Inductance and capacitance meter adapter for DVM

This device is an addon to a digital or analog voltmeter. The device can measure inductance in the 100μH...1H range, capacitance in the 100pF to 1μF range, and it also produces a set of frequencies 100 Hz, 1 kHz, 10 kHz, 100 kHz, 1 MHz.

How the circuit works

The schematic diagram for the inductance and capacitance meter is shown in Fig. 1. The main part of the circuit is a digital IC CD4011 (CD4001 also can be used instead of IC CD4011). It works as an oscillator whose frequency is determined by the RC components R1..R5 and C1..C5 in feedback circuits. A two deck rotary switch S1 selects one of five measurement ranges. The transistor Q1 is used to amplify the signal to provide sufficient current through potentiometer R8 and a load circuit. The signal at the collector of Q1 has 50% duty cycle.

Circuit schematic of the inductance and capacitance meter adapter

Fig. 1. Simple adapter circuit for DVM

The switch S2 is in position "L"

DD1 - CD4011; DA1 - 7806; Q1 - 2N2907; VD1, VD2 - 1N34A;
C1 - 100pF; C2 - 1nF; C3 - 10nF; C4 - 0.1μF; C5 - 1μF;
C6 - 1μF; C7 - 1nF; C8, C9, C10 - 100μF;
R1..R5 - 10K; R6, R7 - 470Ω;
R8 - 470Ω; R9 - 1K; R10 - 3.3K.

A switch S2 is used to configure the measurement circuit. When it switched into position "L", the measurement circuit operates as an inductance meter, see the Fig. 2:

When the switch S2 is in position L, the circuit measures inductance

Fig. 2. The switch S2 is in position "L"

This circuit provides DC output voltage on the clamps X6, X7, and this voltage is directly proportional to the inductance of Lx. The voltage is negative. The source of the voltage is negative pulses across the inductance Lx.

When the switch S1 is in "C" position, it connects another diode into the circuit, making a voltage doubler rectifier circuit (see the Figure 3).

In the position C, the circuit configured as a capacitance meter

Fig. 3. The switch S2 is in position "C"

In this case, the pulses of current through the capacitor Cx are proportional to its capacity.

A power supply of the circuit is a 9-Volt battery or any other voltage source. The 7806 voltage regulator provides 6-Volts stabilized voltage for the circuit.

Adjustment of the circuit

At first, adjust potentiometers R1..R5 to get the set of frequencies 100 Hz, 1 kHz, 10 kHz, 100 kHz, 1 MHz on the output of the oscillator. Then switch S2 into "C" position, and check out the signal at the collector of Q1, it must be a square wave (50% duty cycle). If not, then match the value of the resistor R7 to make shape of the signal as close to square wave as possible.

There is a way to check out the duty cycle at the collector of Q1. Set the switch S2 in "C" position, and connect an RC network to the X3 and X5 terminals (see Fig. 4):

How to check out a duty cycle using an RC network

Fig. 4. RC network

This RC network operates as an integrator circuit. Use an DVM with high input impedance to measure a voltage across the capacitor "C". If duty cycle is 50%, then the voltage must be close to 3 Volts (it is half of the power supply voltage). Check out all five ranges.

Next, switch S1 into 100 pF range, connect to clamps X3, X4 a capacitor with capacitance of 100 pF (use a precise capacitor with ±10% capacitance tolerance or better, it is called "standard capacitor"), adjust the potentiometer R10 to set 1 Volt across X6, X7 terminals. After that, switch the S2 into "L" position, connect a standard coil of 100 μH to the X4, X5 clamps, and adjust the potentiometer R9 to set 1 Volt across X6, X7 terminals. That's all.

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