ESR meter DIY

One way to test electrolytic capacitors is to measure their capacitance. It does not tell the whole story of capacitor condition. Another usually more useful way to measure electrolytic capacitor condition is to use an ESR meter. Here is ESR meter I built years ago based on instructions from http://ludens.cl/Electron/esr/esr.html
but I built the transformer differently from parts I had (took toroidal inductor and wound extra coil).

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Inside

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Circuit diagram

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The design presented here works by applying a 50kHz, 200mV square wave to the capacitor under test, in series with a 10 Ohm resistor.

The circuit diagram shows a transformer with very many turns on primary and secondary. I replaced the transformer with my ow transformer that had the same turns ratio, but when I used a large ferrite toroidal core, I could build transformer with similar other specifications with less turns.

Some more ESR meter material
https://www.epanorama.net/blog/2013/01/17/esr-meters/

This ESR meter has advantage of working better with in-circuit capacitors than smart multi component testers.

Those smart testers can measure nicely ESR from bare components not in circuit board:
https://www.epanorama.net/blog/2019/09/17/component-tester-case/
https://www.epanorama.net/blog/2015/09/09/component-tester-m12864/
https://www.epanorama.net/blog/2018/03/26/transistor-tester-m12864-advaced-version/
https://www.epanorama.net/blog/2014/08/18/diy-electronics-tester-kit-m168/

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