Analogue capacitance meter

This circuit diagram of a very simple analogue capacitance meter was published in the Bulgarian magazine "Radio Televizia Elektronika", September 1977, and later it was reprinted in the Soviet magazine "Radio", June 1978.

The circuit diagram of the capacitance meter is shown in Figure 1. The circuit is based on TTL logic gates and counters. The main parts of the circuit are the pulse generator (it based on logic gates D1.1..D1.3), the frequency divider with decade counters D2..D4, the transistor V1 switching circuit and the measuring circuit with the diode V2, resistor R7 and the microammeter PA1.

Analogue capacitance meter circuit with TTL ICs

Fig. 1. Circuit diagram of the capacitance meter
DD1 - 7400, DD2..D4 - 7490; V1 - 2N2222, V2 - 1N34; PA1 - 100 uA;
C1 - 4.7 nF; C2 - 1 nF; R1 - 470 Ω R2 - 470 Ω; R3 - 820 Ω; R4 - 1 K;
R5, R6 - 100 Ω; R7 - 22K; S1 - rotary switch; S2 - button.

The operating concept of the capacitance meter is quite simple. The microammeter PA1 measures the average discharge current through the testing capacitor Cx, this capacitor is periodically charged by square pulses. The measuring current is proportional to the capacitance of the testing capacitor Cx.

The generator generates square pulses (the duty cycle of them is 50%) with frequency of 100 kHz. The pulses goes through the inverter D1.4 to the frequency divider circuit and to a terminal of the rotary switch S1.

The frequency divider circuit based on three decade counters D2..D4, connected in series. Each decade counter divides the frequency by 10, it provides a set of signals of 100 kHz, 10 kHz, 1 kHz and 100 Hz for each range.

The switch S1 used to select a frequency that applied to the transistor switch.

When a pulse passes through the transistor V1, resistor R6 and the diode V2, it charges the testing capacitor Cx. When the pulse went off, the testing capacitor Cx starts to discharge through the microammeter PA1.

The reference capacitor C2 and the button S2 are used for the calibration of the capacitance meter at the 1 nF range.

The actual accuracy of the capacitance meter depends on the tolerance of the capacitor C2 and how precise adjustment by the potentiometer R7 can be achieved. The accuracy doesn't depend on the clock frequency, it may be slightly differ from 100 kHz, but it have to be quite stable.

A digital multimeter can be used instead of the microammeter PA1, the multimeter should be switched to the "Current" setting (use the 0.1..1 mA range).

The digital frequency divider circuit of the analogue capacitance meter

Fig. 2. The digital frequency divider circuit of the capacitance meter.
D1 - CD4093; D2-D4 CD4017; The power supply voltage is 5 V.

The analogue capacitance meter can be made also using CMOS integrated circuits (see Figure 2). It uses pulse generator based on the Schmitt trigger 2NAND D1.1, all unused gates D2.2-D2.4 are connected in parallel. The divider consist of three 5-stage Johnson counters, connected in series. Note also the value of the resistor R4 - it is doubled.

To adjust the generator, just match the resistor R1 value to get the frequency of 100 kHz, or replace the resistor with a potentiometer of 15K and adjust it. Next, switch the S1 to the range "1 nF". Push the button S2 and hold it down, then adjust the potentiometer R7 to move the needle of the ammeter on the last mark. That's all.

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