Low power relay control circuit

In battery-powered circuits or constant-current powered circuits, sudden current surges that operate relays can cause the supply voltage to drop. This is the result of internal resistance or current limiting effects. This problem can be overcome with the circuit shown in Figure 1, which draws a constant 1mA from the power supply under various conditions.

The circuit shown in Figure 1 controls three Teledyne RF latching relays 72212. The supply voltage of the circuit is 15V. However, the relay coil is rated for 12V operation. Current flowing through the 'a' coil resets the relay contacts to their position. The current flowing through the 'b' coil causes the relay contacts to switch to their operating position. Although the contacts switch approximately 2ms, the coil operating current must be present for at least 6ms. The reason for extending the operating pulse is to prevent false releases caused by contact bounce. Once operating, 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 supply voltage to 12V and provides a current sink when the coil is not energized. Diode D2 limits the current to 1mA and diode D3 prevents reverse current from the capacitor when the 15V supply is turned off.

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

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

The circuit shown in Figure 1 has characteristics that limit the operating rate of the relay. It takes 600ms to charge capacitor C1 from a 1mA constant current power supply to 12V. After every 6ms of discharge, it takes about 150ms to be fully charged again, which can be used for subsequent relay operation. If a shorter period is required between each relay operation, a larger constant current D2 diode can be used instead.

This circuit consumes a constant current even when the relay is not operating. Removing Zener diode D1 can overcome this problem, but it will increase the relay supply voltage. This is not a problem with the Teledyne 722-12 as the maximum coil operating voltage is 16V.

To reduce cost, the constant current diode D2 can be replaced with a resistor. The relay coil operation will cause current to come from capacitor C1, but will also draw current from the 15V supply when discharging. The current from the 15V supply will rise exponentially (due to the exponential fall in current from the capacitor). Adding a capacitor close to the anode 15V supply of D3 minimizes the impact on the 15V supply. To minimize the impact on the power supply, the resistor should be chosen to have a high value (greater than 1kΩ), but if the subsequent periods between relay operations are short, this value must be chosen to be low.