A simple circuit diagram of an inductance measuring device

In electronic production and design, inductor coils with different parameters are often used. The inductance of these coils is not as easy to measure as resistance. Although some digital multimeters have inductance measurement gears, the measurement range is very limited. This circuit measures the inductance value by resonance method. The lower limit of measurement can reach 10nH. The measurement range is very wide and can meet the inductance measurement under normal circumstances. The circuit structure is simple, the operation is reliable and stable, and it is suitable for hobbyists to make.

1. Circuit Working Principle

The circuit principle is shown in Figure 1(a).

The core device of this circuit is the integrated voltage-controlled oscillator chip MC1648, which uses its voltage-controlled characteristics to generate a frequency signal at output pin 3, and can indirectly measure the inductance LX value to be measured with extremely high measurement accuracy.

BB809 is a variable capacitance diode. The potentiometer VR1 in the figure divides the +15V voltage. By adjusting the potentiometer, different voltage outputs can be obtained. The voltage is added to the variable capacitance diode BB809 through R1 to obtain different capacitances. When measuring the measured inductance LX, just connect LX to points A and B in the figure, and then adjust the potentiometer VR1 to make the circuit resonate. An oscillation signal of a certain frequency will be output at pin 3 of MC1648. The frequency value at point C can be measured with a frequency meter, and the LX value can be calculated.

In the formula, the resonant frequency f0 is the output frequency value of the 3rd pin of MC1648, and C is the capacitance value of the varactor diode set by the potentiometer VR1. It can be seen that to calculate the value of LX, the value of C must be known first. For this purpose, it is necessary to calibrate the corresponding value of the potentiometer VR1 scale and the varactor diode.

In order to calibrate the capacitance between the varactor diode and the potentiometer, we need to make a standard square RF (radio frequency) inductor L0. As shown in Figure 6-7 (b), the inductance of the standard coil is 0.44μH. During calibration, connect the RF coil L0 to the A and B ends of Figure (a), adjust the potentiometer VR1 to different scale positions, and measure the corresponding measurement value at point C. Then, according to the above resonance formula, the capacitance of the varactor diode at different scales of the potentiometer VR1 dial can be calculated. The attached table gives the corresponding relationship of the measured samples.

2. Component Selection

The integrated circuit IC can be a VCO (voltage controlled oscillator) chip from Motoroia. VR1 can be a multi-turn high-precision potentiometer. Other components can be selected as shown in the circuit diagram.

3. Production and debugging methods

When making it, you need to make a dial on the multi-turn potentiometer shaft with a pointer. The RF standard coil is made according to the size given in Figure (b). The circuit can work normally if it is installed correctly. Adjusting the potentiometer VR1 to obtain the corresponding relationship between the sliding points and the variable capacitance diode can ensure convenient measurement. This measurement method is an indirect measurement, but it has a wide measurement range and accurate measurement, so it is desirable for electronic enthusiasts and laboratories to detect inductance. If the inductance is fixed, the device can become a signal generator with adjustable frequency.