Capacitive Discharge

The prime concern in capacitive discharge circuits is the ability of the vacuum relay to handle large amounts of surge current stored in the capacitor. Vacuum relays are often used in high voltage circuits to protect personnel by shorting out (bleeding) a capacitive circuit to ground once the high voltage has been removed.

The low resistance contacts of a vacuum relay allow very high peak currents to be handled (up to 200 Amps for up to 50 milliseconds without contact deterioration or welding). Arc contact welding is a function of arc voltage (a constant 18-23 V.), current, and time. Currents of up to 500 Amps have been carried for 10 milliseconds without failure.

Most standard relays will handle discharge pulses of 200 joules. In most applications a series load resistor is used to lengthen capacitor discharge time to reduce peak current carried by the relay. (See Fig. 11)

Some circuits, such as in medical heart defibrillators, involve both capacitance and inductive elements. (See Fig. 12) For these critical applications Jennings has developed a special line of gas filled relays.

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