Trimmer Capacitor is a type of variable capacitor whose value can be varied either mechanically (by turning the shaft using a screwdriver) or electrically (by switching the voltage and frequency of circuits). It is non-polarized (i.e., they do not have any positive or negative terminals) and is usually used in circuits when there is no need to vary the capacitance after initial adjustments are made.

A Trimmer capacitor is used to fix oscillator frequency values, latencies, rise & fall times, and other variables within a circuit. It is utilized in production processes where a specific capacitance value must be matched to a specific circuit. Trimmer capacitor is also commonly used in the production phase and high-frequency applications like frequency tuning of microwave & RF filters.

Pin configuration and characteristics

A Trimmer capacitor has a three-pin configuration where one pin is connected to the active plate (rotating plate), other one is connected to an inactive plate (fixed plate) and the third one is common (which holds the dielectric and the bottom metal plate and keeps them intact when the screw is used to change the capacitance). 

Trimmer capacitor normally has a voltage rating of up to 300 V and its capacitance range varies from 0.5 to 120 pF. It do not have a good capacitance value tolerance but this does not pose a great problem as it can be varied.

Types of Trimmer Capacitor

There are two types of Trimmer capacitor which is classified based on the type of dielectric used.

Air Trimmer Capacitor

In Air trimmer capacitor, air acts as the dielectric medium. Concentric tubular air trimmer capacitor is commonly used in RF applications because of its low leakage current. This means operating losses are minimal in air capacitors, especially in low humidity.

Air trimmer capacitor has self-locking constant torque mechanism that provides uniform torque, less ESR & less dynamic tuning noise. It can be used in the MRI and NMR sectors, as well as in signal processing, RF communications, oscillators, interstate coupling, filter tuning, crystal trimming, and impedance matching.

Ceramic Trimmer Capacitor

Ceramic trimmer capacitor has ceramic dielectric and are very small and thin. It is constructed without using plastic material to provide better soldering heat resistance. This capacitor has high self-resonant frequency of 6.2 GHz making it suitable for high-frequency circuits. It can also be used in crystal oscillators, Bluetooth, crystal filters, and tiny tuner packs for FM and TV, among other devices.

Construction of Trimmer Capacitor

Trimmer capacitor has a dielectric placed in between two parallel semicircular metal conductors/plates. One of the two metal plates is fixed while the other is movable. The dielectric is also fixed. When the movable plate is rotated, the capacitance of the trimmer capacitor changes.

When the semicircular rotating plate overlaps with the stationary plate, the capacitance is maximum and reduces as it moves away from the fixed semicircular plate.

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