Electronic – Measure voltage with no current

capacitorvoltage measurement

Suppose I have a capacitor and I want to observe its charge decay over time. How can I do that without affecting its discharge rate through measurement?

AFAIK a typical voltmeter runs current through a known resistance to determine voltage, but in the process this would discharge the capacitor being measured. With increasing complexity one could reduce the current required to make an accurate measurement, and then reduce the frequency of measurements, but in the limit the measurements will still drain some voltage.

In the hydraulic analogy it's possible to measure the pressure (voltage) by putting a spring gauge on a piston impinged by the two sides of the reservoir. No water flows from one side to the other, but we get a constant reading of the pressure.

So is there a meter, mechanism, or circuit that can do that for voltage on a capacitor or other power supply?

Best Answer

Neat physics solutions aside, the practical way to do this is with a very low input bias current op-amp running in a buffer configuration. One of these op-amps with a properly designed layout can draw down to single-digit femtoamps of current from your cap, making disturbances pretty much negligible, particularly if you only connect the amplifier to the cap when you're taking a measurement.

Analog legend Bob Pease describes measuring leakage of a polypropylene cap using this method:

Now I will charge up some of my favorite low-leakage capacitors (such as Panasonic polypropylene 1 µF) up to 9.021 V dc (a random voltage) for an hour. I will read the VOUT with my favorite high-input-impedance unity-gain follower (LMC662, Ib about 0.003 pA) and buffer that into my favorite six-digit digital voltmeter (DVM) (Agilent/HP34401A) and monitor the VOUT once a day for several days.

[...]

Day 0: 9.0214 V
Day 1: 9.01870 V
Day 2: 9.01756 V
Day 6: 9.0135 V
Day 7: 9.0123 V
Day 8: 9.01018 V
Day 9: 9.00941 V
Day 11: 9.00788 V
Day 12: 9.00544 V
Day 13: 9.00422 V

The first day after soaking for an hour, their leak rate was as good as 2.7 mV per day. Not bad.

If you need to automate such a setup, a good old-fashioned reed relay has basically negligible leakage (better than even modern solid state analog switches) and can be used to briefly connect your amplifier to the capacitor under test in order to take a reading.

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