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In circuit capacitor ESR meter and op amp exercise

QST April 2014 has an article on building an ESR meter. One of the sources with a good explanation of the how and why is by kakopa who has a horrifying story about his denial of entry to the U.S. on a trip to visit an ailing aunt (read and weep for the state of U.S. immigration).

Anyway, capacitor failure is a very common cause of equipment malfunction. It is fairly easy to measure the equivalent series resistance for capacitors to evaluate whether they need replacement. That series resistance soaks up power which shows as heat and the heat destroys the capacitor. Expected ESR values vary by capacitance and voltage ratings. HIgher voltage rating and capacitance usually means lower ESR. Generally, a 3 ohm ESR for capacitors over 22 uf is usually OK although values range up to 20 ohms for a 1 uF electrolytic. You can sometimes tell capacitors that have failed because the heating has caused expansion that shows as bulging or even burst cases.

The ESR tester uses several op amps for different functions. These are (1) to create a split rail power supply, (2) as a 100 kHz oscillator, (3) as a comparator and amplifier, and (4) as a precision rectifier. The idea is to feed a low voltage square wave to the capacitor and see how it changes the balance on a resistive bridge. The voltage is kept low enough so that it is below the barrier junction voltages in connected solid state devices like rectifiers.

This is a good beginner project. Most parts can be found from recycling old electronic stuff, the circuit isn’t very complex, there is a lot of part value leeway, and the result has good utility. The first step is to get out some op amps and see how they work for the functions needed…

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