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555 Voltage Doubler Relay Driver

2015-03-01 16:04  
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555 Voltage Doubler Relay Driver
These novel relay driver circuits have the ability to pick up a relay with a coil voltage rating equal to double Vcc. After pickup, the relay armature is held via the application of Vcc. Under this condition, the coil power is reduced by the square of the voltage ratio, or to 25%. This is the ultimate in relay economy—the reduction of wasted coil power. In typical relay economy circuits, the coil power may be reduced to 25%, but the series dropping resistor dissipates an equal amount of power. In the voltage doubler relay driver, THERE IS NO SERIES RESISTOR! I believe that these voltage doubler relay driver circuits are new to the world. The versatile 555 performs this interface function well. It is configured as a Schmitt trigger inverter /driver with two outputs: Totem pole and open collector. When output (pin 3) goes low, Q1 turns on and sources Vcc to the positive side of the coil. At the same time, the reset line (pin 7) goes low and drives the cathode of C1 to -12V. Thus the difference between the collector of Q1 and the cathode of C1 is 24V momentarily. After C1 is discharged, D2 conducts the coil current to common thus applying 12V minus one diode drop to the relay coil. When the input to the 555 goes low, pin 3 goes high, pin 7 is turned off and the relay drops. At this time C1 starts to recharge—its charge time constant is 0.47sec so it will be ready to fire again in about 2sec. The input logic threshold may be adjusted down to TTL levels via connecting the suggested voltage divider (R4 & R5) to pin 5 (see schematic). This is recommended for only the CMOS TLC555 device. Otherwise, the standard bipolar 555 should work fine.