Cold Switching

Cold switching is a term used to describe a switch (relay) operation where there is no significant user signal present at the moment the relay contacts open or close. Cold switching is significantly less stressful on mechanical relays than hot switching and can be used to maximize the life of a relay. For solid state relays there is usually no difference between cold and hot switch ratings.

Cold switching can also have voltage and current ratings which are different (higher) to the hot switch ratings since the mechanical relay contacts do not to deal with signal arcs and metal migration. The cold voltage ratings are primarily determined by the properties of insulators in the relays, connectors and PCB layout. The cold current rating is similarly determined the metal conductors in the relays, connectors and PCB tracks. PCB tracks are typically determined by a need to keep temperature rise to around 10C.

On some designs the presence of large and rapid voltage changes can disturb the relay control system if the applied voltage has a high rate of rise of voltage that couples into the relay control system. Use of mechanical relays outside the cold switch rated switching system for example can create very high rates of change of voltage (1000's of volts per microsecond) which propagate through the signal path and couple to the control system. When using cold switch ratings it is therefore recommended that the voltage rise time is managed to avoid these voltage transients.

Solid state switches can be used to manage the rate of rise of voltage applied to a switching system, their hot switch transients are less abrupt than those created by mechanical relays and are bounce free.

Many voltage sources also include soft start (or are slew rate limited) to avoid high current surges into capacitors and this start up process is more than adequate to protect the switching system.

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