Voltage surges in relay control circuits

DEVICES which depend for their operation on the linking of magnetic flux with a current-carrying coil produce a transient voltage in their coils when they are de-energized suddenly. The magnitude and duration of such a transient depend on many design and operating factors. Included are size, shape, permeability, and resistivity of the magnetic material, as well as the maximum area to be surrounded by individual eddy currents, the magnitude of the exciting force, coil voltage, number of turns, and the speed of current interruption. In general, the large devices initiate transients having much more energy and longer duration but often of lower voltage than do small devices such as relays.