What is a Control Relay, types, function, difference between contactors and price.

By March 23, 2020Uncategorized
Control Relay circuit diagram

What is a Control Relay?

A Control Relay also known as Relay, is a switch, an electromagnetic switch. This means users are not required to manually turn the switch to isolate or change the state of an electric circuit.  A control relay allows electrical current to flow through a conducting coil that open or closes a switch. It also protects the circuit current.


Currently, control relays play a crucial role in today’s electronic devices. They are electronic parts which powers electronic parts such as motor, power plants, power supply system, transistor and many more.


Different Types of Control Relays

There are different types of Control Relays depending on the operating principle and structural features.


Solid State Relays – It uses solid state components to perform the switching operations without moving any parts.


ContactorA large relay used to switch a large amount of electrical power through its contacts.


Electromagnetic RelaysConstructed with electrical, mechanical and magnetic components and have operating coils and mechanical contacts. Hence, when the coil is activated by a supply system, the mechanical contact is either open or close. The supply system has 2 types AC and DC.

Thermal Overload Protection Relayworks on the principle of thermal effect of electrical energy. When excessive current flows through the circuit, the circuit opens due to the bimetallic strip experiencing an increase in temperature.

How Control Relay works? (Circuit Diagram)

An example of a Control Relay Circuit Diagram

Control Relay circuit diagram

Control Relay circuit diagram

Control Relays permit a low current circuit  to control a high current circuit. Using the above diagram, when electrical current is sent through the coil, it generates an electromagnetic field which will be used to attract the switch downwards. Thus closing the switch, which closes the circuit and allowing to electrical current to flow. When there is no current flowing through the coil, the switch return to its original position which results in an open circuit.


Understanding Contact Type of a Relay

Every Control Relay has a Contact type such as SPST-NO but what does it means?

Poles represent the number of circuits controlled by a switch.

Throws represent the number of positions the switch can adopt.


SPST symbol

SPST symbol

SPST symbol

Single Pole Single Throw, SPST, have two terminals which can be connected and disconnected. Including two for the coil ,such a relay has four terminals in total.


Single Pole Double Throw, SPDT, has a common terminal which connects either one of two others. Including two for the coil, this relay has five terminals in total. Regardless whether coil is active or inactive it, either ‘A’ or ‘B’ is always resting while the other needs to be the coil to be powered.


Double Pole Single Throw, DPST is the equal to two SPST activated by a single coil. Including two for the coil, this relay has 6 terminals in total.

omron MY4IN

omron MY4IN

Double Pole Double Throw, DPDT is equal to two SPDT activated by a single coil. Including two for the coil, this relay has 8 terminals in total.


Difference between Normally Open (NO) and Normally Closed (NC) contacts

NO contacts allow current, when relay is energized. This means when voltage is applied, the contact closes and allows current to flow through.

NC contacts allow current, when relay is not energized. Opposite to NO, NC contact opens and interrupts current flow.


*Change Over (CO) is the same as Double Throw (DT) relay. 

Differences between Control Relay and Contactors

Both of these electrical devices perform the same task of switching a circuit and even Contactors is a term for large relays. Does this mean it is okay to use either Control Relay or Contactors? No, and here is why?


Load Capacitor – Control Relays are classified as carrying loads of 10 Amperes or less. Whereas, Contactors would be used dealing with loads more than 10 Amperes.


Contacts – Contactors are mostly designed to operate NO while Control Relay can operate on either NO or NC.


Auxiliary Contacts – Contactors are often fit with Auxiliary Contacts which are used to perform additional functions but Control Relay doesn’t.


Safety Features – Since Contactors are operate to carry high loads, it is common for them to equip Safety Features such as Spring-Loaded Contacts, Arc Suppression and Overloads.


Applications – Contactors are commonly built and used in a 3 phase applications but a relay is more commonly used in a single phase applications.


How do I know whether I should get a Control Relay or Contactor? 

To summarize which electrical device to pick: 

Control Relay Contactor
10 A and below 9A and above
Max Voltage 250V Max Voltage 1000V
1 phase 1 or 3 phase


Control Relay Prices

You can check out the prices of Control Relay and Contactors at our website, ELECTGO. At ELECTGO we offer a range of comprehensive range of industrial products including Control Relay and Contactors from brands such as Schenider and Omron.





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