Capacitor ESR check
An electrolytic capacitor fails slowly: its electrolyte dries, its capacitance falls and its internal series resistance (ESR) rises. A capacitance meter can still read “close enough” while the ESR has gone from 0.1 Ω to several ohms — and that is what makes a switch-mode supply whine, ripple and die.
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How it works
At 10 kHz a 470 µF capacitor’s reactance is only 33.9 mΩ, so what the meter mostly sees is the resistance in series with it: the ESR.
Rule of thumb for 470 µF, 25 V: under 0.2 Ω is good; above 1 Ω, replace it. C3 has also lost most of its capacitance.
Try it on the bench
- Set the LCR meter to 100 Hz: the reactance now swamps the ESR, which is why ESR is tested at 10–100 kHz.
Parts on the bench
In the course
Electronics Technician, module “Components and testing”.