RELAY INTERFACE

The circuit diagram shows the connection of Relay Driver Circuit. When the logic signal from the Buffer O/P is applied to base of the transistor through resistor 1KOhm the starts conducting, it energizes the relay. The transistor act as a small signal amplifier resistor of 1KOhm is used to provide proper emitter base voltage to turn the transistor to ON state from OFF state.

Relay is an electromechanical switch & it works on the principled energizing an electromagnet. It consists of primary coil, 2 contacts one is normally open contact “NO” & the other is normally closed contact “NC”& pole normally identified a common. When relay is in off state the pole is connected to normally closed (NC contact).The load is connected to the relay as shown in above circuit diagram. The load may be a fan or dc motor or heater coil, when transistor starts conducting current starts flowing through the coil. Which develops its own magnetic flux when the strength of current is suitable, a sufficient flux when produced attracts the pole to make contact with normally open position ‘NO’. Hence the load connected to it performs its operation until the contact is broken. A diode connected in parallel across the primary coil is to eliminate the effect of back EMF on the transistor. Relays have great application in industry. Using the principle of energizing an Electromagnet we can handle large voltages & current application. Without the risk of shocks.