Capacitance meter

Last update: January 21, 2006

No, my meter does not look as fancy as this....

Every home constructor should own at least two pieces of test equipment: a digital (or analog) multimeter and a simple frequency counter. Most digital multimeters can measure capacitance, but the capacitance range is rather limited (in my case, from about 10pF to maximally 10 µF). It is not possible to check the value of large capacitors with such a meter, e.g. bipolar electrolytics in a cross-over filter, or reservoir caps in a power supply. In my attic, I have not only a digital multimeter but also a cheap frequency counter (0...10MHz). So I decided to build a simple tool which would allow me to check the value of large capacitors, using the frequency counter.

The tool is based on a NE555 (or LM555) timer i.c., wired as an astable multivibrator. The frequency of the multivibrator is determined by the value of the unknown capacitor Cx. The circuit can be fed by a small battery. I use myself a 9 V block battery which is connected to pins 4 and 8 of the i.c. via an on/off switch.

The output of the multivibrator is connected to the digital frequency counter. The counter should assess the cycle length of the square wave, not the frequency (on most frequency counters, one can select either frequency measurement or measurement of the cycle length). A wave with a frequency of 1 kHz (i.e. 1000 Hz) has a cycle length of 1 millisecond.

The cycle length of the square wave which is produced by the NE555 can be determined using the following formula:

T = 0.7 * Cx * (R1 + 2 * R2)

[Cx in Farad, R in Ohms, R1 is the resistor between pins 7 and 8 and R2 the resistor between pins 7 and 6 of the i.c.]

When R1 = 1000 Ohms and R2 = 215 Ohms, a capacitor of 1 uF will produce a 1 ms cycle. If you measure large electrolytics (1000 uF or more), you must wait several seconds before you get any display reading on the frequency counter. With this combination of R1 and R2, you can measure capacitors between about 0.1 uF and 5000 uF. Smaller capacitors can be tested if R1 and R2 are made larger (e.g., using 1M and 215k, an 1 nF capacitor will produce a cycle length of 1 ms). However, when small capacitors are measured the capacitance of the wiring must be taken into account (e.g. a capacitor of 1000 pF may give a reading of 1050 pF if the wiring has a capacitance of 50 pF). When Cx is greater than 1 uF, stray capacitance can be neglected. For an accurate reading, R1 and R2 should be 1% metal oxide resistors.