|
New to Ham Radio?
My Profile
Community
Articles
Forums
News
Reviews
Friends Remembered
Speak Out
Strays
Survey Question
Operating
Contesting
DX Cluster Spots
Propagation
Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers
Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net
|
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by KR4WM on June 18, 2009
|
Mail this to a friend!
|
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
|
| |
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by KC8VWM on June 18, 2009
|
Mail this to a friend!
|
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
|
| |
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by WA7NCL on June 18, 2009
|
Mail this to a friend!
|
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.
|
| |
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by K5END on June 18, 2009
|
Mail this to a friend!
|
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.
|
| |
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by KC8VWM on June 18, 2009
|
Mail this to a friend!
|
So, have you ACTUALLY built this, or are you just daydreaming?
-----
Both... :)
|
| |
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by WX7G on June 18, 2009
|
Mail this to a friend!
|
|
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.
|
| |
|
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.
|
| |
|
RE: Homebrew Dummy Load Project
|
Reply
|
|
by N3OX on June 18, 2009
|
Mail this to a friend!
|
"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
|
| |
|
This Topic is Linked
This discussion is regarding an item in the
Articles area
of eHam.net.
View the item being discussed
Email Subscription
You are not subscribed to this topic.
Subscribe!
My Subscriptions
Subscriptions Help
Check our help page for help using
Forum, or send questions, comments, or suggestions to the
Forum Manager.
|
|
|
