Capacitance and inductance meter with digital ICs
This device can be used to measure capacitance in range from 10 pF up to 10 μF, and inductance in range from 10 μH up to 1 H. The whole measurement range is split into six subranges:
- 100 pF / 10μH
- 1 nF / 100 μH
- 10 nF / 1 mH
- 0.1 μF / 10 mH
- 1 μF / 100 mH
- 10 μF / 1 H
The precision of measurement is about 5 percent. The device has a linear scale.
The circuit diagram of the capacitance and inductance meter is shown in the picture below.
D1 - CD4049UB
D2-D6 - CD4017
VD1-VD3 - 1N34 (germanium diodes)
VD4 - 1N4148 (a silicon diode)
X1 - 1 MHz quartz crystal
R1 - 24k; R2 - 300 Ω
R3, R4 - 2k (trimming potentiometers)
C1 - 220 μF x 6 V;
M1 - ammeter with a full scale deflection for 100 μA
SA1 - 6 position rotary switch
It comprises an oscillator circuit (gates D1.1 and D1.2), decade dividers D2-D6, each of them divides the input frequency by 10, buffers D1.3-D1.6, and the measurement circuit with diodes VD1-VD4. The ammeter M1 is used to display results of measurements.
How the circuit works
The oscillator produces pulses with the frequency of 1 MHz. Decade dividers divide the 1 MHz frequency by 10, so there is a set of frequencies 1 MHz, 100 kHz, 10 kHz, 1 kHz, 100 Hz, 10 Hz, they are fed to the rotary switch SA1, it is used to select a range. Any CMOS counters that provide division by 10 can be used in this circuit.
One of the selected frequencies is fed to the buffer, it is made of 4 inverting gates. These four gates are connected in parallel to increase the current drive capability of the output. Resistor R2 limits the output current of the buffer. Through the resistor R2 the frequency is fed to the measurement circuit.
For the capacitance measurement, a capacitor to be measured must be connected to the "Cx" terminals. The frequency goes through the capacitor to the voltage doubling rectifier (VD2, VD3), the capacitor C1 smooths the ripple voltage. The ammeter M1 indicates the voltage that is proportional to a capacity of the measured capacitor. The trimming potentiometer R4 shunts the ammeter, it is used to adjust the readings.
To measure an inductance, the inductor should be connected to the "Lx" terminals. The negative back EMF pulses across the inductor passes through the diode VD1 and the potentiometer R3 to the capacitor C1 and the ammeter M1. The voltage is also proportional to the inductance. Diodes VD2 and VD3 don't affect the circuit. The potentiometer R3 is used to adjust the readings.
In both cases the diode VD4 protects the ammeter from overload.
The power supply voltage of the circuit is in range of 5..15 Volts.
The adjustment of the circuit is easy. Switch SA1 to the middle position (the 10 nF / 1 mH range), and connect a capacitor of 10 nF to the "Cx" terminals. Then adjust the potentiometer R4 to set the pointer of the ammeter M1 on the last mark of the scale. Now connect an inductance of 1 mH to the "Lx" terminals, use the potentiometer R3 to set the pointer to the last mark of the scale. That's all, the adjustment is completed.