The project came from an italian magazine Nuova Elettronica N212. It's very simple but interesting ESR Meter. I've built it and tested some capacitors and it's very useful.
It measures the ESR (Equivalent Series Resistance) of capacitor (electrolytic and not) that s used to tell if a capacitor is good or not. ESR Meter uses a bridge circuit that works at 100KHz.
There are the following possibilities:
1) The electrolytic capacitor is good: (low ESR) the bridge will stay balanced and the meter will indicate the maximum current.
2) The electrolytic capacitor is not good:
(high ESR) the bridge will be unbalanced and that will cause the meter to indicate less current; as less the meter will indicate as higher will be the ESR;
After few measure you'll be able to decide if a capacitor is good or not.
3) There is a short circuit in the electrolytic capacitor:
the meter will indicate the maximum current and the red LED will lamp; capacitor is not good.
4) The electrolytic capacitor is broken:
the meter will not move. Capacitor is not good.
I've closed the circuit in a small plastic box inside which there is the battery, the meter and a ON/OFF switch;
the meter will be 500 uA F.S. but I've used a 1 mA one adding a small NPN transistor as amplifier;
You will notice some interesting thing about capacitor using this meter; for example low value capacitor (1-10 uF) have often higher ESR than high value one.
C1,C2=1 microF electrolytic