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Introduction to Relays – What Is a Relay?

What is a relay?

A relay is composed of (1) an electromagnet and (2) a contact mechanism. It is defined as "a device that actuates a contact mechanism, using an electromagnetic attractive force that is generated when a current of a certain value or more flows through an electromagnet. It opens and closes a contact through voltage and current (input signal) applied to a coil."


Simply speaking, a relay receives an electric signal from a switch, etc., and transmits the signal to an output unit, such as a motor.

Relay race at a track & field day

Relay race at a track & field day illustlation



Relays of control equipment





Relay categories

Two major categories of relays.

Control relays

Contact type

Mechanical relays

The contact opens/closes mechanically.

Non contact type

Semiconductor relays
〈PhotoMOS/Solid-state relays〉

PhotoMOS/Solid-state relaysSemiconductor contact (output section), do not opens/closes mechanically.


Relay applications

Relays are used in various machines and devices.

Digital household appliances

  • Flat-panel TVs
  • Video Games

Office automation equipment

  • Copiers
  • Multifunction fax machines
Office automation equipment

Automotive equipment

  • Car navigation systems
  • Anti-theft security systems
Automotive equipment

Security equipment

  • Security cameras
  • Alarms
  • Entrance/exit monitoring systems
Security equipment

Household appliances

  • Refrigerators
  • Microwave ovens
  • Washing machines
  • Air conditioners
  • Rice cookers
Household appliances

Industrial equipment

  • Industrial robots
  • Machine tools
  • Packaging machines
Industrial equipment

Medical instruments

  • Blood-pressure meters
  • Ultrasonic reflectoscopes
  • Nursing beds
Medical instruments