This is a constant current dummy load I put together recently. It was inspired by the EEVblog version. Mine is simplified, using only one op-amp. It also requires a multimeter to monitor the current.
I used parts I had on hand, except the sense resistors which I had to order. The IRF520 is not a logic level MOSFET. It’s barely starting to turn on at a gate voltage of 5v, so I had to use a 12v power supply. Otherwise, it’s very straightforward. The op-amp will try to maintain its + and – inputs at the same voltage by adjusting the output and therefore the load on the power supply under test. I used ten 10Ω 1% resistors in parallel for the current sense resistor, making the math easy. 1 amp = 1 volt across 1Ω.
I tested it out using the same 12v power supply. (12.6v to be specific) Drawing 500mA results in a power dissipation of about 6W in the MOSFET. With the heatsink in the action shot, it stabilized at about 150F, or 80F above ambient. The sense resistors only showed a few degrees of heating. The circuit seems to be very stable in spite of the breadboard and all of those ridiculously long wires. Apparently, this particular op-amp is very forgiving.
In the future, I’ll add on to this project with an lcd readout and microcontroller. At least, a fine adjustment would be nice. As it is, it’s difficult to control the current setting with a single potentiometer across the full 12v supply. Once it is set, however, it is nice and stable.