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Extensible dummy load. 50 plus watts per transistor. 2013-01-12

()blivion

Active Member
()blivion submitted a new article:

Extensible dummy load. 50 plus Watts per transistor. - I TAKE ABSOLUTELY NO RESPONSIBILITY FOR ANY INJURY, DAMAGES OR UNLAWFUL CONDUCT CAUSED BY FOLLOWING

I TAKE ABSOLUTELY NO RESPONSIBILITY FOR ANY INJURY, DAMAGES OR UNLAWFUL CONDUCT CAUSED BY FOLLOWING THE BELOW INSTRUCTIONS. THOSE INSTRUCTIONS ARE PROVIDED *AS IS* IN THE HOPES THAT THEY WILL PROVE USEFUL, BUT WITH ABSOLUTELY NO GUARANTY OF FUNCTIONALITY OR FITNESS FOR A PARTICULAR PURPOSE. IT WORKED FOR ME, I HOPE IT WORKS FOR YOU. BUT YOU DON'T GET A WARRANTY, AND YOU CAN'T SUE ME IF SOMETHING GOES WRONG. IF YOU DO NOT AGREE TO EVERY PART OF THIS, STOP READING NOW.

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Mosaic

Well-Known Member
Hi,
I have a similar design but with a load shifting improvement.

I am using some 14 awg Nichrome wire as a load on the drain of 2 logic NFets. This allows me to dump the majority of the heat in the Nichrome.
I am dumping 25Amps (const. current) for load testing Lead acid battery reserve capacity. This means I need to ensure 25amps from perhaps 12.6V down to 10.5V. Thus the minimum load resistance must be 0.42 ohms. I am using a pair of IRLB3036 NFETs with a parallel RdsOn of 1.4mOhm and a pair of 30Watt, 0.1ohm precision sense resistors.

Thus the minimum HOT resistance of the wire must be .42-(.05 +.0014 + .002 (wiring losses)) = 0.3666 ohms. Given a ratio of 0.17%/C increase in resistance for the Nichrome C wire and an expected temp of about 700C (ambient air) we have about almost 15% max resistance increase, thus a target resistance of .31 ohm (25C) is required for the Nichrome Load to handle the voltage range desired for the constant 25A.

The peak load thru each NFet @ 12.6V requires each NFET have a linear RDs value of 12.6/25 - (0.31 *1.15)-0.05= .0975 ohm. About .1 Ohm . Therefore the heat generated in each Nfet is 12.5Amps *12.5Amps *.1 ohm = 15.6Watts

Thus the Nfets load is 31.2 watts vs a total load of 315Watts, or 10% power.

Also I have a 3 stage pi filter with a 7Khz 0-5V PWM signal driving the voltage ref of the opamps so that I can dynamically control the load to whatever value I need. Thermistor on the Nfets monitors their temp.
 

()blivion

Active Member
Pretty good, seems like you really worked the numbers out on it. We had talked about doing something like what you just described in *THIS* thread. I even had a working sim for something that wasted all the heat in resistors and none in the transistors. There were issues with it all that were more difficult to address than just using the transistors by themselves, so we abandon the idea.

We had came to the conclusion that while what is normally desirable for any other solid state power electronics is not actually wanted when building a dummy load. Efficient switching isn't needed... you *WANT* to waste power, so there is no point in trying to make the transistors switch efficiently. It wouldn't change performance, it would only change where the heat ended up. With this in mind, as long as your power transistors are heatsinked well enough, they can waste ALL of the heat. You have to remove the watts as heat somehow anyway, so why not try it with something that was intended to be heatsinked? Finally, with the method of soldering the FET backs directly onto copper pipe, then have water flowing through them, as long as you're still in the safe operating area you almost can't overheat them.

Jocanon was able to get his first build up to 1000 Watts without problems. And very little of that power was dissipated in anything other than FETs.
 

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