High Value Capacitor Value Test Circuit

This simple circuit can be used to test the capacity of low voltage, high value capacitors.

Use the well known RC formula: Time = Resistance * Capacitance

Connect a DC voltmeter between TP1 and TP2. Start with the 10 volt power supply switched off and the capacitor discharged. The voltmeter should read close to zero. You can short positive and negative sides of the capacitor with a resistor to discharge it.

You start the timer and switch on the power supply. Stop the timer when the voltage reaches 6.3 volts.

Since time and resistance is known you can now solve for capacitance.

Capacitance = Time/Resistance.

So if you have a 470 uF capacitor, with a 10K series resistor in the circuit, it is supposed to take .000470 * 10000 = 4.7 seconds to charge to 6.3 volts. If, in your test, it actually takes 10 seconds to reach 6.3 volts, you can calculate the actual capacitor value. In this example, 10 seconds /10000 ohms = 1000 uF.

You can increase the value of the resistor to check lower valued capacitors, but it will hard to check real small capacitors with this circuit. You can use a 555 timer circuit to check real small capacitors, but thats a subject for another blog posting.