Week : 15 (fyp sem 2)

Title : Relay

Objective : To connect a relay with automatic spray air freshener

Content / procedure :

What is relay ?

 - A relay is an electro mechanical switch used for driving heavy current loads in electronic circuits. Relay interface the electronic circuits with high voltage AC and there is complete galvanic separation between the two. When current passes through the relay coil, magnetic field develops which attracts a lever and changes the switch contacts. The contacts can be used to connect AC or DC loads which may then turn On / Off the load depending on the contact connections.

- A relay allows one circuit to switch on a second circuit which is completely separate from the first one. For example, a battery operated circuit can operate AC loads through a relay. There is no electrical connection inside the relay between the two circuits so that total galvanic isolation is possible.

Relay contacts 

- Typically a relay has three contacts. A moving contact called Common(Comm) and two fixed contacts. These are Normally Closed (NC) andNormally Open (NO). AC / DC is connected to the Comm. Contact. NO contact is used to connect the circuit / Gadget when the relay triggers. In this case, the circuit/Gadget remains off when the relay is Off. NC contact is used to break the supply when the relay energize. In this case the circuit / gadget remain operating until the relay energize. Contact rating is also important to drive the load. Typical relay contacts are rated for 4 Amps current. If the load is a high current one like Motor, Heaters, Inverters etc, relays having high rated contacts ( 16 Amps or higher) is necessary. These types of relays are called as Industrial type relays. If the contact rating is lesser than the load current, contacts may heat up and fire may break due to sparking.

 

Relay contact symbol

 PCB Relay

-  These are small sized relays that can be directly soldered on the PCB to make the circuit board compact. Like other type s of relays, PCB relays are available with the rating of 5 V, 6 V, 12 V, 24 V etc. Coil ratings may be 100 to 400 Ohms. Pin connection of PCB relay is shown below.

 Result / Analysis : The connection of relay and automatic spray is shown below.