Graham Dale, Director of Research and Development at Pickering Electronics explains ‘hot’ and ‘cold’ switching whilst using Reed Relays. The article is intended to help users maximise the reliability of their designs.
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.
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.
