hobby to go around on the internet trying to prove people wrong for some apparent reason?
Have fun with that ok?
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RE: Home brew Dummy Load Project
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by AB7E on June 19, 2009
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KC8VWM: "Does something that has no continuity exhibit any resistance readings? Can you measure empty air space or something NOT making any contact with your VTM probes? "
What are you talking about? I never said that this element didn't have continuity. I measured the resistance of each section of the element (this particular one had two sections) via the external terminals that you can plainly see in the pictures ... the ones that the wires normally attach to. If you want, I can take a picture of the VOM connected to the terminals with clip leads. It's a 3-terminal connection ... from one outside leg to the middle it reads 15 ohms, from the other outside leg to the middle it reds 23 ohms, and between the two outside legs it reads 38 ohms. I DID say that none of those three terminals has any continuity whatsoever to the tubing, which is true not only of this element but ALL similar water heater elements.
KC8VWM: "Oh and yes, as I said earlier (no secret really) the article was written while I was at work on the night shift (AKA not at home) and so I used "photos" from the internet to demonstate the concept behind my article. ..Uh..so what? "
The "so what" is that you claimed you actually built AND TESTED such a unit, and the reality is that you didn't. The additional reality is that it IS NOT POSSIBLE to build a unit such as you claim you did (when you described the type of resistors you used, the clamps for the resistors, the initial power tests, etc) since the tubing stubs you claim to have connected your resistors to are brazed together and represent a dead short at the base of the element. Why do you think you can keep up this charade? Anyone reading this can stop off at the major appliance section of their local Sears or Home Depot and look at the replacement elements hung on the wall to see for themselves.
KC8VWM: "Curious, do you make it a personal hobby to go around on the internet trying to prove people wrong for some apparent reason? "
Only those who so blatantly try to bamboozle the rest of us.
Dave AB7E
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Homebrew Dummy Load Project
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by CBISBACK on June 20, 2009
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It's amazing to read the comments about this moronic "project".
Don't you people know when you're being "taken for a ride".
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RE: Taken for a Ride?
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by W6EM on June 20, 2009
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OK. I tmatters not whether Chuck had written a "can we make something useful out of this cast off widget" or what he actually did.
The piece stimulates thinking. Perhaps a use for a piece of cast off radiator that would otherwise have been tossed into a dumpster.
Instead of spending time throwing knives back and forth, why not think about the possibilities.
If,indeed, to obtain low SWR in a non-inductive load, it is best to create a 50 ohm characteristic impedance lossy line section, then lets talk inner and outer diameters of the resistor surface as the inner conductor, and a metal tube as the outer coaxial shield. And, take into account the dielectric constant of the mineral oil in designing the coaxial section.
Somehow, me thinks a company up in Solon, OH, with "wings" does just that for their termaline series.
Chuck's project, unlike Bird's designs, would rely upon conduction, convection and radiation to dissipate resistor heat whereas the Bird designs, I think, use conduction and radiation only. I don't think oil moves much in the Bird devices since there isn't a path up and into the fins as is the case with the oil filled radiator.
Think outside the box about this. If we can come up with a decent, screw-in coaxial replacement for the heater element, then ham radio has once again made something useful and practical out of what would otherwise be a cast off.
73.
Lee
W6EM
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RE: Taken for a Ride?
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by CBISBACK on June 20, 2009
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Good grief Lee, you're digging awfully deep while trying to be nice...
It's a silly article and an even more silly idea. But it's a free country and people have a right to their ideas, regardless of how silly they are....
Cheers,
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RE: Taken for a Ride?
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by KC8VWM on June 20, 2009
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Sure are a lot of mindless trolls on here lately... Some apparently don't even have callsigns. I guess we will take their comments quite seriously.. NOT..:)
Thanks for the kind words Lee..
Only if some people would spend their efforts "contributing" something to the hobby and submit ideas like this one instead of being hypercritical of what other people are doing instead... Oh, yeah.. some people don't know how to think on their own.
Well Trolls, please feel free to write your own article so I can jump right in there and throw water all over you, just like you have done with this article submission.
..Can't wait. In the meantime ... Well, I won't say that on here. You get the idea.
73 de Charles - KC8VWM
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RE: Taken for a Ride?
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by WMCO on June 21, 2009
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There are companies out there that make 1200w thin film resistors that have a 2x2 square by 1/4 inch high surface, mount it on a cooling element and either air cool it or fluid cool it it can handle 1200w all day long with adequate cooling and is good to freq of 2gig if it is mounted right. They used to cost about 100 bucks a few years back and where made by KDI. I have no idea if they are still in bussines but there are others out there making them. I use 4 200 ohm resistors on a 14x14x10 heatsink and run 1500w on 2m or HF into it for extented periodes of time if I do stress testing on parts.
J.C.
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RE: Taken for a Ride?
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by CBISBACK on June 21, 2009
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OK Charles..
I think you are the best example of someone "contributing" to the hobby in the last 100 years.
Sorry, don't have a call sign any longer, got tired of all the "johnny come lately" types who think they invented ham radio.
Your "contribution" is wonderful. I went to the dump and collected 100 junked heaters, I'll make them available to anyone who has time on their hands and needs a "project"..
However, I am surprised by your holier than thou attitude. Many contributions have been made by people who don't have call signs. You don't need to be nasty by name calling..
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RE: Homebrew Dummy Load Project
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by N5TGL on June 22, 2009
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K5END:
quote
"No, I read that part.
In any case it's still going to have a hairpin, regardless of which end is cut. "
Ah, then you missed the part that says cut it so only a stub is left, i.e. no hairpin.
73
N5TGL
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RE: Homebrew Dummy Load Project
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by AB7E on June 22, 2009
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N5TGL: "Ah, then you missed the part that says cut it so only a stub is left, i.e. no hairpin. "
You can cut it wherever you want ... it's still going to be a dead short at the base. This entire article is bogus. It's conjecture masquerading as a project, and the author never built it or tested it as he claims. Read the whole thread.
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RE: Homebrew Dummy Load Project
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by KD7YVV on June 22, 2009
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Hi Charles,
Nice to see you again. :)
Question for you....
How about replacing the bottom piece that screws into
the radiator or modifying the one you have to take
bolts? Then, since you have a hole and you can't
really fit resistors that are spread out, what about
a bunch of them with the leads shaped curly-que like
a telephone cord? Solder one lead to a bolt, and spiral
it up, then when you have enough spiraling up, fold it
over, and make another spiral going down within the
first spiral? Then you can solder to the other bolt.
Or, if you want a spiral that's higher
than the radiator, bend it into an L or U shape to
get it inside the radiator?
Nice idea though. I never though of using something
like one of those heaters for such a project.
If you use many resistors, there would be more
surface area to dissipate the heat right?
--KD7YVV, Kirkland, WA
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RE: Homebrew Dummy Load Project
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by KB2CPW on June 23, 2009
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Ok,
Here is one for you guys, I need a UHF dummy load capable of handling anywhere from 1500 to 7000 watts.. Looking to homebrew if possible or a really good deal on a bird.. LOL
Richy N2ZD
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RE: Homebrew Dummy Load Project
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by KE6RAD on June 23, 2009
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> I need a UHF dummy load capable of handling anywhere
> from 1500 to 7000 watts.. Looking to homebrew if
> possible or a really good deal on a bird.. LOL
:)
Funny you should mention that! I was just looking at this paper.
http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/77433/1/chd070_1_045.pdf
433 Mhz, 600kW (1.2MW max)
home brew dummy load.
page 3 and 4 have a pic
I actually was curious about the home made carbon/cement block which was one attempt. I was wondering if anyone ever tried making a big carbon/cement resistor from copper pipe embedded in a cement/graphite mixture.
So got me wondering about this whole homemade resistor thing.
Read the paper. Interesting.
I was also reading more about "water resistors" that the HV guys use. Evidently they like copper sulfate rather than salt in solution, because it doesn't corrode the copper. Lots of other issues with using salt water. So that sounds like a bad idea.
Copper sulfate is pretty poisonous too.
Lots of stuff if you search for "water resistors"
example: http://timsherri.dynalias.net/waterresistor/
Interesting graph of resistance vs temp for straight tap water.
referenced in this book:
http://books.google.com/books?id=nHqQnSM71_cC&pg=PA131&lpg=PA131&dq=%22water+resistors%22&source=bl&ots=ozsJuo4xUz&sig=bZQgoHV6zrUQMm2G4fikvGda8jY&hl=en&ei=ZxJBStC3BI7atgOktuHxCA&sa=X&oi=book_result&ct=result&resnum=10
they say stability of +-10% is rare. Using copper sulfate though, can increase the stability "somewhat"
still, I think those $32 resistors resold by MFJ are a good bargain. They sell the clips too.
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Homebrew Dummy Load Project
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by W3KM on June 23, 2009
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Hello all,
Dummy load power dissipation and match issues can be dealt with by connecting the transmitter to the dummy load with a long piece of coax. Many hams operating the UHF and microwave frequencies routinely use the `Cantenna` and other HF rated dummy loads by simply using a spool of coax to connect the TX to the load.
Using coax for loss, allows you to tune and make tests with higher power than the load can handle when used directly. Check the web for coax cable power ratings and loss specs.
I have 2 spools that I use. 100 ft of RG-8 and 100 ft of RG-58. I have used them individually depending on the power level and frequency. But normally I just leave them connected in series, the RG-8 section is connected to the TX. Due to the loss of the 2 cables, the input of my Heath cantenna `looks` like 50-ohms even at UHF/Microwave frequencies.
While you always try to use low loss coax cable to feed your antennas, the old lossy coax that you were going to throw away can actually be used for something.
Dave W3KM
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Homebrew Dummy Load Project
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by EI7IR on June 25, 2009
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Hello all.Some years ago I constructed a "low pwr" dummy load and before anyone starts cutting the ground from under me I know there is room for improvement.
I used 20 1k ohm 3watt metal oxide film resistors in parrallel.Worked a treat for me.
http://www.qsl.net/ei7ir/dl.html
Paraic
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Homebrew Dummy Load Project
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by YD8CJY on June 26, 2009
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I have built one off dummy load using 50watt russian resistor buy on ebay..
i built it using brass pipe..
the dummy load was great when the first tested.. it show swr 1.1-1.2 for 143-170mhz..
to increase the maximum power i submerged the dummy load in to the refrigerant vacum compressor Oil.. it's very dificult to find mineral oil in my country...
and then the swr ratio became bad it's 1.4-1.7 from 143-170 mhz...
i think maybe the oil cause that.. any other friend have same problem with me??? what is the best solution..
i have tested the dummy load with my 8877 1500 watt amplifier... the dummy load work's great just a little bit warm when i pressed the ptt more than 5 minutes..
but with the high swr 1.4-1.7 from 143-170mhz
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RE: Homebrew Dummy Load Project
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by WA2JJH on June 27, 2009
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Dave W3KM....tnx fer reminding me about the RG-8 coax/attenuater number.
I used to work on a TV live truck in my youth.
Those trucks used pnematic masts of up to 40 feet high.
Heliax for the 2.1ghz ENG band would be to brittle for mast use.
All the ENG(live) trucks did the same trick. use a 1-3W 2.1ghz TX inside the truck. Then over 40 feet 0f RG-8 would coil around the pole.
After the 40 foot run, a mere 250mw was left from the TX. So a 12W amp with 2 "GOLDEN RODS" are connected by a 1 foot section of RG-8.
Chunks of RG-8 make great dummy loads/attennuators for 800mhz-2.5ghz. Dirt cheap, if you ever seen what pro microwave stuff goes for.
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Homebrew Dummy Load Project
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by WA7PRC on July 1, 2009
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Interesting article! I own a Heath Cantenna that I refurbished after the Carborundum resistor climbed to over 90 ohms. After locating a replacement, I still had to find a source of coolant. I learned mineral oil isn't as good as transformer oil, and I was able to obtain a gallon from the local utility free gratis. But, after all was said and done, I was left with a load that can handle a KW for only a few seconds... and still wants to weep oil.
Fortunately, I work in the RF Lab for a major CO2 laser OEM, and perfectly useable but old (to us) parts abound. I had a pile of 50 ohm / 400 watt RF resistors... RF Power Components (now Anaren) pn RFP400-50R. These are flange-mount devices, about 0.5" x 1.1" in size. I also scrounged two good-sized heatsinks, with 3.5" x 5.5" mounting surfaces & 136 sq in of cooling surface each. I mounted two resistors to each heatsink. The resistors are then connected series-parallel, for a combined value of 50 ohms at 1600 watts.
The heatsink assemblies are mounted face-face, to keep the connections short. A network analyzer says they look very close to 50 ohms +/- 0j, past 50 MHz.
With the heatsink fins oriented vertically, a (scrounged) 100 cfm fan blows air up through the heatsink for cooling. An LM34DT temperature sensor IC (also scrounged) mounted near one of the resistor pairs is used to monitor temperature. Driving the load with 650W at 40.68MHz (ISM frequency), the temperature stabilized at 55°C. The resistors can handle 100°C at 100% rating. I haven't seen them get anywhere near that warm (just over 70°C) during extended test sessions, driving it with my Heath SB220 (1KW output).
The total cost to me was zero. If I had to buy the parts, it'd probably cost a few hundred dollars... about $50 each for the resistors, etc.
73,
Bryan WA7PRC
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