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Author Topic: What oil to put in an oil dummy load?  (Read 7916 times)
AD5X
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Posts: 1430




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« Reply #15 on: September 06, 2010, 04:09:39 PM »

Check out these dummy loads/peak detectors on my website:  www.ad5x.com/images/Articles/LoadDetector.pdf

Phil - AD5X
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W1ITT
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Posts: 146




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« Reply #16 on: September 06, 2010, 06:34:37 PM »

MFJ sells transformer oil by the gallon, their part number MFJ-21 for about $30 plus shipping. It's used to refill their version of the old Heath Cantenna.
73
Norm W1ITT
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KB1GTX
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Posts: 460




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« Reply #17 on: September 07, 2010, 01:51:09 AM »

You can use "JB weld" to epoxy several resistors to a used computer heat sink.
If you keep the lead short it will work  to the 6m freqs
The old JB weld is quite heat conductive but is a good insulator.
Also I've soldered several resistors to a pl259, covered the resistors with the epoxy then slid a 1/2 copper tube with some epoxy in the tube over the end of the resistors and pl259..  Works really well with about an 6"tube.
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ZENKI
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« Reply #18 on: September 07, 2010, 02:14:40 AM »

For the those interested in a cheap  sources of mineral oil they should look in the following  places.

Farm stores sell mineral oil as a laxative for horses. You dont need medical grade mineral oil  for a dummy load!

Paint shops also sell mineral oil for oiling wood and as a paint additive. They can order it  in for  you for a very good price. A price thats
far better than the average drug store price.

Henry radio also sells a ceramic 800 watt heat sink mounted microwave resistor for a very good price.


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VK2TIL
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« Reply #19 on: September 07, 2010, 03:10:29 AM »

Oil should only be used if it's really necessary.

Oil has a higher dielectric constant than air; this increases the stray capacitance and degrades the performance of the load at higher frequencies.

Oil was common before high-quality non-reactive resistors were available; it was a "necessary evil".

Nowadays, high-power ceramic resistors are available from makers such as Florida RF;

http://www.rflabs.com/IMG/pdf/Florida_RF_Labs_-Terminations-2.pdf

They sometimes appear on ebay; that's where I got the one I used in this homebrew load;

 

It loafs at 100W.

WA7PRC has described his QRO load here;

http://forums.qrz.com/showpost.php?p=1921061&postcount=14

So oil is certainly overkill for a QRP load and, with the availability of modern ceramic resistors, unnecessary for higher powers (unless they are very high powers  Smiley  ).
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K1CJS
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Posts: 6034




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« Reply #20 on: September 07, 2010, 04:43:46 AM »

One thing to keep in mind if considering motor oil.  The 'detergents' in motor oil can break down most resistors after a while.  Most stores sells 'non-detergent' motor oil for lubricating things other than an internal combustion engine.  That oil is more suitable for dummy loads than detergent containing motor oil.
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AA4PB
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« Reply #21 on: September 07, 2010, 05:23:38 AM »

Three 150 Ohm, 2W carbon resistors in parallel will give you a 50 Ohm dummy load that will handle 6W. Why bother with oil or any liquid coolant for QRP?

The cooling method doesn't have any impact on the "stress" of the transmitter finals as long as you don't let the dummy load burn up and change resistance significantly.

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W5LZ
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Posts: 477




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« Reply #22 on: September 07, 2010, 07:19:37 AM »

Something people don't think about is that your local power company may have 'transformer oil' for refilling transformers.  Mine does/did.  Try asking if you can have, or buy some of it, you might be surprised.
I started at the front office of the local maintenance department.  Got send to a couple of other offices till I wound up in the supply department.  Told them I wanted some transformer oil for a project, not much.  Was asked how much, and when I said a gallon, the guy told one of the workers to go get this guy a can of oil.  Didn't even charge for it.  (They probably loose that much just from the spigot leaking.)  No PCBs or that stuff in it for a lot of years, so is 'safe'.
Or, ask your local pharmacist if they carry mineral oil in gallon containers.  Careful with the answers if they ask you why you need that much!  I told them I had this constipated elephant... and had to look somewhere else for the mineral oil.  I don't care, it was worth the looks on their faces.  Smiley
Paul
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KE4DRN
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« Reply #23 on: September 07, 2010, 05:11:41 PM »

hi,

visit your local tractor supply store
they sell mineral oil in gallon jugs $15

or your local ag or vet supply is good too.

cooking oils are more expensive then mineral oil.

73 james
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KC3JV
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Posts: 20




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« Reply #24 on: September 07, 2010, 10:27:28 PM »

I worked in the power industry for 14 years and transformer oil is basically mineral oil.    The mineral oil in transformers is now 100% synthetic but the drug store variety which is natural will work well.

Mark Rothstein, P.E.  KC3JV
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W0BTU
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« Reply #25 on: September 08, 2010, 04:09:29 AM »

I worked in the power industry for 14 years and transformer oil is basically mineral oil.    The mineral oil in transformers is now 100% synthetic but the drug store variety which is natural will work well.

Mark Rothstein, P.E.  KC3JV

Hi Mark,

Any idea why mineral oil is not used in power transformers?
And do you know for sure whether the thermal conductivity is the same for both types?

I assumed the transformer oil had better thermal conductivity, but I don't really know. My decision to use transformer oil in my dummy loads was based on that assumption.

A higher thermal conductivity oil would lower the temperature of the resistor and help prevent it from heating beyond the point where its resistance would permanently change. This would not matter so much with cold oil, but as the oil gets hot (say that you are building and testing a legal limit homebrew amplifier and need to use the dummy load for a longer period of time),  thermal conductivity could really become an important consideration.
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N1DVJ
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« Reply #26 on: September 08, 2010, 05:11:42 AM »

The original cantenna I bought from Heath back about 1981 came with thermal rating charts for power.  How long you could key-down at different power levels with the can dry, with oil, with good oil, etc.  So I assume it makes a big difference.  I seem to recall one of the thermal charts even printed on the side of the can.
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W0BTU
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« Reply #27 on: September 08, 2010, 06:16:34 AM »

The original cantenna I bought from Heath back about 1981 came with thermal rating charts for power.  How long you could key-down at different power levels with the can dry, with oil, with good oil, etc.  So I assume it makes a big difference.  I seem to recall one of the thermal charts even printed on the side of the can.

Oh Yeah! I just looked at the chart on the side of my Cantenna. I don't know if I ever noticed that before. It clearly shows that transformer oil is unmistakably superior to mineral oil. 10 minutes at 1000 watts, using transformer oil. But only 650 watts for 10 minutes using mineral oil!

You can see that in the photo at http://www.cliftonlaboratories.com/images/High%20Power%20Loads.jpg. The solid line is mineral oil; the dashed line is transformer oil.

Wonder what the curve is for 1500 watts?

Here's some more images of the power rating chart:
http://www.google.com/images?hl=en&newwindow=1&biw=1151&bih=795&gbv=2&tbs=isch%3A1&sa=1&q=heathkit+cantenna
« Last Edit: September 08, 2010, 06:32:22 AM by Mike Waters » Logged

NR4C
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Posts: 308




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« Reply #28 on: September 08, 2010, 07:36:27 AM »

Every CANTENNA I've seen has been vented.  This is to let out the gases when the oil gets warm under use. 

Every CANTENNA I've seen has an oily film on the top from venting.

Therefore, I don't think an oil cooled dummy load is very portable due to potential leakage.

there are several sources for kits for dummy loads for QRP.  Very reasonable prices and fun to build.  Even some carbon resistors, (2X100, 4X200 etc) in parallel will work great.

...bc
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W0BTU
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« Reply #29 on: September 08, 2010, 07:59:43 AM »

Every CANTENNA I've seen has been vented.  This is to let out the gases when the oil gets warm under use. 
Every CANTENNA I've seen has an oily film on the top from venting ...

Shortly after I built my Cantenna, I saw the oily film on the top, like everybody else who ever owned a Cantenna did. I fixed that by raising the vent with a 2" long piece of 1/2" copper pipe, with a brass disc (with the Heathkit vent in it) on top of it all. No more oil problem.
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