‘Hot’ versus ‘Cold’ switching: Page 2 of 2

March 02, 2017 //By Graham Dale
‘Hot’ versus ‘Cold’ switching
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.

When calculating the delay time between switching on the relay coil and applying the current to the switch, it is important to consider the effects of high ambient temperature if this is likely to be encountered. The maximum operate time and bounce figures given on the data sheets are at a 25°C ambient level. At higher temperatures, the resistance of the coil winding will increase at a rate of 0.4 % per °C, this being the coefficient of resistance of the copper coil wire. There will therefore be a corresponding fall in coil current and the level of the magnetic field that is generated to operate the reed switch. This lower drive level will increase the operate time slightly. The timing figures on Pickering data sheets are normally quite conservative so this is unlikely to be an issue up to the normal ambient specification of 85°C. However, if there is any additional self-heating within the relay due to a high carry current and the switch resistance (I2R Watts), it will be necessary to consider this and allow a little more time before turning on the current through the switch.


About the author:

Graham Dale is Director of Research and Development at Pickering Electronics - www.pickeringrelay.com

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