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Home made dummy load questions

B

Beakster

Member
May 27, 2008
14
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Hi,

I'm looking to build a Dummy load suitable for peaks of 200 watts. I've ordered a 200w 50ohm resistor: Amazon product ASIN B01MXMBMIC
I'm planning to wire this to an SO-239 using some 12 gauge solid core eletrical wire. Then I was thinking about putting in it a small paint can filled with oil. Do I need to take any precautions to keep the resistance fixed when I do this? Like should I heatshrink the terminals so they stay dry?

What type of oil to use? I have loads of used engine oil from my motorbike, mostly semi-synthetic.

Thanks
 

Use a non-petroleum based oil. Too low of a flash point.

(Best to stick with mineral oil as CK said. It's cheap too!)

73
David
 
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CrossbowDummyLoad.jpg


Not sure the max wattage. But may give you an idea.

I'm sure someone with better knowledge here can define it and give proper directions.
 
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Double check the load you're going to use, can it be immersed in oil? Some composition types are
reactive or not suitable for immersion...

Depending on the SO-239 you use, a thread thru flange or flat flange mount -

upload_2020-4-17_10-51-57.png

Shameless plug...​

One of the biggest bumps, if that - will be the internal length of that 12# wire - anything shorter than 1 inch, you should be ok.

Just use a good solder - both Center lead it's return and Crimp (both if possible) the Resistor leads onto the SO-239 if you need to use a lug ring and one mounting Bolt (not screw - BOLT with LOCKNUT) - for the load itself will certainly get hot.

Don't try to fold over the load unless you have to - just keep lead distance equal from Center to Load and Load to Return (Shield case/Can) so the load can be suspended in the oil - covered with oil. Vertical support is ok if clearance problems - but you may not be able to keep oil covering all the parts of the load.

There was a time I had done the "vertical orientation" into a copper pipe capped both ends but if it tilted - the load was not completely submerged in oil - it can blow because of the composition wasn't immersed and the oil evaporated from the heat - a gentle POP PING and done...

So with that lesson, you may wish to think about a simple hole plug cap and drill a hole in which to allow any pressure to escape so the lid of the can, so it can vent air pressure or the oil itself - won't vaporize in the event of a catastrophic failure or arcing and possibly erupt and spray out...

if you make this too airtight to prevent leaks - the pocket of air trapped inside will build in pressure and even warp the can thru the thermal cycling it will go thru - so I highly recommend some form of venting to keep the pressures stable...
 
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Hi, I made one years ago, just like in the description and diagram above, but I constructed the dummy load with carbon resistors, not wirewound higher power types.
 
BrutonStreetTailor said:
Hi, I made one years ago, just like in the description and diagram above, but I constructed the dummy load with carbon resistors, not wirewound higher power types.
Click to expand...


You cannot use wirewound types as they are very inductive at RF frequencies. Even the so-called non-inductive wire wound types still have enough inductance to be useless.
 
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Captain Kilowatt said:
You cannot use wirewound types as they are very inductive at RF frequencies. Even the so-called non-inductive wire wound types still have enough inductance to be useless.
Click to expand...

Oh dear, this is bad news. Here's the spec of the one I ordered:

uxcell 5% 200W 50 Ohm Wirewound Aluminum Housed Clad Resistor Gold Tone 12cm Long
by uxcell
Product Name: Aluminum Resistor; Type: RX24; Resistance Value: 50 Ohm
Power Rating: 200W; Resistance Tolerance: +/- 5% (J); Lead Thread Dia: 4.5mm/0.18"
Total Length: 12.8cm/5"; Body Size: 10 x 5.1 x 2.8cm/3.9" x 2" x 1.1"(L*W*T)
External Material: Aluminum; Color: Gold Tone
Net Weight: 188g; Package Content: 1 x Aluminum Resistor

619H2VyynSL._AC_SL1100_.jpg
 
So back to the drawing board in terms of resistor choice, glad I asked before using wire wound :) . I was thinking about this:

50 pcs 100 Ohm Carbon Film Resistor, 2W 5% Tolerance Carbon Film Resistors, Axial Lead

61fCQYS7CML._SL1100_.jpg


They come in a pack of 50, so I was thinking groups of 10 in parallel is 10 ohms, and if I put 5 groups in series then I'll have 50 ohms.

I'm not sure exactly how evenly the power is disappated across them, maybe that arrangement it would handle short bursts of 100W dry? Then once in a can with mineral oil it could handle a couple of minutes of 200W? Thoughts?

Thanks
 
I like the last option. Using several dozen low power resistors means you need to keep the lead lengths as short as possible to avoid stray capacitance and/or inductance which will make the load worse as you go higher in frequency. I have a pair of 100 ohm 800 watt Florida RF Labs ceramic chip resistors wired in parallel to provide 50 ohms at 1600 watts. They are mounted on a huge heatsink that measures about 10 x 15 x 2 inches. With 100 watts continuous into it I can hardly raise the temperature of the heatsink unless I lock the key down and walk away to watch a movie.
 
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upload_2020-4-17_23-13-45.png


https://www.ebay.com/itm/383339481383

Might want to read the fine print near the bottom of the page - its' a little too delicate for many applications unless it's nailed down somehow - not suspended in a fluid...uses a Silver Strip - be gentle...for its part of the technology that gives it it's ability to be a low inductance resistive path. Even into the UHF which many of the larger Carbon Compositions cannot be due to size and substrate.

upload_2020-4-17_23-17-56.png
 
So MFJ used to sell the MFJ 115-1500A resistor for their cantenna 1.5K dummy load. Its a 50 ohm 90 watt resistor they say is good for 1.5k and good luck finding them.
There are 2 models the 1500 and the 1500A. DO NOT USE THE 1500. USE THE 1500A ONLY!
Now, good luck finding them. Wire wound is no good. You need noninductive resistors that are meant to be immersed in oil of you want a wet unit.
Food grade mineral oil will work but if you want real transformer fluid ill warn you now its not available to purchase by the gallon and its around $150 for a 5 gallon jug shipped.
Here is some info on the MFJ cantenna and the safety issue with the 115 1500 resistor.
 

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For simplicity, I built one like this quite a few years ago.
https://www.qsl.net/kf8od/load.html


If you need help locating this type of resistor, here are a couple places.
http://www.usresistor.com/ - These guys make the rod resistors and many others. Not sure if they sell direct or if there are vendors for their products.

Mouser has a couple sizes on stock, but it appears in quite limited numbers.
The 90W resistor submerged in a paint can of mineral oil or transformer oil would be fine for 1000W PEP. Just be mindful of the duty cycle. They also have the model 888 and 889 of this type which are 190W and 275W respectively. Larger cans of oil would be required for those larger ones but they would handle larger amounts of power with proper cooling.

https://www.mouser.com/ProductDetai...BOLa8n/M5RY4JxXvzRpt9AQ5xL7Gg%2BhhdA4y2Hf0g==
 
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