AS with many tinkerers and junk electronics collectors, a variety of acquired power supplies wind up on the authors shelves to await attention. But are they worth keeping? Testing them with a resistive load is messy and difficult, and with high current supplies it is nearly impossible, unless you have a carbon pile! The tester whose circuit diagram is shown in Fig controls supply currents to 20A, and voltages from 1·7V to over 50V. Current control is so stable that once the current is set, a supply voltage can be varied across this range and the current will remain constant. Maximum power will depend upon how well the pass transistors utilize heatsinks.