Computing stuff tied to the physical world

Measuring capacitance

In Hardware on May 6, 2012 at 00:01

Capacitors are all about storing and releasing charge. The main difference with batteries is that this charge is stored directly as electrical energy, whereas a battery converts to / from chemical energy in some form.

In an ideal capacitor, charge and discharge follow an exponential curve. Charging takes place when connected to a fixed supply via a resistor, discharging is a matter of placing a resistor across the capacitor:

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The “time constant” is the level when the discharge reaches 36.8% or the charge reaches 63.2% of the original voltage. It can be calculated using the formula: T (seconds) = R (ohm) x C (farad).

This property makes it easy to measure the value of a capacitor: charge it up to a known voltage, then discharge it through a known resistor and measure the time it takes for the voltage to drop to 36.8% of the original voltage.

This is the approach taken by this capacitance meter kit by Radio Hobby Store:

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There is excellent documentation including very detailed assembly instructions, leading to this:

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And sure enough, it works as expected – measuring a 10 µF cap in this case.

The only two drawbacks I found is that it doesn’t measure caps larger than 50 µF, and that there is no on-off switch. With a tool like this, you tend to want to use it from time to time and put it away after use. Without the switch, you have to disconnect the battery each time – a bit awkward and inconvenient.

This meter is based on a pre-flashed PIC controller. There’s one button to calibrate its zero value, and a convenient “auto-zero” mechanism, which keeps adjusting for exactly 0 pF when no capacitor is connected.