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Dummy load

Posted: Sun Jul 28, 2019 2:32 pm
by alfaeire
Looking to build or buy a dummy load that can take up to 300w comfortably.
So probably 500w or so..
Anyone built anything like it before and or have plans for one?

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Re: Dummy load

Posted: Sun Jul 28, 2019 6:14 pm
by Radiokaos
building a dummy load for that kind of power could be a bit tricky, maybe best to purchase on TBH

Re: Dummy load

Posted: Sun Jul 28, 2019 6:17 pm
by alfaeire
Tbh?

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Re: Dummy load

Posted: Sun Jul 28, 2019 6:27 pm
by Albert H
To Be Honest

My first load that would handle that kind of power (for a short time) was a "Cantenna". It was full of transformer oil and could handle a couple of hundred Watts for three or four minutes. I bought it at a ham rally and it was really cheap (back in about 1977). The first one I had was made by Heathkit, but the recent ones I've seen have been either made by MFJ or are cheap No-Name types (which I'd be reluctant to trust).

https://www.ebay.co.uk/itm/MFJ-250X-Wet ... 2667401367

Re: Dummy load

Posted: Sun Jul 28, 2019 9:10 pm
by sinus trouble
Microset CF-800?

Not sure how reliable they are? But forced cooling could be used!

Re: Dummy load

Posted: Sun Jul 28, 2019 9:12 pm
by sinus trouble
As albert said, The oil cooled ones are very reliable but can get messy if you get a leak! :lol:

Re: Dummy load

Posted: Mon Jul 29, 2019 1:18 am
by nrgkits.nz
My first high power dummy load was simply a bunch of RF resistors mounted on a heatsink and submerged in a tray of water.

Here's the RF resistors, they are rated at 250w each and the resistance is 100R. Combine them in parallel and series to get 50R and increase the power rating - also use thermal paste under each one - CPU thermal paste from your local PC shop is fine.
https://www.aliexpress.com/item/1672496026.html

The heatsink doesn't need to be massive, 100mm x 60mm x 40mm will be fine - make sure its got a lot of fins. Mount the RF resistors on it, mount an N or SO239 socket on the side. Make sure to minimize the distance between the connector and RF resistors as much as possible or you'll get stray inductance. I used 16SWG tinned copper wire to connect all the tabs and the center pin of the connector.

Place the heatsink in a tray of water (obviously not covering the Resistors). I ran 400W into it and the heatsink/water was only slightly warm after about 10 minutes. Mounting the resistors to outside of a large metal container filled with water would be far more effective

Re: RE: Re: Dummy load

Posted: Mon Jul 29, 2019 12:04 pm
by alfaeire
nrgkits.nz wrote:My first high power dummy load was simply a bunch of RF resistors mounted on a heatsink and submerged in a tray of water.

Here's the RF resistors, they are rated at 250w each and the resistance is 100R. Combine them in parallel and series to get 50R and increase the power rating - also use thermal paste under each one - CPU thermal paste from your local PC shop is fine.
https://www.aliexpress.com/item/1672496026.html

The heatsink doesn't need to be massive, 100mm x 60mm x 40mm will be fine - make sure its got a lot of fins. Mount the RF resistors on it, mount an N or SO239 socket on the side. Make sure to minimize the distance between the connector and RF resistors as much as possible or you'll get stray inductance. I used 16SWG tinned copper wire to connect all the tabs and the center pin of the connector.

Place the heatsink in a tray of water (obviously not covering the Resistors). I ran 400W into it and the heatsink/water was only slightly warm after about 10 minutes. Mounting the resistors to outside of a large metal container filled with water would be far more effective
You hardly have a picture of that knocking about somewhere do ya?

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Re: Dummy load

Posted: Mon Jul 29, 2019 12:12 pm
by alfaeire
https://www.nonstopsystems.com/radio/fr ... _dummy.htm

I found this.. Looks like it could work for what I want.. It's going in a mineral oil bath as well..

Question, could I not just air cool the heatsink like on an amp?


I also found the following on fleabay.
https://rover.ebay.com/rover/0/0/0?mpre ... 3878459497



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Re: Dummy load

Posted: Mon Jul 29, 2019 2:20 pm
by thewisepranker
Well essentially it's the same thing. Putting the heatsink in oil gives you a large thermal mass which means that your load can absorb a much larger amount of thermal energy than it would be able to in air, but for a limited time. This is facilitated by the large surface area at the interface between the heatsink and the oil. However, the interfaces between the oil and the can, and the can and the air are quite low in surface area, relative to the heatsink.
While initially the oil will absorb a lot of power with a barely noticeable external temperature increase, after a while the oil will start to convect and the temperature distribution will become more uniform the more it does so. The oil continues to absorb more energy and increases in temperature more and more uniformly, whilst the can also increases in temperature. Because the power can be transferred into the oil at a far greater rate than it can transfer it to the air, it ends up insulating the heatsink - your resistors will end up at a much higher temperature than if the heatsink were in free air, potentially resulting in the lid popping off and probably throwing hot oil around.

In other words, the oil merely serves as a reservoir that is "filled up" by thermal power over time. Once it's full, the thermal reservoir does nothing, and the entire system is a dummy load with an empty paint can as the heatsink.

Re: Dummy load

Posted: Mon Jul 29, 2019 4:00 pm
by Albert H
Pranker's description is spot-on. If you run my old load for a while with enough power, the "safety-valve" pops, and you get a jet of extremely hot oil shooting into the air!

Some years ago, one South London rig builder made a dummy load out of a heap of resistors and used a large glass jar to contain it. He used cooking oil rather than transformer oil, which worked quite well, but was secondhand, so every time he fired up a rig into it, there was a smell of frying chips!

Re: Dummy load

Posted: Wed Jul 31, 2019 7:49 am
by darklife
Salt water dummy load? Not sure if they work good on VHF and above though but I'd imagine they would test out when powering up lowly and then hit them with higher power. They tend to radiate a bit though unless in a metal tin. I use them often for HF and below so I can't imagine they wouldn't work for VHF. Best of all they cost you a pickle jar, some salt, a knife to stir it up and an RF connector. Basically scraps found around the shop. Search "salt water dummy load". I've used them up to 27MHz US CB radio band no problem and low power FM 88-108MHz broadcast band. Beats expensive power resistors and amateur radio operators have used them with no problem for eons.
Image
Check out The Free Radio Forum, yeah I'm plugging, it's our US forum here besides HFU. I'd love to see if anyone tries the salt water dummy, it works for me no joke. Looking to share info.
You guys over there have so much info on FM transmitters and even mediumwave. It's sickening how little we get over here. I stalk this forum for info because you people have incredible schematics and ideas that we just don't have here.
I didn't know much about PLL circuits until this forum. Sorry I don't mean to hijack the thread but this place is a learning outlet.

Re: RE: Re: Dummy load

Posted: Thu Aug 01, 2019 10:27 am
by nrgkits.nz
alfaeire wrote:
Mon Jul 29, 2019 12:04 pm
nrgkits.nz wrote:My first high power dummy load was simply a bunch of RF resistors mounted on a heatsink and submerged in a tray of water.

Here's the RF resistors, they are rated at 250w each and the resistance is 100R. Combine them in parallel and series to get 50R and increase the power rating - also use thermal paste under each one - CPU thermal paste from your local PC shop is fine.
https://www.aliexpress.com/item/1672496026.html

The heatsink doesn't need to be massive, 100mm x 60mm x 40mm will be fine - make sure its got a lot of fins. Mount the RF resistors on it, mount an N or SO239 socket on the side. Make sure to minimize the distance between the connector and RF resistors as much as possible or you'll get stray inductance. I used 16SWG tinned copper wire to connect all the tabs and the center pin of the connector.

Place the heatsink in a tray of water (obviously not covering the Resistors). I ran 400W into it and the heatsink/water was only slightly warm after about 10 minutes. Mounting the resistors to outside of a large metal container filled with water would be far more effective
You hardly have a picture of that knocking about somewhere do ya?

Sent from my ANE-LX1 using Tapatalk
Here it is:
B92653FF-9647-4D3A-97E8-DE267F02B3FE.jpeg

Re: Dummy load

Posted: Thu Aug 01, 2019 10:35 am
by nrgkits.nz
The SWR is around 1.15:1

I haven’t tried putting anything more than 400W into it although it should handle a lot more. If you’re going to run more than 1kW, oil would be a better choice instead of water, bigger RF resistors, and possibly also a 7/16 din connector with some LMR or Andrew coax to deal with the power.

Re: Dummy load

Posted: Thu Aug 01, 2019 11:05 am
by thewisepranker
darklife wrote:
Wed Jul 31, 2019 7:49 am
Salt water dummy load?
...
Beats expensive power resistors and amateur radio operators have used them with no problem for eons.
High power RF resistors are much, much cheaper than a new output device!

Re: Dummy load

Posted: Mon Aug 05, 2019 8:12 am
by darklife
thewisepranker wrote:
Thu Aug 01, 2019 11:05 am
darklife wrote:
Wed Jul 31, 2019 7:49 am
Salt water dummy load?
...
Beats expensive power resistors and amateur radio operators have used them with no problem for eons.
High power RF resistors are much, much cheaper than a new output device!
Of course. Was just sharing info on another option that seems to work okay for hams in a pinch if operated at low power first to get the mix of salt and water right for lowest SWR then crank up power. I've seen pictures of amateurs using huge buckets for this purpose handling hundreds of watts with ease and little heat but as I said I don't know how well it will work at VHF and above. Still an option for HF and below though. Thought it was interesting information to share. Didn't mean to hijack the thread, sorry.