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Before leaving the plant, power supplies are always subject to performance testing. These tests include line regulation at full load and operating temperature.

The problem with these ideal testing conditions is that there's no guarantee that the operating conditions of a power supply will be consistent with the test conditions. One reason for this is that electrical components have performance tolerances – the most common being 5, 10 and 20 percent of the nominal value. This component tolerance effect is additive, which can keep the power supply from working properly.

Once you've ruled out tolerance issues, here is how you can diagnose a power supply problem.

No Load. Start by testing the power supply on a bench with no load. If it doesn't work, you'll know right away to contact the manufacturer for another unit. If the power supply works, you should test it with a resistive load to see if that's the problem.

Resistive Load. Failure with a resistive load can mean that your tolerances have caused a process or component failure. Other possible reasons for failure with a resistive load are if the component is out of specification or has a cold soldered joint. If the power supply still works with a resistive load, you'll want to test it with an active load.

Active Load. If the problem comes from the active load, it could be due to an unfamiliar signal caused by high capacitance. A large capacitor on initial power-up can act as a short circuit, forcing the unit into short circuit protection. At this point, you should check the manufacturer's specification to see if your unit has exceeded the maximum capacitive load.

If your power supply still doesn't work, you have two other options: either lower the capacitive load, or get a different unit with circuitry that can handle higher currents.