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Components & Materials

Vacuum Capacitors

Air and vacuum variable capacitors for comparison: The air capacitor shown is variable from 34 to 864 pF (25:1 capacitance range), and has a plate spacing of 1.6 mm giving a voltage rating of 5 kV peak (3.5 kV RMS). The dimensions of the capacitor frame (excluding protruding studs and mounting brackets) are: 260 × 126 × 135 mm. The vacuum capacitor (shown to comparitive scale) also has a voltage rating of 5 kV peak, but is variable from 10 to 1000 pF (100:1 range). It is 77.5 mm in diameter at its widest point, and is 171 mm long excluding the control shaft. Notwithstanding its advantages in terms of dimensions and variation range, the vacuum capacitor can be expected to have an ESR considerably smaller than that of the air capacitor, and being more compact has a much smaller inductance.

Jennings CMV1-1000. 8 - 1000 pF, 3 kV. Shorting type.
Max rated RMS current: 40 A. Control shaft diameter is 0.25" (6.35 mm).

35 - 2300 pF
7.5 kV

Control shaft diameter is 0.5"

250 pF, 7.5 kV

EEV U1000/3.

Russian Vacuum Capacitor. 10 - 1200 pF, 4 kV.
Control shaft diameter is 12mm.

Fixed Vacuum Capacitors. Used in medium-power broadcast transmitters (several kilowatts). The bottom two capacitors are of Russian origin and have Cyrillic markings: πΦ = pF, kB = kV. The Jennings unit (50 pF) has silvered electrodes, the others are aluminium. The Jennings capacitor uses pale-green-coloured uranium glass for the glass-to-metal seals (see: en.wikipedia.org/wiki/Glass-to-metal_seal).

Death of a Vacuum Capacitor

In capacitors with clear glass envelopes and copper electrodes, air ingress is indicated by slow discolouration of the copper. Such discolouration however will take some months to appear after the glass has cracked, and you should beware of purchasing units that have been badly handled or poorly protected during transit. An air-filled capacitor at atmospheric pressure (760 Torr, 1.01 Bar) has a voltage rating about 13 times lower than that of an otherwise identical vacuum capacitor (at 10-7 Torr).

Text and Photographs © D W Knight 2007, 2013.
David Knight asserts the right to be recognised as the author of this work.

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