Using an idea originally described in an EEVBlog video (youtube-EEVBlog), I built a circuit that can “dial-up” a specific load (from 5mA up to 2A) and hold it even when the DUT input voltage changes. This will come in handy for testing power supplies. It uses an LM324 op-amp configured as a voltage follower that drives an N-channel power FET. A 1 Ω current sensing power resistor in series with the source lead provides feedback to the inverting input of the op-amp. Since the resistance is 1 Ω, the voltage across the resistor represents the actual current of the DUT. The FET is essentially acting as a variable resistor because when the gate voltage changes due to a change in output from the op-amp (controlled by changing the voltage of the non-inverting input), the FET drain to source current will change accordingly. Maximum power dissipation is ~25W so a heatsink with fan (from an old computer power supply) was added to keep the FET and resistor from frying.
The blue trimmer pot next to the op-amp sets the desired load. This current is "expressed" as a voltage across "TP +" and "TP -" and is monitored with a multi-meter.
Initial tests revealed a severe oscillation at the op-amp output since they aren’t designed to drive high capacitive loads (ie. FETs). Searching for answers on the internet, I discovered this problem had already been addressed by others and simply added a small capacitor from the output to the inverting input (ie. negative feedback) to dampen the instability. Problem solved! Pretty cool test jig.
