How to detect variable capacitors?

Variable capacitor, usually called variable capacitor, is a kind of capacitor whose capacitance can be adjusted within a certain range. By changing the relative effective area between the pole pieces or the distance between the pieces, its capacitance will change accordingly . Usually used as a tuning capacitor in a radio receiving circuit. It is composed of a moving piece, a fixed piece and an insulating medium. The capacitance can be changed by changing the relative angle of the moving piece and the fixed piece. The moving plates of several variable capacitors can be assembled on the same shaft to form a coaxial variable capacitor (commonly known as double-connected, triple-connected, etc.).

1. Characteristics and functions of variable capacitors

There are many types of variable capacitors, which can be divided into air dielectric variable capacitors and solid dielectric variable capacitors according to the dielectric materials used. There are single-connection variable capacitors, double-connection variable capacitors, differential double-connection variable capacitors, etc., which are common in our lives. The appearance is shown in Figure 1 (a).

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Figure 1 (a) Shape of variable capacitor

Air dielectric variable capacitors are divided into air single-connect variable capacitors and air double-connect variable capacitors, which are generally used in radios, electronic instruments, high-frequency signal generators, communication equipment and related electronic equipment. Domestic air single-connect variable capacitors include CB-1-X X series and CB-X-X X series, and air double-connect variable capacitors include CB-2-X X series and CB-2-X-XX series.

Solid dielectric variable capacitors are divided into sealed single-connected variable capacitors, sealed double-connected variable capacitors and sealed four-connected variable capacitors. Sealed single-connect variable capacitors are mainly used in simple radios or electronic equipment, sealed double-connected variable capacitors are used in transistor radios and related electronic instruments and electronic equipment, and sealed four-connected variable capacitors are commonly used in AM/FM multi-band radios. .

2. Detection method of variable capacitor

The capacity of variable capacitors is generally very small and cannot be measured with a multimeter, but it can be judged whether there is a contact or leakage between the moving and fixed pieces, as shown in Figure 1 (b).

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Figure 1 (b) Detection of variable capacitor

Set the multimeter to the RX10KΩ block, connect the two test leads to the moving piece and fixed piece of the variable capacitor respectively, and slowly rotate the shaft several times back and forth. The pointers of the multimeter should be in the "variable capacitor characteristics and detection method" position. move. If the pointer sometimes points, it means that there is a short-circuit point between the moving piece and the fixed piece; if it is rotated to a certain angle, the pointer points to a certain resistance value, indicating that there is leakage between the moving piece and the fixed piece.

The leakage resistance of the sealed single-connection or double-connection variable capacitor becomes smaller, which may be caused by moisture. If the resistance becomes larger after drying, it can be used continuously.

Variable capacitor is a capacitor whose capacitance can be adjusted within a certain range. By changing the relative effective area between the pole pieces or the distance between the pieces, its capacitance will change accordingly. Usually used as a tuning capacitor in a radio receiving circuit. If you encounter a variable capacitor that does not work in your life, you can try the above methods to test it with a multimeter.