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Basic Operation on How Relays Work

By Donald Kerr
Basic Operation on How Relays Work

The primary use of a relay is to create versatility in wiring designs using many different combinations to create large load operations with small voltage control.

A Relay allows you to:

  • Use one voltage to operate the coil and another voltage to operate the load (machinery)
  • To create automation using one machine to activate a second machine or drop it out.
  • To create large load operations with small voltage control.
  • You can control for example a three phase load by using a single phase control.


A - Electromagnet
B - Spring to retract the contact when the magnet is not energized
C & D - Controlled circuit, on when the magnet is energized and off when the magnet is not energized.
E - Power to energize the electromagnet

NOTE: C & D connections are completely isolated from the E contacts.

Relays are rated according to:

  • Control trigger: voltage and current (for the electromagnet)
  • Load Circuit rating: voltage, current max. on the load contacts.
  • Number of contacts: In the diagram there is only one contact. In real life you can get relays, that can switch several independent contacts at the same time, and some ever have contacts in the off position as well as the on position, so the load circuit route in such a case would have a common connection , and would connect to a second connector in the off position only and connect to a third connector in the on position only.

Applying Power

In the diagram if you apply dc power to the E contacts , a magnetic field is created at A, this magnetic field is strong enough to overcome the strength of the spring B, and attracts and pulls down the flexible armature connected to C towards the electromagnet and thereby closing the contact which connects the circuit between C and D. When power is removed from E (supplying power to the electromagnet ), it collapses the magnetic field on the electromagnet A, and the spring B will pull the armature back disconnecting the contacts and breaking the circuit between C & D.

The following diagram is also a basic relay but configured in such a way that momentary contact buttons can be used to engage or disengage the relay.

Latching Relays are also referred to as "Holding Circuits"

F - The trigger power source.

G - A push to disconnect button, normally is always on unless actually pushed, will reconnect when you let go of the button

H - A push to connect switch, only connects the circuit when pressed , when you let go of the button , the connection is broken.

Look on the first diagram on this page for the remaining legend references.

Operation of Relay

Pushing the button at H will energize the electromagnet, close the load contacts in the relay, then will stay latched after the H button is released because the load contact has now provided another route to complete the trigger power for the electromagnet. To unlatch , push the G button which will cut the trigger power and unlatch the relay. These types of relay wiring configuration would have multi sets of contacts within them , and only one set of latching contacts would be used for the latching circuit , and another isolated set of load contacts would be the actual relay switch for an independent circuit.

To wire a 2 button on / off switch for such a latching relay, for would need a 3 wire cable that is rated to handle the type of voltage/current needed for the trigger voltage only and NOT the load voltage / current that relay is controlling.

Alternative Type Relays

A self locking relay that takes power to change from the open to closed or closed to open but no coil power is required to keep it in a certain position.

There is also electronic type relays that is packaged in the form of an IC chip that is mounted to a heat sink to dissipate heat that is generated , this electronic type has no moving mechanical parts, the theory behind how the electronic version works is beyond the scope of this article.

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