An in-circuit Equivalent Series Resistance meter

An Equivalent Series Resistance Meter Some time ago I was trying to repair a switched mode power supply (SMPS) in my oscilloscope. I’d been quoted £800 for a new PSU, so trying to fix it first was well worth my while. It’s pretty common for electrolytic capacitors to develop faults in an SMPS; specifically they develop an higher than normal internal resistance. So while the capacitor may hold a charge and measure as the correct capacitance, it will not behave correctly in a filter or PSU circuit. The ESR of a large high voltage, high capacity electrolytic capacitor should be fractions of an Ohm, smaller capacitors have ESRs of a few Ohms typically. As the capacitor degrades in use the ESR can easily climb to several hundred times the normal value, while exhibiting no changes to voltage and capacitance ratings Suspecting the capacitors, and not having a way to measure their series resistance, I set out to design and build an ESR meter.