Solidstate Relay



The basic function of a relay is to switch on power to a load using an electrically isolated, low-power control signal. Hitherto, electromechanical relays have been the components of choice to perform this function. The advances made in the semiconductor technology have resulted in the emergence of solidstate relays.

Fig. 1: Solidstate relay
Fig. 1: Solidstate relay

Solidstate relays provide the advantages of almost infinite switching, bounce-free operation, immunity to electromagnetic interference (EMI), higher operating speeds, low-voltage control, small package size and multifunction integration.

Here we describe a DC-operated solidstate relay that uses readily available components (see Fig. 1). It operates off a 3V DC battery or TTL-compatible PWM intput. Pin configurations of MOSFET IRF540, transistor BC547/BC557 and optocoupler are shown in Fig. 2.

Fig. 2: Pin configurations of MOSFET IRF540,transistors BC547/BC557 and MCT2E
Fig. 2: Pin configurations of MOSFET IRF540,transistors BC547/BC557 and MCT2E

The load voltage may be supplied by an automobile or tubular battery of voltage anywhere between 24V and 96V for driving up to 10A current through the load. Here we have used a 24V DC battery. The value of series resistor R6 (330 ohms) will change depending on the battery voltage to supply 30-35mA current. The value of resistor R6 can be calculated as:
R6 = 1000 x (Bat.2 V–12V)/35 and its power dissipation=(35/1000)2 x R = 0.001225R

(Note. The wattage of R6 should be at least twice its power dissipation.)

When S1 is pressed, the optocoupler input is high (logic 1) and the MOSFET triggers to activate the load. When S1 is open, the optocoupler input is low (logic 0) and the MOSFET doesn’t trigger. As a result, the load remains inactivated.


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