Handling Guidelines
Fixed and Variable Vacuum Capacitors

Properly handled and applied, vacuum capacitors manufactured by Jennings Technology will be highly reliable, long life components. The external appearance of these units, especially those packaged in a ceramic envelope suggest a very robust component. It is indeed a sturdy assembly, but one that is at the same time quite unforgiving if handled incorrectly.

Figure 1 illustrates the construction of a typical Jennings variable vacuum capacitor. Two sets of concentric cylinders, one on a sliding shaft, the other fixed, are enclosed in an evacuated ceramic or glass envelope with OFHC copper seals located at both ends. A flexible metal bellows, attached to a sleeve type bearing, maintains the vacuum while allowing the capacitance to be varied.

The linear sliding motion required to vary capacitance is converted to rotary tuning via a thread shaft; in many capacitors direct pull tuning is an alternative.

Internal breakdown voltage is primarily determined by the spacing of the opposing plates.

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

As shown in the illustration a vacuum capacitor is a complex assembly of constituent parts. It is manufactured with extremely tight alignment requirements between a series of concentric fixed and variable plates, bellows and bearing surfaces. Moreover due to the brazing process utilized to join the glass or ceramic, metal and copper, the copper plates become annealed and very soft. The spacing between these concentric plates is often very tight and on some units as close as 0.010 inch. On most of the units there is an evacuation port also known as a tabulation. Damage to the tabulation can cause the units to loose vacuum.

All of these capacitors have to be handled with care. The glass envelope units are more susceptible to breakage and loss of vacuum if dropped or banged. Even the rugged looking ceramic units can sustain permanent damage if dented, dropped, or even banged slightly. A blow in the wrong direction, can cause a shorting of the internal plates, which is non-repairable. Yet when properly installed in equipment, the units are highly reliable and have extremely long life.


Handle carefully-Do not bang, Drop, or Dent

Leave the units in their original shipping containers until they are ready to use. All units are individually packaged to protect against damage.

Do not put the capacitors in bins where they can rub against each other. This is especially important with the glass envelope units. They can easily damage the tubulation or crack the units.

Do not remove any protective sleeve on gas filled units. These units are at very high pressure and the sleeve will help prevent injury if the unit is dropped.

All variable capacitors are shipped in the C minimum position. Rotate the unit counter clockwise to take the unit towards C max position.

Do not alter the capacitor housing or turning screw (shaft). Any machining of the capacitor assembly can cause an internal failure.

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