GB1291573A - Improvements in or relating to intrinsically safe circuits - Google Patents

Abstract

1291573 Constant voltage devices ROSEMOUNT ENG CO Ltd 28 Jan 1970 [23 April 1969] 20826/69 Heading G3X [Also in Division H2] An intrinsically safe circuit arrangement for coupling a load 16 to an A.C. supply 13 comprises a transformer 10 having an inductor 12 in series with, and a capacitor 14 in parallel with, the primary winding 11; the winding 11 and the capacitor 14 constituting a parallel tuned circuit which is resonant at the supply frequency and which has a Q-factor at resonance such that the current flowing in the primary winding 11 at resonance is sufficient to partially saturate the core of the transformer whereby the load current is limited to a safe value, the inductor 12 being effective to limit the primary current in the event of an open-circuit or short-circuit of either the capacitor 14 or the primary winding 11. The circulating current in the parallel resonant circuit 11, 14 depends on the Q-factor thereof which will be modified by the reflected impedance of the secondary circuit 20. The Q is chosen so that over the required range of secondary current, the current in the primary winding 11 results in partial saturation of the core 10. This results in a reduction in the permeability of the core 10 with a consequent change in the Q of the resonant circuit 11, 14 and the system therefore provides a constant output voltage as the load current increases-up to a limit where the inductance of the primary winding has fallen to a point where the circuit can no longer support resonance. At this point the primary inductance falls rapidly to a low value and the secondary voltage then collapses almost to zero. An earthed screen is provided between the primary and secondary windings of the transformer 10 and the elements 10, 12, 14 may be encapsulated to form a single unit.