I recently decided I needed a DC current load. I looked though posts about DIY designs. I ordered a Xiaolin LM324 electronic load based on enut11's experiences as shared in this thread:
Upgraded LM324 Based 300W 72V 20A Electronic Load
Before the kit arrived, I realized I wanted to go by a different path as modern MOSFETs are generally not designed to work in DC region and all designs seem to use them past their specifications.
This DC load will sink 15A in the range of 2-30V. The LM723’s V- is negatively offset to about -2.5V with a TL431 to allow it to regulate the voltage emitter resistor voltage down to 0. Regulated voltage is averaged across the four emitters of the shut transistors, and the current limiting transistor in the LM723 is used to shutdown in case of overheating. Otherwise it’s a pretty standard 731-design.
I built it using as many parts from my stocks as possible, which explains the choice of cabinet, heat sink, passive cooling, and complexity of the power supply. I adapted a 7107 based panel meter to provide direct current read out.
Fitting of the transistors on the heat sink is critical. Silicone isolators don’t work (I tried). 0.47R emitter resistors are on the high side, but eliminate the tendency to thermal run-away provided mica-shims and good heat conductive paste is used. Only one TIP3055 died during my testing... RIP
I saw significant differences in running temperature under heavy load when measured directly at each transistor, but this was down to mounting problems (screws needing tightening).
My design draws a constant current down to about 2V. At lower voltages it becomes resistive. Being MOSFET based, the Xiaolin load works below 1V.
A few ideas for improvements are ruminating: An Arduino with a 12-bit AD/DA could make the load fully programmable. It just needs a +/-5V symmetrical power supply which could be made simpler from a stacked pair of cheap 5V switch mode modules. With active cooling and by adding more TIP3055’s, the design could be extended past 300W. Temperature compensation should be added as the resistance of the emitter resistors drift upwards with increasing temperature, so the load will reduce the current when it heats up (and the current read out will not show). Individual PCBs under the TIP's could make replacement easier.
I’ve shared my design here in the hope it may be useful for others. Feedback is much appreciated!
/Anders