Variable capacitors are mostly use in radio tuning circuits and sometimes called “tuning capacitors”. They have very small capacitance values, typically between 100pF and 500pF. Many of them have short spindles, which are not suitable for the standard knobs use for variable resistors and rotary switches.
Variable capacitors are not use in timing circuits because their capacitance is too small to be practical. Their range values available are very limit. Instead, timing circuits use a fixed capacitor and variable resistor if it is necessary to vary the time period.
Other types are Trimmer Capacitors.
Trimmer capacitors (trimmers) are miniature variable capacitors. They are design to be mount onto the circuit board and adjust only at the process of building the circuit.
However, the process of adjustment requires patience if adjustment is needful. Because, the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer.
Trimmer capacitors are only available with very small capacitance, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2 – 10pF. Trimmers are the capacitors equivalent of preset which are miniature variable resistors.
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