INDEX

Relay Driving using Transistor

This circuit is used to turn on or turn off relays. Suppose if you use an LDR and a series resistor to turn ON and OFF light in your room, if the intensity of the light become LOW.Now let's discuss about the circuit to turn ON/OFF relays using a microcontroller or a computer.

Here you can see that I am using a 6V,100ohm relay. The circuits given in this book are all tested. The output voltage of my adapter when I put it into 6V is 6.86V. A 6V relay will switch properly only when Vcc>6V. If you use the output of 7805 then you can hear only a small sound or sometimes no
sound meaning that relay is not getting enough magenetisation. So remember it.See the relays troubleshooting section. Now come to the series resistor in the base of the transistor. It can vary from 100ohm to 10Kohm. But 1Kohm is good. If I use a 100ohm resistor in series then the relay won't be switched properly. If you use 10Kohm then also relay will not be switched. If Rb<100ohm then you can see that the adapter light dims(Ic=Ib). You can verify it by connecting and removing the base voltage to Vcc. If it is not switching then your circuit is not working.You can see that Ic=Ib and Ic=hfeIb are the limits.

Here we can see we are using a protection diode,sometimes called free wheeling diode. The purpose of the protection diode is to protect the transistor from the burning of transistor because relay is a coil, i.e an inductor, the property of the inductor is to oppose its cause . So when you switch off the circuit the discharging of the inductor occurs in opposite direction(remember the properties of inductors), so if you use a protection diode, then it will discharge through relay+diode circuit, otherwise transistor got damaged. But if you use power transistor, this problems won't occur.