Ferrite Rod Switch

(Russian, ferrod), a contactless electromagnetic telephone switching device that operates on the principle of magnetic saturation of ferromagnetic materials—the same operating principle as used by a transformer with magnetic biasing. Such switches are used for logic functions in the control apparatus of quasi-electronic automatic telephone offices, for example, to indicate the operating state of a subscriber’s line.

The principal elements of a ferrite rod switch are a core in the form of a slug or rod made of a square-loop ferrite with a low coercive force; two control windings connected in series; an excitation winding; and a reading winding. Bipolar current pulses, usually with an amplitude of 0.5 ampere and a duration of 3–5 microseconds, are supplied to the excitation windings from an interrogation circuit. If no current is present in the control winding, the core undergoes magnetic reversal under the influence of the excitation pulses; voltage pulses induced in the reading winding (with an amplitude of approximately 0.2 volt) are fed to the operational memory of the automatic telephone office. If DC is present in the control windings sufficient to magnetize the core to saturation (usually from a few milliamperes to several tens of milliamperes), no pulses are induced in the reading winding.