Types of Capacitor-Variable value capacitors

A variable capacitor is a special type of capacitor, most commonly used for tuning radios, which allows the amount of electrical charge it can hold to be altered over a certain range, measured in a unit known as farads. Regular capacitors build up and store an electrical charge until it's ready to use. While a variable capacitor stores the charge in the same fashion, it can be adjusted as many times as desired to store different amounts of electricity.
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When altering a variable capacitor, the user is actually changing its capacitance. Capacitance means the amount of energy the capacitor can store. A bigger capacitance means more stored energy. This energy is measured in farads, but because a variable capacitor typically has a very small capacitance, a smaller unit known as a picofarad is used instead. 

Types of Variable Capacitor

Air-gap capacitors:

These capacitors use air as the dielectric medium. The distance between the plates can be varied to change the capacitance. The capacitance values offered are high and can be used with high voltages. These are used for high frequency operations in communication systems.




Vacuum capacitors:

These capacitors have glass or ceramic encapsulation and vacuum as the dielectric. Their complex construction makes it very expensive. Theoretically, it has less losses and are used in RF applications.



Functioning

While each performs the same function, one uses a high vacuum instead of air to insulate the capacitor. This allows for a higher capacitance to be produced in a smaller-sized capacitor. Variable capacitors can also be controlled mechanically or electronically. The electronically controlled capacitors change their capacitance based on DC voltage applied to it, while the mechanically controlled versions are designed so the parts can be moved to increase or decrease capacitance.

Uses

Common uses for variable capacitors is in radios to allow the radio to tune to different stations. The capacitor is part of an LC circuit where the L stands for an inductor and the C stands for a capacitor. This inductor/capacitor combination uses the variable capacitor to alter the frequency passing through the LC circuit and thus connect with radio stations, each of which operates on a different frequency that the LC circuit must match to receive.

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