Capacitance and inductance meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed pointer meter allows you to determine the parameters of most of the inductors and capacitors encountered in the practice of a radio amateur. In addition to measuring the parameters of elements, the device can be used as a fixed frequency generator with a ten-day division, as well as a label generator for radio engineering measuring instruments.

The proposed capacitance and inductance meter is characterized by simplicity and low labor intensity of manufacture. The measurement range is divided ten days into six sub-ranges with capacitance limits of 100 pF - 10 μF for capacitors and inductance 10 μH - 1 H for inductors. The minimum values ​​of the measured capacitance, inductance and the measurement accuracy of parameters at the limit of 100 pF and 10 μH are determined by the constructive capacitance of the terminals or sockets for connecting the leads of the elements. On the remaining sub-ranges, the measurement error is mainly determined by the accuracy class of the pointer measuring head. The current consumed by the device does not exceed 25 mA.

The principle of operation of the device is based on measuring the average value of the discharge current of the capacitance of the capacitor and the EMF of the self-induction of the inductance. The meter, the schematic diagram of which is shown in fig. 1, consists of a master oscillator on the elements DD1.5, DD1.6 with quartz frequency stabilization, a line of frequency dividers on microcircuits DD2 - DD6 and buffer inverters DD1.1 - DD1.4. Resistor R4 limits the output current of the inverters. A circuit of elements VD7, VD8, R6, C4 is used when measuring capacitance, and a circuit VD6, R5, R6, C4 is used when measuring inductance. Diode VD9 protects the PA1 microammeter from overload. The capacitance of the capacitor C4 is chosen relatively large in order to reduce the jitter of the needle at the maximum measurement limit, where the clock frequency is minimal - 10 Hz.

Rice. 1 (click to enlarge)

The instrument uses a measuring head with a total deflection current of 100 μA. If you apply a more sensitive one - by 50 μA, then in this case you can reduce the measurement limit by 2 times. The ALS339A seven-segment LED indicator is used as an indicator of the measured parameter, it can be replaced by the ALS314A indicator. Instead of a quartz resonator at a frequency of 1 MHz, you can turn on a mica or ceramic capacitor with a capacitance of 24 pF, however, in this case, the measurement error will increase by 3-4%. It is possible to replace the D20 diode with D18 or GD507 diodes, the KS156A zener diode with KS147A, KS168A zener diodes. Silicon diodes VD1-VD4, VD9 can be any with a maximum current of at least 50 mA, and the transistor VT1 can be any of the types KT315, KT815. Capacitor C3 - ceramic K10-17a or KM-5. All element values ​​and quartz frequency may differ by 20%.
The drawing of a printed circuit board made on foil-coated fiberglass with a thickness of 1,5 mm is shown in fig. 2.

Fig. 2

The device setup starts in the capacitance measurement mode. Switch SB1 to the upper position according to the diagram and set the range switch SA1 to the position corresponding to the measurement limit of 1000 pF. By connecting an exemplary capacitor with a capacity of 1000 pF to the terminals XS1, XS2, the engine of the trimming resistor R6 is brought to a position in which the pointer of the PA1 microammeter is set to the final division of the scale. Then the switch SB1 is switched to the mode of measuring the inductance and, having connected an inductor of 100 μH to the terminals, in the same position of the switch SA1, a similar calibration is performed with a tuning resistor R5.

Naturally, the accuracy of instrument calibration is determined by the accuracy of the reference elements used. It is advisable to start measuring the parameters of the elements with a device from a larger measurement limit in order to avoid a sharp overshoot of the arrow of the device head. To power the meter, you can use a DC voltage of 10...15 V or an alternating voltage from a suitable winding of the power transformer of another device with a load current of at least 40...50 mA. The power of a separate transformer must be at least 1 W. If the device is powered by a battery of batteries or galvanic cells with a voltage of 9 V, it can be simplified and more economical by eliminating the diodes of the supply voltage rectifier, indicator HG1 and switch SB1 by bringing three terminals (sockets) to the front panel of the device from points 1, 2, 3 indicated on the concept. When measuring capacitance, the capacitor is connected to terminals 1 and 2; when measuring inductance, the coil is connected to terminals 1 and 3.

The accuracy of an LC meter with a pointer indicator to a certain extent depends on the section of the scale, therefore, the introduction of a switchable frequency divider by 2, 4 or a similar change in the frequency of the master oscillator (for the version without a quartz resonator) into the circuit makes it possible to reduce the requirements for dimensions and accuracy class of the indicating device.

Author: E. Terentiev, Naberezhnye Chelny, Tatarstan

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