Relay as we know is an electromagnetic component. They operate by applying current to the coil ends of the relays. This power can be AC or DC. That is relays can be operated by Alternating Current (AC) or Direct Current (DC). The voltages can be 5,6,12 or 24 and 110 as far as DC is concerned while it can be operated through AC voltages also. AC voltages can be 6,12,24,48,110 & 230 Volts.
AC operated relays are used where high power is to be controlled while DC operated relays are used where relatively low power is to be controlled. They are generally bulkier then their DC counterpart. Due to low efficiency of the coil used in AC relays, it is usual to heat up the coil more than DC operated relays.
These AC relays have to be operated above their minimum pickup voltage otherwise humming occurs which is very annoying. Care has to be taken to maintain voltage level if more relays are to be operated at a time. This is because of coil's inherent property of consuming more current ( called inrush current) at the time of starting. Though this stabilizes once relay is on.
AC relay are generally controlled through mechanical switches or are controlled through small DC relays. That is, only the AC power applied to coil is controlled through smaller DC relays. So the scenario becomes like this. An electronic circuit controls a small DC relay. This DC relay switches on & off the small AC power required for the coil of AC powered bigger relay & this AC relay in turn controls very high power.
DC powered relay is operated by supplying DC current to the coil. These current & voltages are relatively smaller than AC powered relays. Their size is also smaller as they consume less power. DC relays are used mostly in instruments & gadgets having small sizes. Here also the power applied to the relay has to be above minimum operating value, otherwise chattering occurs. This affects operating of the instruments. To reduce chattering near minimum operating value, a small capacitor is connected adjacent to coil. A diode is also placed near to coil for removing back EMF ( ElectroMotive Force) generated while cutting off power to the coil. This prevents the driving transistor ( an electronic component which acts as an electronic switch to operate the DC relay) from destroying itself.
As referred above the DC relays are operated by an electronic component called Transistor. This is a kind of electronic switch, which is in turn controlled by an electronic circuit. The circuit decides when to operate the relay depending upon the defined criteria. The circuit gathers data through various inputs & sensors and then according to conditions & requirements controls relay or group of relays.
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