Introduction to Relays.

A relay is a switching device used to control the path of one or more circuits.

Whereas a person must manually turn a switch on or off, a relay switches a contact on or off with a magnetic solenoid. This means an electrical circuit can change the path of other electrical circuits.

There are many types of relays with different contact arrangements and power ratings.

There are also many types of coil ratings.

The circuit used to energize the coil must match the coil.
1)DC Volts
2)AC Volts and HZ (Hertz)

The contact rating must be large enough to handle the load current and it must be rated for the voltage.

The same contact ratings will be different for a Resistive Load than for an Inductive load.

A Resistive Load will have the same rating for Make, Break and Continuous amps.

An Inductive Load will have a different rating for each Make, Break and Continuous amps.

A relay with a contact rating of 30 amps (up to 240VAC [Resistive]) will have a different rating when switching an Inductive circuit.

It will have a continuous rating of only 10 amps; the MAKE rating @120VAC will be 60 amps, and @240VAC it will be 30 amps, however, the BREAK rating is only 6 amps @120VAC and 3 amps @240VAC.

When a circuit is applied to a winding, and that winding creates a magnetic field, induction occurs.

When the circuit is broken the magnetic field collapses.

This generates a voltage spike and creates an arc at the breaking point of the circuit's path.


To the right is an illustrated animation showing the same relay circuit in two ways.

The top half shows a relay with a magnetic solenoid placed below the contact switch just as a real relay.

The bottom half shows the same circuit in the schematic form.

In a schematic diagram a single component such as a relay may be at many different locations throughout a schematic diagram.

The label identifies the component; in the bottom half of this schematic CR1 is the label for the relay.

CR1 stands for control relay one.

This circuit will latch and form a momentary normally open push button (Start) and will unlatch from a momentary normally closed push button (Stop).

The relay is used to create a latch by diverting the path of the circuit through the normally open contact of the relay directly to the coil. When the coil of the relay is energized the normally open contact will close and provide a maintained path to the coil.

Until the stop button breaks the path to the coil the coil will remain energized.

Lights are connected to each relay contact to help show how the relay contacts can divert the path of the circuit.