Electromechanical Relay

An electromechanical relay is an electrical or electronic switch that is typically operated by using electromagnetism to operate a mechanical switch mechanism.


Relay Technology Includes:
Relay basics     Reed relay     Reed relay specs     Relay circuits    


A relay is an electromagnetically operated electrical switch - an electromechanical switch. A relatively small current is used to create a magnetic field in a coil with a core and this is used to operate a switch that can control a much larger current.

In this way an electromechanical relay can use a small current to switch a much larger current.

Relays come in a variety of different sizes and they can be of a variety of different types using slightly different technologies, although they all use the same basic concept.

Although relays may be considered in some respects to use old technology, and solid state switches might be thought to be a more effective means of switching current. Nevertheless relays have some unique properties that make them ideal for many applications, where other types may not be as effective.

Typical electromechanical relay switch

Relay circuit symbol

The circuit symbols for relays can vary somewhat - like most circuit symbols. The most widely used format shows the relay coil as a box, and the contacts are placed close by as shown below.

Electrical / electronic relay switch circuit symbol
Circuit symbol of a relay
Note that on this symbol, both normally open and normally closed contacts are shown. Where one is not used, it is often not shown.

Other circuits, especially new that may be a bit older may show the relay coil as an actual coil. Although this does not conform to the latest relay circuit symbol standards, it may nevertheless be seen in some instances and it describes the inside of e relay well.

Electromechanical relay switch circuit symbol
Circuit symbol of a relay
An older style showing the relay coil.

It is possible for there to be further sets of switched contacts. In the same way that it is possible to have multiple poles on a switch, the same can be done with relays. It is possible to have several sets of switch contacts to change over multiple circuits.

Circuit symbol for a two pole relay switch
Circuit symbol of a relay
An older style showing the relay coil.

Relay switch basics

A relay is a form of switch that is operated by electromagnet which changes over the switching when current is applied to the coil.

Basic concept of the operation of an electromechanical relay switch
Basic concept of an electrical relay switch operation

When a current flows through the coil an electro-magnetic field is set up. The field attracts an iron armature, whose other end pushes the contacts together, completing the circuit. When the current is switched off, the contacts open again, switching the circuit off.

Advantages and disadvantages of relays

As with any technology there are advantages and disadvantages to the use of electromechanical relays. When designing circuit, it is necessary to weight of the positives and negatives to select the right technology for the given circuit.

Advantages

  • Provides physical isolation between circuits
  • Can usually withstand high voltages

Disadvantages

  • Mechanical nature of the relay means it is slow when compared to semiconductor switches
  • Has a limited lifetime due to the mechanical nature of the relay.

Sometimes another option that can be considered when requiring electrical isolation between two circuits may be an opto-isolator.



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