Vacuum capacitor

A vacuum capacitor is an electric capacitor, in which a high vacuum with a gas pressure of about 10-7 Torr or 1.33 x 10-7 mbar is used as dielectric.

• 3.1 goodness
• 3.2 Temperature dependence of capacitance

General

Vacuum capacitors are required in RF transmitters, as a resonant circuit capacitors, as well as in applications where high voltages can occur, for example in performance LC resonant circuits for radio and television and in RF amplifier positions in magnetrons for pulse shaping in the output circuit, in high-frequency welding equipment and high frequency drying equipment, plasma coating, and plasma etching in the semiconductor industry and in nuclear magnetic resonance devices (MRI) as a non-magnetizable capacitors.

These applications require a very high dielectric strength and extremely high current carrying capacity of the capacitors. There are up to 1000 A vacuum capacitors manufactured with dielectric strengths up to 90 kV and current capacities.

The low pressure gas of the vacuum reduces the likelihood of impact ionization of the air molecules ( avalanche effect), the square of the falling air pressure. This relationship is described in Paschen's Law. In addition, the low air pressure of the high vacuum reduces the appearance of spark discharges through peak efficiency of mechanical irregularity of the electrodes. This results in the very high dielectric strength of capacitors with a Vakuumdielektrikum. It is depending on the gas pressure between 20 and 500 kV / mm is indicated with an average of about 40 kV / mm which is about ten times higher than that of a capacitor with air dielectric at normal air pressure.

Main advantages of vacuum capacitors compared to ceramic capacitors ( high frequency capacitors ) and air capacitors of the same specification of the performance are:

• Smaller dimensions,
• Significantly lower ohmic losses (better quality factor Q)
• Higher current capacity,
• Self-healing ( no insulation available that can be damaged by arcing ).

Vacuum capacitors are produced in two designs, as solid capacitors with constant capacitance value and a variable vacuum capacitors, whose respective capacity is mechanically adjustable within defined limits.

A discussion about the term "vacuum" as a material-free ideal case and its relation to the term " dielectric " in the sense of " material constant " is not necessary here, because even in a high vacuum still 100 to 1000 molecules per cubic centimeter than there are " material."

Concept and characteristics

Vacuum capacitors consist of two concentrically arranged cylindrical electrodes, usually with several mounted on a base plate cylinders. These cylinders are concentric electrodes in capacitors with solid capacitance value, without touching each other, pushed into each other. In vacuum capacitors with adjustable capacitance value, a cylindrical rotor electrode is concentrically rotated into a stator electrode. By the cylindrical construction of the electrodes, a maximum utilization of the volume of the likewise circular, hermetically sealing the housing is achieved. As the material of the surrounding housing glass or a special ceramic used. The capacitor is similarly common in the tube technology, evacuated and sealed with suitable pumps. Depending on the current carrying capacity of the capacitors may be provided with tubes in the housing for an air or water cooling.

The distance between the electrodes of opposite polarity to each other depends on the required dielectric strength of the capacitor. However, not only the electrode gap is defined, the dielectric strength, the surface characteristics of the electrodes plays a role. Mechanical bumps on the electrode surface can favor by field emission spark discharges. To reduce this effect, the surfaces of the electrodes can be mechanically honed and additionally passivated or electrolytically.

Because of the very high alternating currents that can occur in the condenser in the operation of a transmitter tuned circuit and have not only high electrical but rather high mechanical stresses on the internal structure of the capacitor results in the mechanical structure of the electrodes must be very robust and stable. In order to keep this mechanical stress as low as possible, are often called " non-magnetic " vacuum capacitors whose electrodes are made of special non-magnetic alloys, are used.

Vacuum capacitors with fixed capacity

As described above consist vacuum capacitors with solid capacitance value (fixed value vacuum capacitors ) consists of two concentric cylindrical electrodes arranged with usually several, mounted on a base plate cylinders are concentric, without touching each other, pushed into each other. Vacuum capacitors with solid capacitance value are used as RF power capacitors ( high frequency capacitors) in transmitters with power ratings above about 10 kW in resonant circuits for generating radio frequencies for commercial and industrial channels. They are available with capacitances of 10 pF to 6000 pF, are mostly with the capacity tolerance of ± 5 %, specified and have nominal voltages up to 90 kV. Vacuum capacitors with solid capacitance own due to robust design with its very low internal ohmic losses, a very high current capacity. They are available as high-frequency capacitors in this application in competition with RF power ceramic capacitors. However, vacuum capacitors can be operated at high loads even with convection cooling when ceramic power capacitors would have to be already operated with forced cooling. Vacuum capacitors can also be easily used to at very high altitudes. By hermetically sealed vacuum in the condenser, these components are almost independent of the external air pressure used.

The disadvantage is the effect of the higher price of vacuum capacitors over ceramic high frequency capacitors. Also the temperature behavior of the capacity that is specified depending on the housing material with about 50 to 100.10 -6 K-1 could be a disadvantage if the temperature dependence of the inductance in the LC circuit requires a different course to obtain a temperature-stable frequency.

Variable vacuum capacitors

Variable vacuum capacitors are variable capacitors whose capacitance value can be continuously adjusted within defined limits. They resemble structurally the " diving trimmers ", an invention by Philips from the 1930s, but from a complete power range fro much more in the field of air - variable capacitors. However, variable vacuum capacitors are much smaller than air - variable capacitors.

Have variable vacuum capacitors as the vacuum capacitors with a fixed capacitance value, cylindrical electrodes with a plurality of generally -mounted on a base plate cylinders. One of these cylinder- shaped electrodes, the stator is mechanically firmly connected with the housing. The second electrode of the rotor, is screwed with the aid of a thread on a central axis in the cavity in the stator electrode. To maintain the vacuum during the adjustment, the screw threads of the rotor is enclosed within the housing with a hermetically sealed metal bellows sleeve.

Variable vacuum capacitors are designed for frequent operations. The thread on the rotor axis makes it possible to set the desired capacitance value with a rotation angle greater than 360 degrees. The adverse geometrically induced linear capacitance behavior of the immersed rotor that does not meets the resonant circuit laws, and is offset by the large angle of rotation again. In addition, variable vacuum capacitors can have very large, continuously variable capacitance ranges. The ratio of the minimum to the maximum capacity can up to be 1: 150. The drive of the rotor which must often be effected during the transmission operation can be done manually, but is usually often accomplished via a controlled motor.

Variable vacuum capacitors are of its dimensions significantly smaller than variable capacitors with air dielectric. Hermetically sealing the housing also prevents the air takes place unprotected variable capacitors contamination of the spaces between the electrodes due to dust and insects, which significantly reduces the withstand voltage of the variable capacitor. The influence of a changing humidity, which certainly causes changes in the capacitor characteristics at air condensers is also excluded in the hermetically sealed vacuum capacitors.

Variable vacuum capacitors are used in high-frequency transmitters with power ratings above about 10 kW for the precise control of the station frequency. They have adjustable maximum capacitance ranges 3-6600 pF, can be operated with up to 90 kV and carry currents up to 1000 A. The largest and most capacitors have a device for water cooling.

Properties

The mechanically robust design of vacuum capacitors has the positive effect is that the ohmic losses in the capacitor are very small due to the large cross -sections. And the compact structure of the vacuum capacitors, compared to air-cooled condensers, to reduce the internal resistance loss contributes. Results from the very low R ESR of vacuum capacitors, resulting in a very high current carrying capacity, together with the very low dielectric losses of the vacuum.

In general, the ohmic losses of a capacitor with the Resr, usually abbreviated ESR ( equivalent series resistance), or the loss factor tan δ specified. In vacuum condensers to be reciprocal, the quality factor Q is specified in place of the loss factor. A large value of the quality corresponding to a small width B at the resonant frequency f0 of the condenser. Since the curve of the impedance curve is steeper in the resonance range, the smaller the tan δ, large numbers of quality provide a more vivid indication of the ohmic losses in the capacitor.

The extremely low ohmic losses of vacuum capacitors lead to very high Q values. There are quality values ​​from 1000 to 5000 or even higher still specified in the relevant data sheets. These quality values ​​correspond to ESR values ​​5-20 milliohms. In comparison, NP0 Class 1 ceramic capacitors with a nominal capacity ≥ 50 pF a quality of 500 to ( minimum value according to the applicable standards IEC 60384-8/-21 ). However, it is more useful if the cooling of a system is to be calculated to consider the ESR of the capacitor. The ESR can be used directly by the equation P = I2 · ESR, the heat loss P are calculated at current I.

The ESR of vacuum capacitors is frequency dependent.

Temperature dependence of capacitance

For accurate compliance with a transmission frequency generated by a LC - resonant circuit over a wide temperature range, the characteristics of the components must, for the capacitors is substantially the capacity, have a very low dependency upon the temperature. Vacuum capacitors have this rather low temperature dependence of the capacitance value. In the data sheets of the manufacturers values ​​are called, corresponding to the values ​​N33 to N150 of the class -1 Ceramic Capacitors about. For vacuum capacitors with ceramic housing, temperature coefficient to 50.10 -6 K-1, named for capacitors with glass enclosure to 100.10 -6 K -1.