30-08-2013, 01:09 PM
Switches & Relays
Switches & Relays .ppt (Size: 1.31 MB / Downloads: 55)
What is a Switch?
A switch is a device that allows you to stop the flow of current entirely. These are usually mechanical devices that separates two bits of metal (contacts). When the metal doesn't touch, current doesn't flow. When the metal touches, is called a closed circuit. When the metal doesn't touch, is called an open circuit. (closed = ON, open = OFF)
SWITCH TYPES
Single Pole, Single Throw = SPST
Single Pole, Double Throw = SPDT
Double Pole, Single Throw = DPST
Double Pole, Double Throw = DPDT
Push Button Switches (NO/NC)
Single Pole, Double Throw = SPDT
This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position.
What is a Relay?
A Relay is an electromechanical switch, operated by passing current through a coil of wire wound around a steel core, which acts as an electromagnet, pulling the switch contact down to make or break a circuit.
RELAY OPERATION
Electrically controlled switch
Current through coil
Magnetises soft iron core
Soft iron armature moves
Contacts join to closed
Why are they used?
Relays are essential for automation systems and for controlling loads.
Relays are the best way for galvanic insulation between high and low voltage portions of a circuit.