Nowadays Relays are generally controlled through electronic circuits. Electronic circuits consist of many electronic components like Integrated Circui...
Nowadays Relays are generally controlled through electronic circuits. Electronic circuits consist of many electronic components like Integrated Circuits (ICs),
Transistors, Capacitors,Resistors, Microswitches, Sensors etc. These all components are assembled on a board generally called as Printed Circuit Board (PCBs). This PCB is specially designed board having thin copper layer on one side or both side of it. This copper layer is etched to make conducting lines to connect different components placed & soldered on PCB.
We know that relay is controlled by applying power to the two ends of coil. This can be AC power in case of AC relay while it should be DC in case of DC relay.
Now let us see how AC relay is controlled. If it is manually controlled, AC power is applied through mechanical switch. So by making mechanical switch on & off the relay can be made on & off.
The other way in automatic controlling system is to control this AC power applied to coil of AC relay by a small DC relay. In this kind of system, the AC power also supplied energy to the small circuit. Here the AC power (in some countries it is 230 Volts while others are having 110 Volts power grid) is first converted to small quantity like 24 or 12 volts with the help of a electrical component called Transformer. This reduced power is converted to DC power by using 4 diodes. This is called Full Wave rectification. This power is supplied to a component called Electrolytic Capacitor which acts like a tank, storing the power. This is then supplied to circuit as & when required.
This rectified DC power supply is then used to power the ICs in a circuit. IC itself is a kind of circuit board built on a very small scale. They are as small as a head of a small office pin. This is placed in a small plastic case, having small pins brought out for connections. This IC is connected with microswitches & sensors. Depending upon the condition of these switches & inputs from sensors the IC decides to make relay on or off.
A pin from IC, which is often called Output pin is connected to the pin of a Transistor (referred as Base) to switch it on or off. This transistor is in turn connected to Relay. One pin of the coil of the relay is connected to positive DC power supply and other pin is connected to a pin of transistor (referred as Collector). The other pin of transistor (referred as Emitter) is connected to Negative of power supply. So when the output pin of the IC is made High (the level of voltage is equal to DC positive power supply), transistor is switched ON and this in turn will switch on the relay.
As we turn on & off the relay the Back EMF (ElectroMotive Force) is generated. This is dangerous for transistor. It can destroy the transistor if not properly taken care off. A diode is connected in reverse order in parallel way to the coil to pass this Back EMF back to the power supply, thereby saving transistor from destruction.
Though transistors are used to operate a single or two relays, it becomes cumbersome to operate more relays like 8 or 16 relays. For this also ICs having inbuilt Transistors & reverse Diodes are used. This saves much more precious space on PCB & helps to reduce overall volume of Instruments. Also saves cost due to smaller PCBs.
Nowadays as the cost of MCUs (special ICs in a smaller footprint just like chips, that our computers & laptops are made up of) are considerably coming down, lots & lots of instruments & gadgets using relays are controlled by them. But the basic controlling operation remains same as explained above.
