Final Year Projects: WHAT IS RELAY? RELAY TYPES,WORKING OF RELAYS

RELAY

A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. Relays are used throughout the automobile. Relays which come in assorted sizes, ratings and applications, are used as remote control switches. A typical vehicle can have 20 relays or more. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier.

Operation

When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.

By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an opto coupler can be used which is a light-emitting diode (LED) coupled with a photo transistor.

Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways:

Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact.
Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact.

Change-over, or double-throw, contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact. If this type of contact utilizes “make before break" functionality, then it is called a Form D contact.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts.

The relay's switch connections are usually labeled COM, NC and NO:

COM = Common, always connect to this; it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off.
NO = Normally Open, COM is connected to this when the relay coil is on.
Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.
Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

Relay Variations

Other relay variations include three and five pin relays. A 3-PIN relay instead of two B+ input sources, this relay has one B+ input at pin 1. Current splits inside the relay, supplying power to both the control and load circuits. A 5-PIN relay has a single control circuit, but two separate current paths for the switch: One when the relay is de-energized (OFF - no current through the control coil) and the other the energized (ON - current is flowing through the control coil). When the 5-PIN relay is de-energized (OFF), pins 4 and 5 have continuity. When the relay is energized (ON), pins 3 and 5 have continuity.