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A relay useful article

07 October 2019

There are many circumstances in which a high current or voltage load needs to be controlled by the operation of a low-power circuit. In these cases, a relay can provide the necessary isolation between the high-power and low-power parts of the system. Will Darby, Managing Director of Carlo Gavazzi UK, outlines how to select the most appropriate solid-state relay for different applications.

There are two main types of relay: electromechanical relays and solid-state relays. An electromechanical relay (EMR) uses a physical moving part to connect contacts within the output element of the relay. The contact’s movement is generated using electromagnetic forces provided by the low-power input signal to enable completion of a circuit containing the high-power signal.

By contrast, Solid-State Relays (SSRs) do not have physical moving parts, they are switched electronically when an external voltage is applied to the unit’s control terminals. An SSR uses a low power electrical signal to generate an optical semiconductor signal, typically with an octocoupler, that transmits and energises the output circuit. When activated, the input optical signal acts as the “switch” that allows a mains voltage power to pass through the SSR’s output component to turn a device on or off. This article is about the selection of SSR’s.

An SSR’s lack of moving parts enables it to switch much faster than electromechanical relays and, because SSRs have no moving parts or contacts that can wear out they generally last longer and require less maintenance than EMRs.

Read the full article in the October issue of DPA.

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