What are Control relays?

Basic working principle:

An electromechanical relay is an electrical switch actuated by an electromagnet coil. As for switching devices, they exhibit simple “on” and “off” behaviour with no intermediate states. Relays are very useful electrical switches through the coil of relay creates an electromagnetic field which attracts a lever and changes the switch contacts.

Electromagnetic induction is the operating principle of the relay switches. which is the production of an electromotive force (i.e., voltage) across an electrical conductor in a changing magnetic field and vice-versa.

When a voltage is applied to the electromagnetic coil system in the relay, current flow in the coil produces a magnetic energy which pulls the armature down. When pulls down the circuit gets closed as the armature touches the No contact lead.

Types of Relay switching contacts:

SPST (NO contact) :

spst relay switch

SPST, that is ‘single-pole, single-throw’ relay which is a normally open contact ( NO contact) which means the switch will be in the open (off) state when the relay coil is de-energized. When the coil is energized the magnetism generated pulls down the armature and close the switch. This type of switch is synonymously know as Form-A.

There is also SPST normally closed type switches

SPST (NC contact):

SPST normally closed type switches

The armature is arranged as shown in above picture so that the switch is normally closed and when an electric voltage is applied to the coil, the switches open and the circuit brakes. This type of switches are also known as Form-B in SPST switches.


The single-pole, double-throw (SPDT) relay contact, otherwise known as a “Form-C” contact. This design of switch provides both a normally-open and normallyclosed contact set in one unit, actuated by the electromagnet coil.


DPDT relays are some of the most common found in industry, due to their versatility. There are two SPDT switches arranged in the DPDT switch circuit and both are electrically isolated from each other. Each contact set offers the normally open and normally closed contacts.

There are 4 pin and 5 pin relays available:

5 pin switch most commonly contain normally open and normally pen contacts.

4 pin relay contain either normally open or normally closed contacts.


  • Relays can operate in DC or AC.

  • Relays can switch many contacts at once.

  • Relays can switch higher voltages.


  • The contacting becoming worn, resulting in a limited lifespan

  • Would perform well when it comes to switch high inrush currents.

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