RELAY

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"What is A Relay"

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

"Working Principle of a Relay"

Each relay has two mechanical parts inside. The first one is the contact(s) of the relay. The contacts operate similarly to the contacts of a simple switch or push button  You should consider the contacts as a pair of metals like the following diagram:

The two terminals operates as a switch. When the contacts are 'in contact' then the current flows from Terminal 1 to Terminal 2. There are two types of contacts: the NO and the NC. NO stands for Normal Open contact, while NC stands for Normal Closed contact.

The Normal Open is a contact like the one showed in the previous illustration. When the contact is still, then no current flows through it (because it is an OPEN circuit). 

On the other hand, a Normal Closed contact allows the current to flow when the contact is still. Bellow i illustrate both of these contacts:

Applications....

Relay Circuit...

Circuit Description

Here are some circuit diagrams for driving relays from a micro-controller  Make sure you’re using a 5-volt relay (this refers to the coil, not the load circuit), and make sure that the relay has a high enough rating for the load that you’re driving.

 

This circuit is necessary if you are using a relay with a coil that needs more power than the microcontroller can supply (this includes most miniature electromechanical relays):

Troubleshooting:

• Make sure that you have the pins correctly labeled on the relay; different relays have different configurations.

• The diode protects the circuit in case the polarity is reversed. The stripe on the diode should be towards the 5v side.

• The base pin on the transistor is usually the center pin; consult documentation to distinguish the collector from the transmitter.

• If the relay is working correctly, you should be able to hear a little ‘click’ when you trigger it (except when using a solid-state relay).

Some relays (such as reed relays and solid-state relays) have coils that can be switched directly from the microcontroller, in which case you can use a less complex circuit:

Finally: if you’re switching an AC device, PLEASE BE CAREFUL. Make sure you’ve got the coil working BEFORE you hook up the AC load. Make sure you have correctly labeled all the pins on the relay – NEVER connect AC voltage directly to the relay coil. Build your AC circuit BEFORE you plug it in. If you’re not sure, ask someone to check it for you first.

About the Author

I am Kashif Mirza, the founder of Student Training Lab (STL). I am working on projects since 2005 before that I just search things and now I am sharing my knowledge through this plateform.

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