Line supervisory circuit

Claim 1

An electrical circuit for monitoring current in a monitored circuit comprising a magnetic element of a material exhibiting substantially rectangular hysteresis characteristics and presenting a closed and a partial flux path, an interrogate winding coupled to said closed flux path for selectively establishing remanent flux in said closed flux path to a first state and to a second state, means for generating an inhibiting flux in said magnetic element for preventing said remanent flux from switching from said first state to said second state comprising a first and a second drive winding coupled to said partial flux path, said monitored circuit completing a conducting path between said first and second drive windings when in one operative state, said drive windings being individually coupled in a sense and in a number of turns so that a current of a predetermined value in said monitored circuit is sufficient to induce said inhibiting flux in said partial flux path, means for sensing said switching of said remanent flux comprising a readout winding coupled to said closed flux path, and means for inhibiting the induction of an inhibiting flux in said partial flux path due to current in only one of said drive windings comprising first and second magnetoresistor means electrically connected respectively in parallel with said first and second drive windings and inductively coupled respectively to said second and first drive windings, each of said magnetoresistor means having first and second resistance values in the presence of first and second predetermined magnetic fields, respectively.