| 摘要 |
The coil of a relay is activated by a DC voltage directly through a primary switch, which voltage is also applied through a diode to charge a capacitor. A transistor biased by the same DC voltage is connected across the charging diode, but poled for conduction in the opposite direction. When the power is removed from the relay coil and the charging diode, the transistor is forward biased and conducts the charge from the capacitor through the relay to keep the relay energized until such time as the capacitor charge equals the transistor bias; then the transistor cuts off and the relay opens immediately. Relay energization is independent of the transistor; if the transistor fails open, the relay is disenergized instantaneously; if the transistor fails shorted, the relay is deenergized when the capacitor voltage drops below the dropout voltage of the relay; therefore, the circuit is fail safe. Relay dropout as function of transistor cutoff causes the timing to be very accurate; the capacitor charging voltage being the same as the bias supply voltage for the transistor causes the accurate timing to be independent of variations in supply voltage.
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