Vacuum Capacitors





Air and vacuum variable capacitors for comparison: The air capacitor shown is variable from 34 to 864pF (25:1 capacitance range), and has a plate spacing of 1.6mm giving a voltage rating of 5KV peak (3.5KV RMS).The dimensions of the capacitor frame (excluding protruding studs and mounting brackets) are: 260 × 126 × 135mm. The vacuum capacitor (shown to comparitive scale) also has a voltage rating of 5KV peak, but is variable from 10 to 1000pF (100:1 range). It is 77.5mm in diameter at its widest point, and is 171mm 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 - 1000pF, 3KV. Shorting type.
Max rated RMS current: 40A. Control shaft diameter is 0.25" (6.35mm).



Jennings
UCSXF-2300-7.5S
35 - 2300pF
7.5KV

Control shaft diameter is 0.5"



Jennings
UCSF-250-7.5S
250pF, 7.5KV



EEV U1000/3.



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


Fixed Vacuum Capacitors. Used in medium-power broadcast transmitters (several kilowatts). The top two capacitors are of Russian origin and have Cyrillic markings: πΦ = pF, kB = KV. The Jennings unit (50pF) has silvered electrodes, the others are aluminium.

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 which 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).
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