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Electromechanical & SSR Relays


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Basically relays are of two types

  1. Electromechanical and
  2. Solid State Relays


Electromechanical Relays

As these types of relays have physical moving parts involved due to which it is called Electromechanical Relays. Electromechanical Relays works on the principle of electromagnetic Induction that is why it is also called as Electromagnetic Relays.

Electromagnetic Induction –

Electromagnetic induction is applicable various products in motors, contactors, transformers, generators, relays.

An Electromagnetic relay consists of coil, spring movable contacts, and stationary contacts. It has two circuits, one is low voltage control circuit which includes electromagnetic relay coil and the second is high voltage power supply on contact side. 

As per principle of electromagnetic induction supply to the relay coil induces the magnetic field around it. Contacts moves towards or away due to the induced magnetic field causing contacts to connect and disconnect. Coil voltage is determined from the coil design (wire size, coil diameter, number of turns). Similarly contact ratings are determined from the contact material and dimensions of the contact material.


Relays are distinguished on number of changeover available-

1) Single NO contact relay (SPST)

Refer below image for single NO contact relay.



NO (normally open) contact is normally in open condition after coil actuation contact connects to common.


2) One Changeover contact (SPDT)

Refer below image for single NO contact relay.


This Relay have one common and respective NO and NC. In power off condition common is connected to NC contact. After coil is charged common connects to NO contacts. At a time common connects to only one contact.


3) Two Pole 1NO  contact (DPST)

Refer below image for two pole 1 NO contact.



It has two poles and respective NO contacts. NO (normally open) contact is normally in open condition after coil actuation contact connects to common


4) Two Changeover contacts (DPDT)

Refer below image for double pole double throw.



It has two poles, and each pole has respective NO and NC signal. Two individual supplies can be applied to the each pole .


5) Four Changeover Contacts (Four Pole Double throw)

Refer below image for double pole double throw.

Double Row 

In above configuration it has four poles and respective NO and NC. Four individual supplies can be connected to each pole.


Solid State Relays

Unlike Electromagnetic/Electromechanical Relays;   Solid state relays (SSR) do not have any moving parts instead it uses semiconductor properties to isolate input and output.


As per above diagram SSR provides a complete isolation between input (coil voltage) and Output (Switching/contact voltage) voltages effectively due to the use of optical isolation. It has infinite resistance when it is OFF and negligible resistance when it is ON. Unlike Electromagnetic relays the contact side cannot be designed to work on both AC and DC voltages at a same time whereas SSR is designed as per switching voltages.  Also due to the mechanical movement involved in the parts, electromechanical relays cannot be used for higher frequency whereas due to Semiconductor technology SSR can be used for higher switching frequency.


Heat is generated in the process of switching of semiconductor devices; to overcome SSR is designed with heat sink.

For higher ratings the size of SSR and Heat sink increases which makes it expensive to use the SSR for higher ratings.

Following is the difference between electromagnetic relays and solid state relays.


Connectwell Interface Module -

Connectwell Relay and SSR Modules can be mounted on the all DIN type rails/ panel mounted, making  it suitable for the applicable in all the types of industries. Electromagnetic and SSR modules available in all ratings.

Electromagnetic relays are used where there is a requirement of higher power.

Solid state relays are used where there is requirement of fast switching and can be used in vibrating area due to non-moving parts. Also it generates minimum electrical noise by zero voltage turn on zero current turn off.

Both the relays are  used in every industry namely Automation, Automobile, DSC System, SCADA system, PLC protection system, Machines, Oil & Gas, Building system, process industry.


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