Capacitance Bridge

The purpose of this project is to study the capacitance bridge which is used to measure unknown capacitance values between about 0.001 uF and 10uF.

This circuit uses many of the same concepts which are used in expensive commercial capacity bridges which sell for over $100. The circuit is an audio oscillator combined with a modification of the Wheatstone Bridge circuit.

The oscillator circuit is of the common pulse tone type which we've talked about before see Oscillator Section. The bridge circuit is a modification of the Wheatstone Bridge. The modification to the bridge involves substituting Capacitors for the two top adjacent arms of the bridge, and then using the 50K Control for the remaining two arms.

Operation of the bridge is as follows:

1. Connect unknown capacitance Cx across the Probes.

2. Close the key and adjust the 50K Control for minimum earphone signal.

3. Compare the Control dial setting with settings obtained by measuring Capacitors of known value.

You can use some of the Capacitors on the board, such as the 0.001, 0.01, 0.02, 0.05, both 0.05's in parallel to obtain a 0.1, 3.3 and the 10uF. Use the space below to make a chart or graph of capacitance versus Control dial settings. Once you've calibrated this capacitance bridge you can use it to check those mysterious color coded Capacitors you have salvaged out of old radios and TV's.