How to Drive a Relay
Category: Analog Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is designed and simulated in Proteus 7.10. To simulate this circuit you must have Proteus 7.10 installed in your system or you can draw the copy of this circuit. This is a very important circuit and its working is given below. 1. To understand this circuit you should know how to use transistor as a switch (see Transistor Logic category). 2. Relay is a device to switch between the input signals, i.e, to select which input signal should connect to the output. 3. Normally a relay has 5 pins, 2 pins for energizing coil, one pin is common (output), one pin is normally open, called NO (input) and other input pin is normally closed (called NC). 4. By default the common pin of relay is connected with normally close pin. 5. A free wheeling diode across the relay energizing coil is necessory to disharge the electrical energy stored in energizing coil. 6. When the switch is open, the transistor Q1 is OFF, so the green LED glows, because the path of current is completed through normally close pin. 7. When the switch is closed, the transistor Q1 is turned ON, now the relay coil is energized and it became electromagnet, therefore it attracts the common to to connect with normally open pin. Now green LED will turn OFF and yellow LED will turn ON. 8. When the switch is open the relay coil is de-energized through free wheeling diode D1 and green LED will again turn ON and yellow LED will turn OFF. Application: This circuit has many application, some examples are given below. 1. This circuit is used in AVR (Auto Voltage Regulator) of generator for selection between Utility and Generator electrical supply. 2. This circuit is used in automatic voltage stabilizer.