Relays are very interesting and useful electronic component. They are a kind of switch, like those we use everyday in our home and offices to turn on and off electrical devices like bulbs, TVs, fans etc. The function of relay is to control the switching of a relatively heavier load which demands high voltages (like 240 V AC mains) or high current, from a relatively low voltage control signal (like 5v or 12v)
In the image above, the area surrounded by dotted rectangle is the high voltage area. As you can see one wire from AC mains wall socket is connected directly to the load (the bulb). The another line goes to the load via relay. When the relay is in off state, this path is broken and the current from mains socket cannot flow to the load, and as a result the bulb remains off.
When a control voltage of 12v is applied to the input of relay, it energizes the relay. As a result, the output circuit is completed and current from AC mains wall socket can flow via the relay to the load and the load is turned on.
From the image above it is clear that a relay should have 4 pins. Two for the input and two for the output. If you are also thinking the same, you are right (almost). The input side is called the coil pins (why? you will come to know latter in this article series).
In contrast to the image shown above, the output side has one more pin called the NC pin. NC means "Normally Closed" It is connected to the common pin (COM) while the relay is in off state. And gets disconnected when the relay is turned on. This behavior is exactly opposite of the function we learned above. That pin which we discussed earlier is called the NO (Normally Open) pin. Because it remains open when the relay is off. Open circuit means the current path is open and no current can flow through it.
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