Hi all - recent discussion here on testing electrolytic capacitors prompted me to dig out the schematic for an ESR meter that I designed numbers of years ago.
For those who are unaware of ESR, the term refers to the fact that all capacitors have an "Equivalent Series Resistance" internally in series with the capacitance within the body of the capacitor.
Normally, we ignore this resistance as it has little consequence in non-electrolytic caps. However, electrolytic caps are constructed with a paste of electrolyte between the capacitor plates, and this paste can (and does!) dry out over time and as the cap is exposed to elevated temperatures.
This means that a cap can measure ok with a capacitance test, but still be faulty in actual service. The fact that numbers of us are using recovered electros from old computers means that some of these caps may have high ESR although read ok with a capacitance meter. In fact, I find my ESR meter is the best tester for the performance of a cap as it tests both capacitance and series resistance.
If you are interested in building your own ESR meter, I attach the schematic here. As you can see it is pretty simple, using just one quad op-amp. One amp generates a 100Khz signal that is applied to the cap under test via a voltage divider with protection diodes across the test terminals. The voltage across the terminals is then amplified with two more amps before being rectified and after a DC amp applied to a meter. I used an old VU meter, but any old analogue meter can be used. Just adjust the resistor in series with the meter to provide full scale deflection of the meter when the test probes are not connected to a cap.
Using the meter is pretty intuitive and you can test caps pretty quickly. It is useful for testing recovered caps from computers and monitors, and also for testing old caps in vintage amps. The real plus about the design is that you test the cap in circuit - you don't need to remove the capacitor to test it
If anyone is interested in building their own ESR meter, I'd be more than happy to provide any further details
For further info on the tester, and a vero layout see http://www.guitargear.net.au/discussion/index.php?topic=22554.0