The Effect of Voltage On Vacuum Capacitors

Two voltage ratings are given in our product spec sheets: Peak Test Voltage and RF Working Voltage. Ratings at 60 Hz are applicable at 50 Hz.

Peak Test Voltage is the maximum 60 Hz voltage that can be applied to the capacitor without breakdown occurring, as indicated by either internal or external arc-over. Capacitors are tested at this voltage as a means of determining the general condition of the capacitor. Such a test is frequently used by customers in incoming inspection to check for damage in transit.

RF Working Voltage is the maximum peak RF voltage that can be applied continuously to the capacitor without affecting its ability to withstand instantaneous overloads. It is generally 60% of the Peak Test Voltage rating. The difference between the 60 Hz and the RF working voltage values is the recommended operating safety factor.

For variable capacitors, the voltage rating is essentially constant from maximum capacity to a point near minimum capacity, where it increases significantly. Within the normal accuracy of instrumentation (3%), voltage ratings should not be exceeded. Jennings RF testing facilities monitor the above characteristics on a production basis; and, in addition, can do special application testing when required.

DC—Vacuum capacitors should not be operated in DC applications above the peak RF working voltage.

DC plus RF—For DC plus RF applications, the sum of the DC plus the peak RF voltage should not exceed the peak RF working voltage.

Capacitors for DC plus RF applications are tested for dc emission on a dielectric strength tester. To meet Jennings Quality Assurance standards, the DC emission current must not exceed ten microamps at the rated working voltage.

Amplitude Modulation—The peak output power of an AM transmitter which is 100% sine wave modulated is 4.0 times the unmodulated carrier power. The peak RF voltage will be twice that of the unmodulated carrier and the capacitor should have a RF working voltage rating equal to or greater than this voltage.

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