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 25°C 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
· 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.