One Relay for Multiple Supply Voltages

Mechanical relays continue to enjoy a growing demand and so far can't be replaced by solid state relays (SSR). Compared to SSRs, the lower cost, contact resistance and the galvanic isolation of mechanical relays is a convincing design for most applications. However, relay drivers, can profit from the use of dedicated semiconductor ICs. With a special relay driver two significant disadvantages of mechanical relays can be eliminated. In the turn-on phase a much higher current is needed to overcome the air gap of the magnetic loop, than during hold phase when the loop is closed. Without a dedicated driver, relays are usually driven with the turn-on current -- quite a waste of power. Furthermore, in conventional unregulated applications, the coil has to be designed for the given operating voltage. This, together with the resistance of the coil, defines the coil current. It always requires specially designed relay coils for different supply voltages.

Possible solutions range from simple transistor circuits with time constants defined by resistor/capacitor networks (allowing higher current to be supplied during the turn-on phase), to integrated solutions with pulse width modulation (PWM). Up to now, integrated solutions mainly offer an open-loop PWM control (see Figure 1). To reduce the initial turn-on current to a lower hold current is easily achieved by simply changing the duty cycle to a smaller value. The actual current though still depends on the operating voltage. This means that both, the coil and the PWM control (frequency and duty cycle), have to be designed for a specific operating voltage. Although, in some integrated solutions an attempt is made to control to track the supply voltage with the PWM.