[Kerry Wong] had some extreme MOSFETs (IXTK90N25L2) and decided to create a high current electronic load. The result was a two-channel beast that can handle 50 A per channel. Together, they can sink 400 W and can handle a peak of 1 kW for brief periods. You can see a demo in the video below.
An electronic load is essentially a load resistor you can connect to a source and the resistance is set by an input voltage. So if the load is set to 10 A and you connect it to a 12 V source, the MOSFET should look like a 1.2 ohm resistor. Keep in mind that’s 120 watts–more power than a common incandescent light bulb. So you are going to need to carry some heat away.
The circuit is pretty simple. The FETs accept a voltage on their gates that sets them to look effectively like a resistor that varies with the voltage. A very small source resistor develops a voltage based on current (only 75 mV for a 50 A draw). That voltage feeds a comparator which generates the gate voltage after looking at the input control voltage. Each millivolt into the comparator translates to an additional 1.33 A through the load.
Using a small current-sensing voltage is a good thing as it leaves most of the voltage for the load. However, it does mean you have to carefully select the op amps to have very low input offset voltage as even 1 mV is more than a 1% error at this level.