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. Suspecting the capacitors, and not having a way to measure their series resistance, I set out to design and build an ESR meter.
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.