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Dummy load

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Posted by Nick Gammon   Australia  (23,016 posts)  [Biography] bio   Forum Administrator
Date Sun 05 Feb 2012 01:09 AM (UTC)

Amended on Mon 12 Mar 2012 08:21 AM (UTC) by 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.

There is an optional indicator LED so you realize you have your battery plugged in. For an extra milliamp drain a high-power LED is very bright. (Added since the photo was taken).


It's all a bit rough and ready, but since that is a 7W resistor, and the MOSFET can handle 30A, 60V, and we are running it from a 9V battery, I would estimate:

  • Maximum load voltage: 9V (if you put 12 VDC into the "power" plug you can probably put 12V into it).

  • Maximum current: around 4.5A (the 9V from the potentiometer goes through a voltage divider so the maximum voltage into the second op-amp would be 4.5V).

  • Maximum load: 7 watts (because of the resistor)

  • Minimum current: around 20 mA

Test points

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.

Computer control

This suggested circuit (not tested yet) would let you generate the reference current by computer command:

By doing an analogWrite() you could determine the voltage (in effect) sent to the op-amp, and thus generate the desired load.

- Nick Gammon

www.gammon.com.au, www.mushclient.com
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