Relay, Transistor Wiring
A relay is basically an electromagnet that is used to magnetically pull down a spring loaded shaft so that alternate make and break contacts may be established in between the moving shaft and its two fixed poles. In an absence of a coil voltage each of these poles are normally open (N/O) and normally closed (N/C) respectively.
To power a relay or configure it with an electronic circuit, a small output circuit is generally incorporated and is known as the relay driver circuit.
As shown in the diagram, the section basically consists of a transistor T1, resistor R1 and a flyback diode D1 connected across the relay coil.
Resistor R1 is used to bias the transistor and this biasing voltage is in fact the triggering voltage, which is generally received from a source such as an IC.
As soon as the transistor receives the trigger voltage, it instantly conducts and activates the relay. This happens because the end of the relay which is connected to the transistor is pulled to the ground potential, so that the entire supply voltage passed through the coil to energize it.
Every inductor has an inherent property of switching back an opposite EMF equal to the magnitude of the supply voltage, the moment it’s switched OFF. This is what exactly happens when the transistor is switched OFF. The relay coil instantly kicks a dangerous EMF back into the transistor. This may cause permanent damage to the transistor unless and until some precautionary measure is adopted. Diode D1 performs the important function of neutralizing this EMF by short circuiting it and avoiding its passage through the transistor.