





Current The current rating given in the tables of our spec sheets is the maximum current the vacuum capacitor at maximum working voltage capacity can handle continuously under normal convection cooling at an ambient temperature of 25°C. Accurate measurements of RF currents and temperatures at specific frequencies and various capacity settings have been made. In the interests of simplicity, the operating temperature was allowed to vary a few degrees over the capacity range so that a single current rating could be provided at a given frequency. The "RMS Amperes vs. Frequency" Curves, based upon maximum allowable current, are independent of voltage. Operating current for a given working voltage can be determined by Ohm's Law or the nomograph (see Nomograph diagram). In those instances where current ratings deviate from the normal, I(rms) = (.707) (Ew) (2pfC), as in the case of some glass vacuum capacitors, curves for maximum working voltage are provided. The slope of these curves will provide general guidance for determining operating curves for lower voltage/capacity relationships. Pulse RatingsContinuous RF current ratings may be exceeded for short periods if the working voltage rating is not exceeded. This applies particularly to pulse and peaksofmodulation applications. Momentary currents may exceed the catalog continuous current rating by a factor of the square root of the duty cycle, provided the working voltage is not exceeded.
Amplitude Modulation RatingsCapacitors in AM service must be able to withstand peaksofmodulation voltage and current. Current ratings are based on temperature so the heating effects of the modulated currents determine the capacitor requirements. The average output power of an AM transmitter which is 100% sine wave modulated is 1.5 times the unmodulated carrier power. The average modulated carrier current is 1.225 times the unmodulated carrier current, therefore a capacitor current rating of 1.2 times the carrier current will be sufficient even though the peaksofmodulation currents are twice the carrier current. Forced CoolingIf higher current ratings are required, capacitors are available with forcedair cooled bellows to operate safely at 200% of the convection cooled rating. Watercooled capacitors are also available which are normally limited only by voltage. Typical water cooling figures are 3.5 gallons per minute at 15 psig, the high pressure being necessary to prevent steam pockets from forming in the bellows convolutions. Detailed watercooling specifications are available (see your local Jennings representative). On standard convection cooled capacitors current rating may be exceeded for short periods of time, providing the rated temperature rise is not exceeded. Under no conditions should the current exceed 150 % of convection current rating. Fixed capacitors can carry more current because they have shorter RF impedance path. Fixed capacitor current ratings may be increased by forced air cooling up to voltage limitations.

