ESR (equivalent series resistance) is one of the characteristics that define the performance of an electrolytic capacitor. Low ESR is highly desirable in a capacitor as any ripple current through the capacitor causes the capacitor to heat up due to the resistive loses. Typical capacitor checker measures the capacity of the test capacitor. Some advanced units also test for leakage current. Most of these testers require that the capacitor be removed from the circuit. Unless the capacitor has totally failed, they will not detect a high ESR value. Having to remove each one for testing is very tedious and there is a great risk of damaging circuit boards.
To overcome this problem, Lawrence P. Glaister has built a Capacitor ESR Tester. It uses a low voltage (250mv) high frequency (150 kHz) A/C current to read the ESR of a capacitor in the circuit. The in circuit testing is possible because of the low voltage used for obtaining the measurement. The voltage is low enough that solid state devices in the surrounding circuitry are not activated and do not affect the low resistance reading we are attempting to obtain. A lot of capacitor checkers will be damaged if you happen to test a charged capacitor.
This Capacitor ESR checker circuit is A/C coupled and will withstand up to 400vdc of charge on a capacitor. The ESR checker will not detect shorted capacitors as they will read with a very low ESR value. If you are trouble shooting a circuit, you will have to use several instruments including your nose, voltmeter and oscilloscope to locate all the possible failure modes.
- Tests electrolytic capacitors > 1uf in circuit.
- Caps may be tested in circuit or by bridging them across the terminal posts or using test leads.
- Polarity insensitive testing.
- Tolerates charged capacitors up to 400vdc.
- Low battery draw (approx 25ma) resulting in about 20 hours of battery time when using 4 cheap AA nicads.
- Easy to read analog meter.
- Measures ESR range from 0-75ohms with an expanded scale A/C ohmmeter.