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Topic: Dummy load (Read 8 times) previous topic - next topic

Nick Gammon

I was inspired by Dave Jones blog about making a simple dummy load for testing stuff (power supplies, batteries, etc.) ...


Not having a pre-printed circuit board around I assembled one on a small piece of protoboard:


I didn't bother with a load read-out like Dave did. All you need to do is clip a multimeter across the big current sense resistor, and as it is 1 Ohm, then 100 mV on the meter will represent 100 mA current.

It seems to work pretty well. I could only get the lower limit down to around 20 mA but the op-amp wasn't a rail-to-rail one. The pot is a 10-turn wirewound, which gave reasonably sensitive control.


Looks good...I'm getting tired of slapping together resistors to simulate random loads...
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Interesting circuit. I will simulate this circuit and see what happen. I was thinking of building a variable dummy load just my ex-employer ( when I was working as a technician fixing / testing Switching Power Supply ), they where usign a bigger variable load system.

Thank Nick.

Nick Gammon

All credit to Dave Jones for describing it. Although I put the thing on paper, with his blogs you tend to have to copy stuff down from the screen. But he does explain what everything is doing which is great.

You can buy these of course, but there is a certain satisfaction in doing it yourself.

Out of interest I measured the current consumed by the 9V battery powering the circuit. It was about 1.5 mA. So, small consumption makes it nice and portable.

Nick Gammon

Feb 05, 2012, 05:12 am Last Edit: Feb 05, 2012, 06:06 am by Nick Gammon Reason: 1
It's hard to resist over-egging a design like this. :)

I added an indicator LED so I would realize I had my battery plugged in. For an extra milliamp drain a high-power LED is very bright.

Also I marked two test points on the circuit:

  • TP:A - a voltmeter between here and Gnd shows the input voltage to the second op-amp. Effectively this becomes the "set current" (whether or not this is achieved). So for example, 500 mV would be a set current of 500 mA. So you could attach a meter, and dial-up the current you want, before powering up the (main) circuit.

  • TP:B - a voltmeter between here and Gnd shows the actual voltage dropped by the 1 ohm current-limiting resistor, and thus the current actually being drawn by the circuit under test.

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