‘Hot’ versus ‘Cold’ switching

March 02, 2017 // By Graham Dale
Reed Relays generally have a higher Carry Current rating than their ‘hot’ Switching Current rating. It is usually during ‘hot’ switching where contact damage occurs due to the resulting arc across the contacts as they open or close.

A severe current overload will quickly melt the contact area causing the two surfaces to fuse together creating a hard weld as soon as the contact closes. Less severe current inrushes will cause a milder weld or gradually build up a ‘pip’ on one contact and erode a ‘crater’ on the other according to the direction of current flow. These can eventually lock together. Arcs can occur when contacts open, particularly when the load is inductive and Back EMFs from inductive loads should always be limited, usually by a simple diode in the case of DC loads or by a Snubber or Varistor in the case of AC loads.

One way to reduce or remove these issues is to ‘Cold’ switch. This is a common technique in Test Instrumentation, where the current or voltage stimulus is not applied to the switch until after the relay has been operated and contact bounce finished. In the same way, the stimulus is removed before the contact is opened. In this way, there will be no arcing or switched current inrushes and the relay will achieve maximum life, often into billions of operations.

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