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ESR meter

kalshagar - ESR meter

Theory

In a perfect world, a capacitor has only capacitance. In the real life, a capacitor (same goes for inductors and resistors) has a capacitance, a resistance and an inductance value. Of course, capacitor companies minimize the resistance and inductance to be the closest possible to the "ideal case".

One important thing to note is that a capacitor can retain its capacitance intact but buid up a resistance 100 times its original design and lead to different failures (circuit stops working, capacitor explodes, capacitor shorts, ...).
Conclusion: measuring capacitance is not the key to measure a failed capacitor (and anyway you can't do it with the resistor in place in the circuit).

ESR (Equivalent Serie Resistance) is the measure of the resistance of that component. As per the litterature on the net it is typically :
  • For electrolytic capacitors :
    • Big capacitance low voltage (ie. 470 uF 10v) : [0.01-0.1] ohm
    • Low capacitance high voltage (ie. 4.7uF 400v) : [1-3] ohm
  • For ceramic, tantalum,... : << 0.01 ohm, meaning nothing really measurable
When the capacitor goes bad the ESR value goes up, and that's where the problems start.

How to measure ESR ?

I'm basing my design on what's explained here : all credits go to them http://ludens.cl/Electron/esr/esr.html
According litterature, facing an AC at high frequency (>= 50kHz) a capacitor will basically behave like a resistor only (other coponents become neglectable).

Build a circuit with the capacitor C and a 10 ohm resistor R in serie. Feed the circuit with a square wave signal with f >= 50 kHz and amplitude = 200mV and measure the voltage difference around the resistor R.

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All content on this site is shared under the MIT licence (do what u want, don't sue me, hat tip appreciated)
electrogeek.cc ~ Formerly known as Kalshagar.wikispaces.com (AlanFromJapan [2009 - 2017])