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.
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.
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.
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.
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.
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.
- Provides physical isolation between circuits
- Can usually withstand high voltages
- 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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