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Power tussle?

Electromechanical versus solid state. Is the relay debate a done deal? By Vanessa Knivett.

A few years ago, you may well have read headlines predicting something like ‘Smart solid state devices set to demolish the electromechanical relay market!’
However, the electromechanical (EM) relay market has proved that, far from being threatened by solid state, it can survive and thrive. Today, EM relays continue to be specified for many applications and are being manufactured in larger numbers than ever before. The reason is largely cost – the EM relay market, like others, has experienced severe price erosion, bringing the average 10A relay down to roughly 40p from a price of around £1, 10 years ago.
But there are also technical reasons for the continuing success of EM relays. An electromechanical relay consists of a coil mechanism to which an input voltage is applied. This magnetises the core, pulling the armature towards it, causing the output contacts to touch and the load circuit to close. When the input voltage is removed, the armature springs back and breaks the load circuit connection. A solid state relay, in contrast, has no moving parts. SSRs consist of a low current control and high current load which are typically optically, or occasionally transformer, isolated. A photo sensitive MOSFET with light emitting diode (led) is generally used to actuate the device.
SSRs are renowned for being faster than EM versions. Their switching time depends upon how fast the led can be turned on or off – 0.1 to 5ms, compared to the average EM relay’s physical switching speed of 10ms for a typical “sugar cube” relay.

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Vanessa Knivett

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