Capacitor internal resistor meter

Posted on Sep 23, 2012

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 AC voltage appearing across that resistor is measured and displayed on a meter. So the whole thing is nothing else than a simple ohmmeter that uses ultrasonic AC for measurement instead of the usual DC used by every common ohmmeter. Since the Ac voltage used is so low, it does not make semiconductor junctions enter conduction, which further helps to make this meter suitable for checking capacitors mounted in a circuit.

Capacitor internal resistor meter
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An additional beauty of an ESR meter is that in almost all cases it can check capacitors while they are in the circuit! This is so because a good capacitor would measure almost like a short circuit, and so any other parts connected in parallel will have minimal influence on the measurement. These are the features that make an ESR meter an irreplaceable tool for troubleshooting electronic equipment. One section of a dual low power operational amplifier is used as a square wave oscillator. A small ferrite core transformer is used to step down the voltage and provide the necessary low impedance output. A 10 Ohm resistor loads the output to absorb inductive spikes from the transformer, which could cause a false reading for low value capacitors. The other section of the op amp amplifies the signal that gets through the capacitor being tested, and its output is rectified and applied to a 50µA galvanometer through a calibration potentiometer. A small 5 Volt regulator maintains the supply constant while the instrument is being powered from anything between about 7 and 15 Volt. I power the meter from the 13.8V bus which I have in my workshop, but if you prefer, you can use a 9V battery instead, connected through a switch. The power consumption of this circuit is so low that a 9V battery should last at least 100 hours. Building this ESR meter is simple and straightforward. I assembled the circuit on a scrap piece of project board,...

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