Relay is another interesting electronic component, lets see how it looks ...
This big black box, is this a relay ?... but what is this for ??
I know these type of questions jumbling in your mind, but you will really appreciate this device and its importance once you understand its requirement and functionality.
Basically, relays are used as switches. These are different from those manual switches where we push button to turn them on and off. Relays are electromagnetic device, and very useful in scenarios where we want to control a high voltage device through our small low-voltage circuit.
For example, assume that you have designed a small water pump controller which works on 12V DC. When water level on the overhead tank goes low, the controller outputs 5v on the output pin indicating that the pump should be turned on now. Similarly, when the tank is about to overflow, the circuit outputs 0v on output pin indicating the pump to turn off. Now in this case how will you control a 110V AC pump motor? The answer is very simple .. Use relay as show in the diagram below.
Now lets understand the principle of relay and how it works. Relay works on the principle of electromagnetism, look the diagram below
Working of relay is very simple. When small voltage is applied to the copper coil, since this coil is coiled over a iron rod, the iron rod gets magnetized. Due to this magnetism iron-rod attracts the iron-strip above it. As the iron strip is pivoted, and due to magnetic force of iron-rod, right-end of the iron-strip turns toward the iron-rod. Due to this turning right hand side of metal contactors come contact, and this completes the load-line circuit. As long small voltage is supplied to the coil, magnetic strip will inclined, metallic-contractor will in contact, and load-line circuit will be ON.
But, as the small voltage is removed from the coil, the iron-rod will get de-magnetized and will not be able to pull iron-strips. Now the spring force will come into picture which will cause iron strip to move away from iron rod. As a result metallic contactors will also get apart, and this will break the load line circuit (turning load-line circuit OFF ). Relay will also be in off state if 0V is applied, or no connection is made to the coil. Since in both the cases there won't be any magnetism in the iron-rod to pull the strips.