Capacitor capacity comparator

The capacitance comparator allows you to compare the capacitances of two capacitors and indicate the equality or inequality of these capacitances.

The monitoring of changes in capacitance of capacitors under the influence of external factors (e.g., capacitor sensors) can be used to indicate the distance to various objects, in contactless capacitive switches, capacitive liquid level indicators, security systems, etc.

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The device for comparing the capacitance of the reference capacitor C1 and the capacitor of the sensor Cx, Figure 1, contains a dynamic measuring RC bridge, to the diagonal of which the inputs of the comparator U1 LM324 are connected. The unbalance of the bridge is indicated by LED1 and LED2 indicators.

Figure 1 A capacitive comparator circuit with a dynamic measuring RC bridge that contains two RC circuits (R3, Cx and R5, C1).

The measuring RC bridge is made of two RC circuits: R3, Cx and R5, C1, where R3=R5, and Cx≈C1. Rectangular pulses from an external generator with a frequency of 10 kHz, a voltage of 10 V and with a fill factor D on the order of 99% are fed to the measuring RC bridge. Capacitors C1 and Cx are simultaneously charged exponentially. At the end of the input pulse, both capacitors are instantly discharged through diodes D1 and D2.

The inputs of the comparator U1 are connected to the diagonal of the measuring bridge. If Cx≠C1 the charge rates of the capacitors C1 and Cx are different, an unbalance voltage will be present in the diagonal of the bridge. The unbalance of the bridge will cause the state of the comparator U1 to switch.

In the case of Cx>C1, LED1 will light up; in the case of Cx<C1, LED2 will light up. The device reacts to the unbalance of the bridge when the sensor capacity changes by hundredths of a percent.

When correcting the values of resistors R2 and R7, you can turn on the LEDs of optocoupler controlling external devices.

Michael A. Shustov is a doctor of technical sciences, candidate of chemical sciences and the author of over 750 printed works in the field of electronics, chemistry, physics, geology, medicine, and history.

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