A relay is just a simple switch with an electromagnet use to change the state of switch. As other switches come into types like SPST, SPDT, DPDT etc, in the same way relay has types like these switches. Moreover as they have electromagnet/current carrying coil inside with the addition of simple switch that’s why its rating contains two types. One rating shows the voltage at which coil gets fully energized and other rating shows the value of current or voltage that the switch of relay is able to bear.
If something like this is written on the relay:
DC 6V 110VDC / 220VAC 5A
It means its coil get fully energized by giving merely 5V on the terminals of coils and you can apply 220V AC or 110V DC on the terminals of switch and that switch can bear 5 Ampere of load.
1- It is coil of relay which should be energized by applying 5V on the terminals . The silver thing inside number 1 circle is in fact connections going to the terminals and in the perfect relay it is connected to the yellow color coil. Here in this picture those terminals are not present because during opening of relay those terminals got broken.
2- These are six terminals of the switch. Basically these are two circuits so you can apply your two electrical loads at a time here. To check the state of terminals, apply and disconnect 5V to the coil terminals and in both cases check with DMM that which terminals are short.
3- This is the switch having three terminals, when coil gets fully energized the middle terminal get connected with the terminal near to the coil.
This phenomenon is according to the Biot Savart’s law that there is an interaction between current carrying conductor present in a uniform magnetic field. As the switch terminals have current and current carrying coil is electromagnet’ which is responsible for the magnetic field. So, this fulfills the law and two terminals near to the coil short and when you disconnect the 5V on the coil terminals, then two switch terminals away from coil will get connected.
Here, the resistor is for the protection of the controlling device to limit the current. Its value varies according to transistor type. Some transistor want more gate current like 10mA to start the transistor action but some transistors want very minor current like 10uA to start the action. So here 1k is enough with BC548. The diode present here is use for avoid back emf action of coil. It force current to flow in one direction in the coil.
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