Relay is basically an electromechanical component used in Electrical & Electronics fields for use as a switch. This is very analogous to the use o...
Relay is basically an electromechanical component used in Electrical & Electronics fields for use as a switch. This is very analogous to the use of a switch. We use switches daily for various purposes like switching our lights,
TV, Computers & other gadgets on & off. The difference between this relay & our age old switch is that the relay needs power to operate while the switch does not need any power to operate. It is operated by mechanical force applied by us on it.
Our trusted & simple mechanical switch is used mainly where direct presence of human being is there & will be operated by him/her. But there are many circumstances where you have to make switch on & off while you are not there physically. Take for example - you want your pump motor to be operated automatically when the upper tank becomes empty. How you do it? You cannot just sit & watch upper tank all the time & switch on the motor as and when required. So what can be done?
Well! The answer is to do it by a controller. This controller uses a relay to switch motor on & off. So basically the relay comes into picture, when you want to control any electrical or electronics item to on or off automatically as & when certain conditions are met. Now you know what is relay & why we need it.
So now the question is what it is made up of? Basically the relay is electrical product. That is, it uses electricity to make it operate. Just like it's mechanical counterpart it has 2 contacts which make or break as per requirement. If you know there is an electrical phenomenon. When an electrical current is applied to a coil, it becomes a magnet and at the same time if you stop the current the coil becomes normal. That is there is no magnetic effect. So by applying & non applying current you can operate the switch that is, you can make it on or off.
So based on this principle of current, making the coil magnet, the relay's construction is done. A coil is wound around a cylindrical non conducting material. This becomes a magnet when current is passed through this coil (because of this, they are also referred as electromagnetic relays). A small lever along with 2 contacts is placed at one end of the coil, which acts like contacts of a mechanical switch. So when you apply power to the two ends of the coil the coil will operate & the contacts are closed there by passing current from lever point to the one of the contacts. This lever is anchored at a point where it attached with a small spring. This will keep the lever having contact with one of the external contacts. This contact is called Normally Closed (NC) contact. The other contact is called Normally Open (NO) contact. So when power is not applied the middle lever is connected with NC pin. When power is applied to the coil, the middle lever, switches over to NO pin. So in general there are 5 contact pins protruding outside relay body- 2 for coil, 1 for the middle lever which is called as Common Contact, 1 for NC & 1 for NO.
Whole of this construction is put in a plastic case and in some cases whole unit is sealed off. Only 5 contacts are protruding outside to make connections with external circuit. When a power (this generally is low power) is applied to the 2 contacts of coil, the power (generally high power) connected to Common contact is available on NO pin.
Having 5 pins outside is a general relay. As the requirement evolved the relay construction has also evolved. Now a days relays come into many flavors. This general relay is also called Single Pole Double Throw (SPDT). Other varieties include Double Pole Double Throw (DPDT), Cube relay, Power Relay, Reed Relay, Automotive Relay, Solid State Relay etc. All these relays do the same thing - making contacts.
Now let us consider the advantages & disadvantages of this kind of mechanical relays-
Advantages -
1. Relays provide complete electrical isolation there by it is easy to control large amount of power by using low amount of power. That is you can control high voltages & current through use of low voltage & current.
2. Apart from electrical isolation, relay is also able to control many appliances or connections at a time.
Disadvantages -
1. Being a mechanical construction, relay has got some inherent limitations. Because of springing action, it cannot be operated fast. That is you cannot make it on/off more than few tens in a second.
2. Though they come in many varieties, their size becomes a main issue in modern instruments.
3. The mechanical contacts are prone to wear & tear due to sparks generated while switching high power.
4. The coil used in relays need fairly large current, which makes them unusable in some battery operated instruments.
Though there are disadvantages, relays are used in millions.
