What is a Relay & How it Works

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What is a relay?
A relay is simply a electrical switch (like you might find in your home). But instead of being operated by someone turning it on or off, it is controlled by another electrical circuit. So, a relay is an electrically operated switch.
Most relays use an electromagnetic to operate the switch. An electromagnet is a device that creates a magnetic field by passing an electrical current through a coil of wire. The magnetic field is then turned off when the current is removed.
Why do we use relays?
Relays are commonly used when we need to control a switch with high voltage, or circuits with large amounts or current passing through them.
Operating these types of switches manually can be dangerous, inefficient and impractical.
By using relays, we greatly increase the operational safety, while also giving us the ability to use smaller, safer and less expensive electrical equipment to control our devices.
Using relays can also allow us to control several devices on a single switch, as opposed to using several bulky switches for each device.
Relays can be combined with timer and a logic circuits to assist in electrical automation.
Where do we use relays?
Relays are commonly found electrical devices where they might be controlling a motor, or a light.
They are also very common in cars, where the 12-volt power supply coming from our battery, means that most everything will need a large amount of current, which are perfect use cases for relays.
In industrial applications they are commonly used in control panels for use in various tasks such as: monitoring power, controlling motors, valves and other electrical equipment and automation.
How do relays work?
First let’s look at the different parts that make up a relay
- The Coil (or electromagnet)
- Armature (which can be attracted by the coil)
- Spring (forces the armature to original state when not being affected by the coil)
- Contacts (when touching, allow electricity to pass through them)
A relay consists of two separate circuits that work together to open or close a switch (or contacts).
The first circuit drives the coil (or electromagnet). The electricity passes through the coil, it creates a magnetic field.
The second circuit contains a set contacts and a separate power source. This circuit is what provides power to the electrical load.
When electricity passes through the coil, it creates a magnetic field. This magnetic field pulls the contact from the other circuit close (or against the stationary contact) which will allow current to pass through the contacts, therefor allowing the load to become energized.
When the coil is de-energized, the magnetic field is gone which allows the contact to be pushed back into its original state de-energizing the load side of the circuit.
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[@johnadams1976]

Clear and simple. Thanks.

[@dimak.9906]

Great concise explanation!!

[@alphab5543]

Many thanks

[@strikemaster1]

Awesome thanks.

[@insylem]

How is the inductive flyback from the relay coil handled in industrial panel circuits? I asked at work and no one seemed to know what I was talking about. Is there a flyback diode in the PLC output channel? Or in the relay base mount that attaches to the DIN rail?

[@ToolsAandLogic]

in the video, i see that you have mentioned that Low voltage for relay coil side and high voltage for Contact side. I think this is not appropriate, because relay coil voltage can be higher than the contact voltage side. For example, Used as an interposing relay, if i have a output of 110 volt from Output card of PLC and i want to switch to something (24 volt valve ) which is operated by 24 volt supply, then in this case the coil voltage will be 110 volt and the contact side will have 24 volt. So, one can not generalize about coil voltage to be low voltage and contact voltage to be high voltage. please correct this.