A detailed look at the 5 transistor ESR meter circuit designed by EEVBlog user Jay_Diddy_B. Here are links to a few threads on the EEVBlog forum that focus on this circuit:
http://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/
http://www.eevblog.com/forum/projects/jay-diddy-5-transistor-esr-meter/

There is a lot of additional detail about the circuit contained in the above threads. The schematic can be found there, as well as a couple of PCB designs.

An ESR meter like this is designed to measure the equivalent series resistance of electrolytic capacitors – a common failure mode in consumer electronics. It works by applying a signal (100kHz square wave in this case) to the capacitor under test, and measuring the voltage drop across the device.

I really like this circuit – there are some very clever things going on. And, there’s some inherent beauty in solving a measurement need with a small handful of transistors. The circuit includes a very interesting emitter-coupled astable multivibrator, a measurement bridge, and an interesting capacitively coupled rectifing measurement circuit.

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