How to Test a DC Power Supply

By John Papiewski

Testing a low-voltage direct current (DC) power supply is something any moderately experienced electronics hobbyist or technician can handle. The supplyâ€™s job is simply to provide reliable voltage and current within its specified limits. You can check most power supplies with a pair of multimeters and a dummy load. The voltage should be clean and steady, not dropping when you put a moderate load on it. If the supply has poor voltage with no load, or if the voltage drops excessively with a load, it needs to be repaired.

Set one multimeter to read DC voltage. Connect its black (negative) probe to the power supplyâ€™s output ground. Connect the meterâ€™s red (positive) probe to any positive power output. Turn the power supply on and observe the voltage on the meter. If the supply has multiple outputs, positive or negative, touch the red probe to each one and compare the meterâ€™s reading to the supplyâ€™s rated output. If the output voltage is variable, turn the supplyâ€™s voltage control up and down and check the results on the meter. If the meter reads a voltage thatâ€™s more than a few percentage points high or low, the power supply is defective.

Select a dummy load based on the power supplyâ€™s specifications by using Ohmâ€™s Law. The dummy load can be a power resistor rated to handle the supplyâ€™s maximum output. For example, a power supply thatâ€™s rated for 10 volts and 1 amp needs a resistor of at least 10 volts times 1 amp = 10 watts. To make sure itâ€™s using 1 amp, you want the resistance to be 10 volts / 1 amp = 10 ohms. A 10 ohm, 20 watt resistor would be a good choice.

Turn the supply off. Set another multimeter to read current in amps. Connect its black probe to the supply ground and the red probe to one lead of a dummy load. Connect the dummy loadâ€™s other lead to the appropriate power supply output. Connect the red lead of the first multimeter (set to read voltage) to the same positive supply output. Turn the supply on and compare the current and voltage readings on the meters with the supplyâ€™s specified output. Let the supply run for a few minutes and check to see if the voltage drops. If the voltage was reading fine in step 1 but now reads too low, the supply has poor regulation.

Warning

DC power supplies are powered by 110-volt household alternating current (AC). This higher voltage will be present at the power cord connection and the supplyâ€™s step-down transformer. Never touch these parts with your fingers or uninsulated metal tools unless the supply is unplugged from the wall.