How to measure capacitance in-circuit

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If you cannot afford the fancy equipment, or cannot take the device out of circuit, here is how to measure capacitance of a capacitor in a parallel RC or in-circuit (ICT or node) circuit.

1. Drive with low-voltage square wave, measure discharge with scope

2. Impedance - voltage divider at signal frequency.

3. Bridge

4. Specialized test equipment (impedance or bridge analyzer)


1. method PRO'S: can drive with one polarity and not activate active devices with another polarity.

2. First, find out DMM resistance check voltage level. Use function generator output voltage of the same setting (peak to peak). Measure resistance both ways. If there are significant differences, check for active devices in circuit. Cannot use method of active devices get forward-biased. This will tell you the "R" value.


Then, add the parallel RC original circuit in series with an external resistor and the signal generator. Measure voltage output of this voltage divider. Determine RC circuit impedance. Then calculate C.

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