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Detailed explanation of the basic knowledge of trimmer capacitors

baijiahao | 21/07/2021

The trimmer capacitor is actually a variable capacitor, but the range of capacitance change is small, usually only a few picofarads to tens of picofarads.

Trimming capacitors are divided into mica trimming capacitors, ceramic-mediated trimming capacitors, film trimming capacitors, pull-line trimming capacitors, and so on. It is often used as a compensation capacitor or correction capacitor in various tuning and oscillation circuits.

Capacitors that can be adjusted within a small range and can be fixed to a certain capacitance value after adjustment are called trimmer capacitors, and semi-trimmer capacitors can also be seen. When adjusting, change the distance or area between the two pieces. The trimmer capacitor is made of two or two sets of small metal shrapnel sandwiched between the dielectric. As shown in the figure, the shape of the semi-adjustable capacitor. Semi-adjustable capacitors generally do not have a handle and can only be adjusted with a screwdriver, so they are often used in places that do not need to be adjusted frequently. Semi-tunable capacitors are used as compensation capacitors or correction capacitors in various tuning and oscillation circuits.

Semi-variable capacitor

Semi-variable capacitors are also called trimmer capacitors and are used as compensation capacitors or correction capacitors in various tuning and oscillation circuits. It is divided into mica trimmer capacitors, porcelain trimmer capacitors, film trimmer capacitors, cable trimmer capacitors, etc.

1. Mica trimming capacitors Mica trimming capacitors change the capacitance by adjusting the distance between the screw and the stator. The movable piece is an elastic copper or aluminum piece, and the fixed piece is a fixed metal piece, and a layer of mica flake is attached to the surface as a medium.

Mica trimmer capacitors are divided into single trimming and double trimming, and the capacitance can be adjusted repeatedly.

2. Porcelain-mediated trimmer capacitors Ceramic-mediated trimmer capacitors use ceramics as the medium. Both the moving piece (ceramic piece) and the fixed piece (ceramic piece) are plated with a semicircular silver layer. By rotating the moving piece to change the distance between the two silver pieces, the capacitance can be changed.

3. Pull-wire trimmer capacitors. Pull-wire trimmer capacitors were used in the oscillator circuit of radios as compensation capacitors in the early days. They used silver-plated ceramic tube substrates as fixed pieces, and thin metal wires (generally thin copper wires) wound on the outside were used as moving pieces. The number of turns of the small metal wire can change the capacitance. The disadvantage is that once the metal wire is pulled off, the original capacitance cannot be restored, and its capacitance can only be adjusted from large to small.

4. Thin-film trimmer capacitors. Thin-film trimmer capacitors use organic plastic film as the medium, that is, add an organic plastic film between the moving piece and the fixed piece (the moving and fixed pieces are irregular semi-circular metal pieces) to adjust the Screws can change the electric capacity by rotating the moving plate.

Film trimmer capacitors are generally divided into double trimming and four trimming. Some sealed double-connected or sealed four-connected variable capacitors are equipped with film trimmer capacitors. The double-connected or quadruple-connected variable capacitors are made into a whole with the trimmer capacitor. The trimmer capacitor is installed on the top of the shell, which is convenient for use and adjustment.

Characteristics and functions of trimmer capacitors

The trimmer capacitor is also called semi-variable capacitor. Its capacity is small, only a few picofarads to tens of picofarads, and the adjustment range is also small. The common ratios of its minimum capacity to maximum capacity are 3pF/10pF, 4.7pF/20pF, 4.7pF/25pF, etc. Trimming capacitors are divided into mica trimmer capacitors, ceramic dielectric trimmer capacitors, film dielectric trimmer capacitors, cable trimmer capacitors, etc. They are often used as compensation capacitors or correction capacitors in various tuning and oscillation circuits for radio tuning or oscillation. In the circuit.

Domestic trimming capacitors include film dielectric trimming capacitor CCW7-2 series and ceramic dielectric trimming capacitor CCWI2-3 series. They all use small screws to adjust the capacity, and are mainly used for precise frequency adjustment and temperature compensation in electronic equipment. There is also the ceramic cable trimming capacitor CCW8 series. The capacitance is changed by removing the outer electrode winding. The capacity is the largest when the cable is not pulled out. When the cable is pulled out and part of the cable is cut, the capacity decreases, so it is generally suitable for different oscillation frequencies. Need to change frequently in the oscillation circuit.

Detection method of trimmer capacitor

The way to use a multimeter to check the quality of the trimmer capacitor is: set the multimeter's range switch to RX1OkΩ, replace the test leads with red and black alligator clips, so as to make firm contact with the trimmer capacitor terminals, and free your hand to turn the screwdriver to make the fine adjustment The moving plates of the capacitor move relative to each other.

While rotating, observe the pointer of the multimeter. When it is normal, the pointer does not move. If there is a certain resistance value, it indicates that the trimmer capacitor is damp or dirty, causing leakage and cannot be used.

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