Mechanical Relays - A Guide on How a Relay Works

A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts.  Relays are found hidden in all sorts of devices.  In fact, some of the first computers ever built used relays to implement Boolean gates.

What Makes Up A Relay?

• Electromagnet
• Armature (that can be attracted by the electromagnet)
• Spring
• Set of Electrical contacts

A relay consists of a coil of wire wrapped around an iron core.  Then electricity is applied to the coil of wire, it becomes magnetic, hence the term electromagnetic.  Another type of relay called a solenoid.  The solenoid electromagnet consists of wire wrapped around a tube containing an iron cylinder called a "plunger"  Then electricity is supplied to the wire coil, the "plunger" moves through the tube and activates a switch.

A relay consists of two separate and completely independent circuits.  The first drives the electromagnet.  In this circuit, a switch can control power to the electromagnet.  When the switch is on, the electromagnet is on and attracts the armature.  The armature acts as a switch in the second circuit.  When the electromagnet is energized, the armature completes the second circuit (which will electrify a light bulb at the end of the circuit).  When the electromagnet is not energized, the spring pulls the armature away and the circuit is not complete.  As a result, the light bulb at the end of the second circuit is not electrified.

The following variables must be considered when purchasing or designing-in relays.

• The voltage and current that is needed to activate the armature
• The maximum voltage and current that can run through the armature and the armature contacts.
• The number of armatures (typically one or two)
• The number of contacts for the armature (generally one or two)
• Whether the contacts (if only one contact is provided) is normally open (NO) or normally closed (NC)

In general, relays are used in cases where a small amount of power in the electromagnet to move an armature that is able to switch a much larger amount of power.  For example, you might want the electromagnet to energize using 5 volts and 50 milliamps while the armature can support 120V AC at 2 amps.  In places where a large amount of power needs to be switched, relays are often cascaded.  In this case, a small relay switches the power needed to drive a much larger relay.  The second relay switches the power to drive the load.

Relays are commonplace in every home kitchen appliance to turn on and off a motor or a light.  Relays are also common in vehicles where the 12V supply voltage needs a large amount of current. 

Contact your Competitive Components sales representative today and they can help you with your relay design needs.  Competitive Components is an authorized distributor doe Picker Components.  Picker Components offers general purpose power relays, Power PC Relays, Signal Relays, Solid State Relays, and Reed Relays.