@▷ 555 Voltage Doubler Relay Driver | Diagram for Schematic

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.

Relay driver circuits that I have previously posted

Flagship 555 Voltage Doubler Relay Driver

555 Voltage Doubler Relay Driver

555 Voltage Doubler Relay Driver Circuit Schematic

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.

Voltage doubler driver via high side contact

High Side Voltage Doubler Relay Driver

High Side Voltage Doubler Relay Driver Circuit Schematic

This version is controlled by a high side switch contact. The switch applies Vcc directly to the relay coil. At the same time, it turns Q1 on which in turn drives the negative side of the coil to -12V.

Voltage doubler driver via low side contact

Low Side Voltage Doubler Relay Driver

Low Side Voltage Doubler Relay Driver Circuit Schematic

This version is controlled by a low side switch contact or perhaps a bipolar transistor or even a 2N7000 MOSFET. The switch (or transistor) grounds the negative side of the relay coil. At the same time, it turns on PNP transistor Q1 which drives the positive side of the coil to +24V via the charge stored in C1.

Advantages

  • Substantial reduction in relay coil power with no power wasted in a series resistor
  • Reliable operation under power supply brownout conditions (V coil = Vcc)
  • 20% (observed) faster operate time (V coil = Vcc)

Disadvantages

  • Relay must remain picked up reliably at half voltage—generally not a problem—relay must be tested
  • Relay cannot operate immediately upon power up—2sec delay required
  • Cycle rate is limited—capacitor must recharge after relay is dropped before subsequent pickup

Relay Driver Oscillograph

Voltage Doubler Relay Oscillograph

Voltage Doubler Relay Driver Photos

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