One Relay for multiple Supply Voltages

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.

With a closed-loop current controlled PWM circuit, both aspects can be taken into account. As with the open-loop PWM control, the effect of the coil storing the current is utilized. However, the current is now measured and used as a control parameter. The switch (T1) is periodically turned-on by an internal oscillator with a fixed frequency, and charges the coil (L1) up to the externally set turn-off current, switching T1 off. The stored coil current now runs through the free-wheeling diode D1 and slowly decreases until T1 is turned-on again. The coil current average is slightly below the defined turn-off current. The important advantage of this setup is the voltage independent current control. For the adaptation to a particular coil only the turn-on current has to be set. The hold-current occurs simply by internally lowering the turn-on current to a value that is approximately two thirds of its initial value. The duty cycle of the PWM is automatically controlled independent of coil parameters like inductance and resistance, or even the supply voltage. With this type of circuit a 6-V relay can be operated from a wide voltage range (for example, 10 to 45-V).

Figure 2 shows a practical implementation of such a closed-loop controlled PWM circuit with the integrated circuit iC-JE [1]. The turn-on current is defined by RSET at pin ISET and is reduced to two-thirds of its initial value after typically 100ms. Timing and current ratios can be modified within certain limits using a resistor/capacitor network in parallel to RSET. In addition to the current controlled PWM driver, the circuit also includes diagnostic functions. They monitor failures of the coil like connectivity or increased internal resistance, on-chip under-voltage and over-temperature. In case of an error the LED output will flash with typically 2.4 Hz. This output can also be used for an automatic error monitoring, when a controlling microcomputer is available.

The integrated Zener diode in the PWM driver output is activated during turn-off and allows a higher free-wheeling voltage and thus a faster demagnetization of the coil. In the shown circuit the relay is simply controlled by the input VIN.

In summary it can be said, that application with mechanical relays, valves and magnets can be made independent from the operating voltage and benefit from an approx. 50% power consumption reduction, when special drivers are used. At the same time the failure monitoring improves the security and reliability of the applications, as well as its serviceability.

Reference:
[1] www.ichaus.de/prod_frame/iC-JE

Author Info:
Uwe M. Malzahn, Dipl.-Ing., did his Master in Solid State Electronics at the University of Darmstadt in 1991 and subsequently joined iC-Haus as an IC designer for mixed-signal bipolar and CMOS ASICs. Since 2000 he is Applications Engineering Manager for DC/DC converters, optical sensors and laser diode drivers.