Relays: Construction & Working of Relays


Relay is an electromagnetic switch which is used  to control switching. It is a low-high voltage switch to be precise. Low to High Voltage signifies Low Control Voltage is used to switch contacts of relays which are comparatively at high potential.

Construction of Relay:

Construction of Relays
Construction of Relays

Relay basically has a coil, to be specific switching coil which switches between contacts of relays. It has two or more than two contacts, we will consider relay with two contacts of which one is NO (Normally open) while other is NC(Normally Closed). And a contact termed as COM (Common). Normally Open means if there is no voltage present at the coil of relay the contact will act as open switch with respect to COM. Similarly Normally closed means it will act as short with COM while there is no voltage present at the coil of Relay. Contacts NO & NC are firmly mounted while COM is mounted in a way it can move. A spring is attached to COM for damping. ie: Getting back normal state once coil is de-energized.

Working Principle of Relays:

Relays are application of electromagnets, when a control signal of sufficient amplitude/voltage is applied to the coil of relay it switches between contacts. Normally open (NO) becomes close, shorted with COM while Normally Closed(NC)’s connection is broken.

Relay has coil with N number of turns, this coil has a operating voltage specified by the manufactures. Common Voltages are 5V, 9V, 12V. When the specified voltage is applied to coil, magnetic field is induced in this coil. This magnetic field attracts the COM contact of the relay. Making a connection between NO & COM. Number of Turns (N) is determined by the magnetic field strength required to move COM contact. It holds the COM contact till the coil is energized. Once there is no voltage present at coil, a spring pulls back COM to NC contact.

Relays are break before make type of switches. That is it first breaks contact with NC and than makes contact with NO.

  Application of Relays:

Relays are used widely in Electronics Control Systems. Here are few examples of its applications.

  1. Switching AC load through DC Control Voltage
  2. To operate various device in conjugation with MicroController
  3. In Electronic Timers etc…


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