What is ESR? Why can a capacitor look 'fine' but be faulty?

The part repairers replace most is the electrolytic capacitor. Often its capacitance is still fine; what has failed is its ESR.

What does it do? ESR is the small series resistance inside every capacitor. In switch-mode supplies the capacitor charges and discharges tens of thousands of times a second, so as ESR grows, output ripple and capacitor heating rise quickly.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/2: Diode conducting

The current pulse from the transformer flows into both the load and the capacitor. The current into the capacitor also passes through the ESR, dropping a voltage across it: the output jumps suddenly.

1. Output (low ESR, healthy)

Just a small ripple from the capacitance charging and discharging: a few tens of millivolts.

On the scope

Multimeter5.00 V (≈30 mV ripple)

🔍 How to spot it on a board

•The large electrolytics on the output side (1000–3300 µF, 10–35 V) and the small ones on IC supplies are hit hardest
•A capacitor with a domed top, leaking base or shrunken sleeve
•Those near heatsinks and hot parts dry out faster

🔧 How to measure it

•With an ESR meter (works in-circuit), unplugged and with the capacitors discharged.
•Look at the output ripple on a scope; a multimeter shows the average and misleads.
•A capacitance meter alone isn't enough: capacitance can be normal while ESR is high.

⚠ Common faults

•Rippled output: the device restarts, picture noise, audio hum.
•A dried-out VCC capacitor: the IC starts and stops, ticking.
•When replacing, choose low-ESR, 105 °C parts; same µF, same or higher voltage.

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