Vacuum variable capacitor

A vacuum variable capacitor uses a high vacuum as the dielectric instead of air or other insulating material. This allows for a higher voltage rating and/or capacitance value using a smaller total volume. In addition to the higher voltage rating a vacuum dielectric greatly reduces the chance of arcing between the plates. There are several different designs in vacuum variables, the most common geometry of the capacitor plates is usually inter-meshed concentric cylinders. The meshed cylinders are contained within a glass or ceramic vacuum envelope, similar to an electron tube. A metal bellows is used to maintain a vacuum seal while allowing positional control for the moving parts of the capacitor.

Invention

Nikola Tesla filed patent 577,671, " Manufacture of electrical condensers, coils, etc." on November 6, 1896. In this patent he claims the following: "It has heretofore been shown by me that the sufficiency and practicability of such devices are very greatly enhanced by the exclusion of air or gas from the dielectric separating the conductors or remote portions of the same conductor; and the object of my present improvement is to secure such exclusion of air in as perfect a manner as possible in a convenient and practical way." He goes on to describe the process in which the aforementioned components may be processed to exclude air.

Original Application

The original use, as indicated in the above cited patent, was to enhance the quality of the electrical components for handling, "currents of high frequency and potential". These components were necessary for the DC impulse research which Tesla was studying for decades after his improvements to the AC electrical system.

Common Applications

Vacuum variable capacitors are commonly used in high voltage applications 5000 volts(5kV) and above. They are used in equipment such as high powered broadcast transmitters, Amateur radio RF Amplifiers and large Antenna tuners. The main applications today are RF plasmas of 2 to 160MHz where the vacuum capacitor is used as the impedance variation part in an automatic matching network.

Other Variations

A 12 pF 20 kV fixed vacuum capacitor

A fixed value vacuum capacitor.

Other variations of vacuum capacitors include fixed value capacitors. Fixed value capacitors are designed very much like the variable versions with the exception of an adjustment mechanism.

Comparison

When compared to other variable capacitors, vacuum variables tend to be more precise and more stable. This is due to the vacuum itself. Because of the sealed chamber, the dielectric constant remains the same over a wider range of operating conditions. With air Variable capacitors, the air moving around the plates may change the value slightly; often it is not much but in some applications its enough to cause undesirable effects. Another common problem with air variables is dust and insects. Dust collected on the plates or a curious insect could have disastrous effects. An insect in contact with the plates is likely to cause a short circuit with the open frame design used on most air variable capacitors.

Vacuum variable capacitors are generally more expensive than air variable capacitors. This is primarily due to their design and the materials used. Although most use copper and glass, some may use other materials such as ceramics and metals such as gold and silver. Vacuum variables also vary in adjustment mechanisms which range from coarse to fine.