Femto Capacitance Meter.
Measuring Large Capacitors.

The Femto Capacitance Meter measures up to 4µF, which about covers everything but large electrolytics. Measuring electolytics does not have to be very precise, because their capacitance varies a lot with frequency (or speed), and drift is high with temperature changes. So I use an ultra simple circuit to measure them, as shown below.

This circuit applies a voltage for five seconds while measuring rate of current flow. A reset button allows restart of measurement.

While the measurement is being made, the output of op amp A is going down at one volt per second. It puts a maximum of 5.4V on the test cap. The other side of the test cap draws current through R2 producing an output voltage of one volt per millifarad.

To get the output of op amp A to slew at 1V/sec, a constant current of 0.22µA goes into the feedback capacitor of 0.22µF. The constant current stems from a constant voltage of about 0.6V across the input resistor, R1, which is 2.7M. The 0.6V is created by a diode (D1) between the inputs ahead of R1. The zener diode of 6.8V creates a reference voltage for the noninverting inputs and volt meter.

If increased precision is desired, measure the exact voltage across the diode (D1) and tailor the size or R1 so that it produces the same number of microamps as C1 has microfarads. Then use a precise resistor for R2.

The reset switch drains the voltage across C1 bringing the output voltage back up for op amp A. The negative side of the test capacitor is connected to the output of op amp A.

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