Juan of ssihla build this DIY electronic load. He writes:
The design is based on some of the various diy electronics loads out there (like the one from Dave Jones). The mosfet is a P45N03LT , most likely I took it from some of the PC power supply I’ve “recycled”. I’m using two 25k potentiometers, one for coarse adjustment and the other for fine adjustment (10 turn pots are kind of expensive…). The control voltage varies between 0 and 5 volts and is divided by two with a couple of 10k resistors. The op-amp is an OPA2336, It has rail to rail output so the load can sink roughly up to 2,5. The op amp is powered with a 7805.