Variable capacitors are designed according to the same principles used for standard capacitor production, with sets of conductive plates arranged in a parallel sequence and separated with dielectric layers composed of materials such as mica, reinforced paper, or certain types of ceramics. Unlike standard fixed capacitors, variable capacitors are configured to allow changing capacitance levels. In most cases, variable capacitance is accomplished by altering the distance between the parallel plates in a capacitor or by shifting the cross-sectional area at which the plates face one another. Due to their normally high allowance for holding charge, these types of capacitors can be found in filter circuits and power supply applications, and are also commonly used in radio frequency networks. In RF applications they are sometimes known as tuning capacitors because of their ability to control signals in radio receivers. HyperPhysics offers more information on the principles of capacitance.
Variable Air Capacitor
A typical variable air capacitor used in radio frequency circuits is composed of two arrays of parallel conductive plates in a single assembly. One set consists of stator plates that are fixed into position and attached to the capacitor’s frame, while the second set is composed of rotor plates affixed to the capacitor’s shaft, which is used to modify the capacitance according to application requirements. These sets of plates are intermeshed, with the rotors moving in relation to the stators. The air between the two plate arrays functions as the capacitor dielectric, so that the relative positions of the plates designates the effective capacitance for the device at each setting. In addition to the shifting plate sets used in air capacitors, each type of variable capacitor uses its own method for adjusting capacitance levels to suit an RF circuit application.
The main components in a compression capacitor include a holder frame, a mica dielectric layer, and a series of inset plates. The conductive metal plates are separated by sheets of the mica dielectric, and increases in the capacitance levels can be accomplished by expanding the area of the plate and mica combination or by increasing the number of alternating mica-metal layers in the unit. Force is usually present on both sides of a pair of plates surrounding a mica sheet in order to preserve unit integrity. The capacitor assembly is typically mounted whole on a holder made of ceramic or similar material. Any holes or screws applied during the mounting process are considered part of the general holder assembly.
A standard variable piston capacitor is composed of a metal piston, a ceramic sleeve, and a metal shell. An inner metal cylinder is positioned coaxially and enclosed within an outer metal cylinder, with a vacuum, air, or ceramic dielectric separating the two. By extending the internal cylinder further into the external cylinder, capacitance levels can be increased. These piston-style, or small compression, capacitors can also be used in conjunction with variable air capacitors. In this combination, the smaller capacitors operate as trimmer devices, which are used to pare the exact capacitance value of the main capacitor. A small compression capacitor can be mounted directly on the air capacitor’s frame or in a nearby location on the circuit. Trimmers are connected in parallel to the main tuning capacitor in a radio frequency network, although an alternative form, known as a padder, can be connected in series.
In a variable air capacitor, the capacitance level at a given setting depends on the degree of shading or coverage of the rotor plates by the stator plates. For example, if the rotor plates are positioned outside the area covered by the stator plates, capacitance will be at its minimal level, while having the rotor set’s cross-sectional area completely shaded by the stator plates will raise capacitance to its maximum. Some variable capacitors are designed with a shaft located at the center of the rotor plate set, meaning their capacitance changes in direct relation to the angle of the rotor shaft. These devices function under straight-line capacitance, and when applied to radio frequency tuners, straight-line capacitance can create an uneven distribution of frequencies. An offset rotor shaft, which works to reduce nonlinearity by positioning the shaft and the plates in a way that produces a linear relationship between shaft angle and resonant frequency, can be used to compensate for the distortion.