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Old 2nd August 2007, 04:07 PM   #1
luka is offline luka  Slovenia
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Question Power load, dummy load (pic)

Hi

I would like to know how you test your power supplys, amplifiers,... to see if they work properly. Problem is when you have a lot of power, few 100 watts, on/with what you test that level of power and maybe for how long.
I mean if supply/amp can work at its power for 10min, there is no reason that it wouldn't work for 1h, 10h, days... is there?

I will show my power load that I'm making now in few days.
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Old 3rd August 2007, 03:18 AM   #2
mzzj is offline mzzj  Finland
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Default Re: Power load, dummy load (pic)

Quote:
Originally posted by luka
Problem is when you have a lot of power, few 100 watts,
Just slab together cheapest power resistors you can find.
For short-time testing water cooling can be very helpfull, single 4w resistor can handle more than 20x normal power rating submerged in water.

And I was trying to figure out how to make easily cheap and small 0.01ohm dummy load rated for 50kW

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Old 3rd August 2007, 04:14 AM   #3
anatech is offline anatech  Canada
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Hi mzzj,
Try using heaters.

Hi luka,
Consumer amplifiers will not deliver their full power for very long. If you manage to keep the heat sinks and amplifier components cool, the thermal fuse in the transformer core will open. Scratch one power transformer. Don't test them any longer than you have to.

I use proper Dale low inductance 8 ohm, 250 watt dummy loads mounted on heat sinks (with grease) that I had resurfaced in a machine shop. The surfacing cost as much as the resistor back then. One of my guys still blew a resistor up testing a Yamaha pro amp (P-2200 I think). He had it bridged and it blew one end out of the resistor.

When I test a consumer amplifier at full power, it's only on long enough for the reading to settle, about 30 sec. Running an amp at 1/3 power will overheat it as well. So if you want to do an extended power test you must check your temperatures with a proper thermocouple temp gauge. Make sure you check the heat sinks and the transformer core. It will not hurt to check a few free standing transistors in the driver stage.

Do not forget that a transformer core builds heat slowly. It holds it for a long time also. I have seen guys wipe out transformers on the bench (thank goodness they weren't my guys!).

-Chris
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Old 3rd August 2007, 04:43 AM   #4
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Quote:
And I was trying to figure out how to make easily cheap and small 0.01ohm dummy load rated for 50kW
Two plates immersed in salt water can be used as a dummy load for high power.

Gajanan Phadte
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Old 3rd August 2007, 05:07 AM   #5
luka is offline luka  Slovenia
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First of all I am doing electronics only for me, so things are rated as they are, not dependent on time, I like that for myself. I don't like big transformers anymore or linear amps, I'm all into switching... so there is almost no heat

Hi mzzj

I was thinking about resistors at first. But one 5w resistor costs 0.38€ here and you need a lot of them, and then you have your power resistor rated at what 400w? And you have to use water, which is on same desk as rectified 230Vac and I don't wan't water to boil and worry about that

Hi Chris

I agree, heaters are the way to go. But I don't feel like buying one big, having only one resistance.... I used heaters from baking oven (some microwave have them too). Each has about 66 ohmes and is rated at 800w free air. Now I have 5 of those. And I will put them in tunnel with 120mm/230Vac fan pushing about 300m^3 per hour. So rating of each should go up a lot, but still won't be so hot that you couldn't have it in room, on desk...

And I have spend less then 2€ by now, to buy two 30cm ALU-profiles and M4 1m long screw

Hi gmphadte

Yea, but you would produce a lot of chloride, if you test with DC, but sure would be a way. But resistance would be unknown and changing...
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Old 3rd August 2007, 04:15 PM   #6
gootee is offline gootee  United States
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I think that the amateur radio people often submerge a resistor in a one-gallon "paint can" full of some kind of oil, to use as a high-power dummy load in place of their transmitting antenna. There was even a commercial product for that, called something like can-tenna.

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Old 3rd August 2007, 07:08 PM   #7
anatech is offline anatech  Canada
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Hi Tom,
Quote:
called something like can-tenna.
Yes. It works well (the concept). Make sure you seal the can so you don't get a messy surprise.

Hi luka,
Quote:
First of all I am doing electronics only for me, so things are rated as they are, not dependent on time, I like that for myself. I don't like big transformers anymore or linear amps, I'm all into switching... so there is almost no heat
That's cool. So it depends on how close you want to be to standard accepted practices.
Quote:
But I don't feel like buying one big, having only one resistance
Look for dead toasters. There ought to be many for free. I have no idea what the element resistance is. They even come premounted.
Quote:
And I will put them in tunnel with 120mm/230Vac fan pushing about 300m^3 per hour. So rating of each should go up a lot,
Yes, the rating will go up and it would be safe to use. For reduced fan noise, place the fan in the center of the tube and run it slower than designed for.

Quote:
But resistance would be unknown and changing...
Yeah, I didn't comment.

-Chris
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Old 3rd August 2007, 07:14 PM   #8
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Click the image to open in full size.

this is what I use. 15x 10w 56 ohm resistors parallel each side.

I figured if the amp would survive bench testing on 3.7ohm loads then it will do 4ohm easy. besides, resistance goes up when they start heating up.
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Old 3rd August 2007, 09:28 PM   #9
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32 x 3,9 ohm; 50W; DALE

2 x PAPST 80x80mm (Metal)

4 x 400W @ 8 ohm

2 x 800W @ 4 ohm

1 x 1600W @ 2 ohm

Regards
zeoN_Rider
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Old 3rd August 2007, 09:32 PM   #10
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