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What Is an Air Capacitor?

Paul Scott
Paul Scott

An air capacitor is an adjustable capacitor consisting of a series of semi-circular, rotating aluminum plates on a central shaft placed between a similarly-spaced set of static aluminum plates. When the moving plates are rotated, the amount of overlap between the moving and static plates changes. The air between the sets of plates acts an an effective dielectric, insulating the sets from one another. As the capacitance of any capacitor depends on the combined size of its plates, this adjustment allows the value of the air capacitor to be adjusted. Air capacitors are commonly used in resonant circuits where adjustable capacitance is required.

Capacitors are electronic components that store electrical energy for short periods. Capacitors consist of thin metal film plates separated by a dielectric, or insulating, material such as glass compounds, paper, or air. The capacitance, or extent of the energy storing capability, of any capacitor is largely dictated by the total area of its plates. Most capacitors are of a fixed capacitance value, component replacement being necessary to decrease or increase the value. Some types of electronic circuits, however, require the values of certain capacitors to be changed repeatedly as part of the circuit's function.

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Man with a drill

In these cases, components such as the air capacitor, also known as the variable capacitor or air variable condenser, are used. These are typically considerably larger than conventional capacitors and are not enclosed in a casing. The air capacitor consists of a series of semi-circular discs with a hole drilled in their centers through which a control rod is passed. Alternate discs are attached to the control rod while it passes freely through the others. This means that the disc set is effectively divided into two groups, one rotating between the other with an insulating body of air between them.

These two groups of discs collectively form the two plate areas of the capacitor. As the discs are semi-circular in shape, rotating the moving set causes the amount which the two groups overlap to change, thus changing the total plate area. As the capacitance of a capacitor is dependent on its total plate area, this change in area causes a corresponding change in the components capacitance, allowing an operator to change the component value at will. The air capacitor is frequently used in LC, or resonant, circuits, which require changes in capacitance. These circuits include frequency mixers, radio tuners, and the impedance matching components of antenna tuners.

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