Low power relay control circuit

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Low power relay control circuit [Copy link]

In battery-powered circuits or constant-current powered circuits, the sudden current surge that operates the relay can cause the power supply voltage to drop. This is the result of internal resistance or current limiting. The circuit shown in Figure 1 overcomes this problem by drawing a constant 1mA current from the power supply under various conditions.

The circuit shown in Figure 1 controls three Teledyne RF latching relays 72212. The supply voltage for the circuit is 15V. However, the relay coils are rated for 12V operation. Current flowing through the 'a' coil resets the relay contacts to their set position. Current flowing through the 'b' coil causes the relay contacts to switch to their operated position. Although the contacts switch for approximately 2ms, the coil operating current must be present for at least 6ms. The reason for the extended operating pulse is to prevent false releases caused by contact bounce. Once operated, the contacts remain in the position determined by the magnetic field of the internal magnet.

The 12V power supply for the relay coil is generated from the 15V power supply. The current flows through diode D3 and constant current diode D2 to capacitor C1 and Zener diode D1. Capacitor C1 holds the charge required to energize the relay coil. Diode D1 limits the relay power supply voltage to 12V and provides a current sink when there is no coil energization. Diode D2 limits the current to 1mA, and diode D3 prevents reverse current from the capacitor when the 15V power supply is turned off.

Applying a logic 1 condition (for 6ms) to the input pin of IC1 causes the relay coil to be energized. IC1 contains the Darlington driver transistor and also contains a diode that conducts the back EMF current to the 12V supply. A logic 1 condition at pin 1 of IC1 turns on the Darlington transistor, energizing coil 'a' of relay RLA. Similarly, a logic 1 condition at pin 2 controls the current flowing through coil 'b' of relay RLA. The relay energization table (see the table at the lower right of Figure 1) shows this relationship.

When a logic 1 condition is applied to one of the inputs of relay driver IC1, the Darlington transistor turns on and drives a current through the corresponding relay coil. This current (approximately 24mA) comes from capacitor C1. After the Darlington transistor turns off, the capacitor is recharged for the next relay switching operation. During these operations, the supply current remains constant (1mA) and the supply voltage does not change during the switching transition. Note that the relay coil is powered with 12V and drops to about 9V after 6ms due to the discharge of C1.

The circuit shown in Figure 1 has the characteristic of limiting the relay operation rate. It takes 600ms for capacitor C1 to charge to 12V from a 1mA constant current source. After discharging every 6ms, it takes about 150ms to recharge fully and be used for subsequent relay operation. If a shorter period is required between each relay operation, a larger constant current diode D2 can be used instead.

This circuit consumes a constant current even when the relay is not operating. Removing the Zener diode D1 overcomes this problem, but causes the relay supply voltage to rise. This is not a problem for the Teledyne 722-12, since the maximum coil operating voltage is 16V.

To reduce cost, a resistor can be used instead of the constant current diode D2. The relay coil operation will cause current from capacitor C1, but will also draw current from the 15V supply when it is discharged. The current from the 15V supply will rise exponentially (since the current from the capacitor falls exponentially). Adding a capacitor to the 15V supply close to the anode of D3 minimizes the effect on the 15V supply. To minimize the effect on the supply, the resistor should be chosen to be high (greater than 1kΩ), but if the period between subsequent relay operations is short, this value must be chosen to be low.