Jennings Vacuum Capacitors

A vacuum capacitor is a capacitor which uses vacuum as dielectric instead of air or other insulating materials. The vacuum dielectric allows a higher voltage rating that an air dielectric.

Jennings Vacuum capacitors

A typical Jennings vacuum capacitor consists of two sets of concentric cylinder plates, one adjustable and the other fixed, are enclosed in an evacuated ceramic envelope with OFHC copper seals at both ends. A flexible metal bellows, attached to a sleeve-type bearing, maintains vacuum while allowing capacitance to vary.

The linear sliding motion required to vary capacitance is converted to rotary tuning via an adjustment screw; in many capacitors, direct pull tuning is an alternative. Internal breakdown voltage is primarily determined by the spacing of the opposing plates and a high vacuum level.

The following are general specifications pertaining to Jennings vacuum capacitors. Current ratings are for normal convection cooling in ambient temperature of 25 °C unless otherwise specified.

Maximum allowable operating temperature — 125 °C (257 °F) for ceramic capacitor
Cooling — natural convection unless otherwise specified
Mounting position — any
Rotation to increase capacity — counterclockwise

If none of our standard catalog models meet your needs, our engineers will work with you to design a custom solution to meet your specific needs.

Features

High voltage rating
High current rating
Wide tuning ranges
Low losses
Self-healing
High altitude operation

Fixed vacuum capacitors

Fixed vacuum capacitors are available with capacitances of 25 to 2,000 pF, withstanding working voltages at 50/60 Hz in the range of 4.5 to 33 kV.

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Variable Vacuum Capacitors

Variable vacuum capacitors are available with capacitances of 30 to 4,000 pF, withstanding voltages at 50/60 Hz in the range of 1.8 to 36 kV.

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Overview

Current/voltage

Maximum operating current for vacuum capacitors is limited by temperature rise and working voltage. At lower frequencies, a capacitor is a current-limiting device as a result of its capacitive reactance. At some frequencies, the internal generation of heat exceeds the device’s heat-sinking capabilities, and its current carrying capacity is limited by thermal considerations. A current vs. frequency chart is provided for each capacitor listed. Peak voltage is limited by mechanical design of the capacitor. It does not vary with frequency. Two voltage ratings are provided in our product specifications: AC test voltage and working voltage. Temperature Jennings Technology vacuum capacitors are rated for a maximum operating temperature of 125 °C (257 °F) with normal convection cooling at an ambient temperature of 25 °C (72 °F).

Capacitance

Fixed capacitors with a nominal capacitance above 50pF shall be within ±5%. Capacitors with a nominal capacitance of 50pF or less shall be within ±10%, or 0.5pF, whichever is greater. For variable capacitors, the low end will be equal to or less than minimum rating. The capacitance change is substantially uniform with rotation, and there are no capacitance reversals. Capacitance is within ±10% of the nominal value of the curves shown (capacity vs. turns), in the linear portion of this curve.

Torque/direct pull

In variable capacitors, the linear sliding motion of the moving electrode assembly is converted to rotary tuning via a threaded shaft. The torque values given in the tables are the maximum torque needed to reach minimum capacitance when rotated with a standard lead screw; the torque required to tune away from minimum may be less than half this value. For most variable capacitors, direct pull tuning is an available option to rotary tuning. Maximum required direct pull force values are also given in the tables. Capacitance range end-stops are built into every variable capacitor. It is recommended that the user install their own external stops to prevent damage from gear-reduction drives.

Mechanical life

The mechanical life of variable capacitors is related to length of stroke, speed of operation, bellows material and total number of cycles. Extensive mechanical life tests have been run, operating units for complete cycles from maximum to minimum and back to maximum capacity covering 95% of the full stroke of the movable plates. Capacitors with a large bellows and a short stroke will have the greatest life expectancy under cycling operation. Our most recent variable capacitor models are rated for >2 million cycles, ideal for the semiconductor processing industry.

Testing standards

Factory
All capacitors are tested for dielectric strength on a 100% basis prior to shipment. Upon customer request, certified test reports will be made available. Dielectric strength is tested using a low current, high potential source at 60 Hz voltage. Capacitors for applications involving applied DC voltage should be tested on a DC dielectric strength meter for voltage and emission current. Jennings will test capacitors to this measure if specified by the customer.

User
Most users will find the 60 Hz dielectric strength test adequate and relatively inexpensive. Jennings does not recommend DC testing being performed by the user because of safety considerations. If DC testing is performed, care should be taken not to exceed 60% of the peak test voltage rating of the capacitor.

Jennings Vacuum Capacitors