To control ON/OFF operation of ac devices we can use electromagnetic relay. This relay has both normally open and closed contacts. When a current is passed through the coil of the relay, the switch arm is pulled down, opening the top contact and closing the bottom contact, as shown in the Fig. 15.15 Electromagnetic Relays in Interfacing.
The relay contacts are rated for maximum current of about 20A – 25A. These relays are also called mechanical relays because mechanical contact makes the circuit to ON or OFF. The mechanical relays having higher current ratings are sometimes called Contactors.
The mechanical relays or contactors, however, have several serious problems. When the contacts are opened and closed, arcing takes place between the contact, which causes the contacts to oxidize and pit. As the contacts are oxidized, they become higher resistance contant and may get hot enough to melt. Another disadvantage of mechanical relays is that when they switch ON or OFF at high-voltage point, they produce large amount of electrical noise, called Electromagnetic Interference (EMI).
To control 220V or 440V ac devices it is not desirable to have a common power circuit ground and control circuit ground. This is very important because if the 220V ac line gets shorted to the VCC line of a microcomputer, it usually bakes most of the microcomputer ICs. To avoid this, the control circuit and power circuit is electrically isolated by separating their individual ground lines.