Switch-arc preventing circuit

Claim 1

An arc preventing continuous current pulsed electrical circuit for telephone networks which provides substantially self-preserving and switch-preserving characteristics over a broad range of inductive coil resistances comprising: a continuous current power source having first and second poles, a protected two-terminal switch having first and second make-break contacts and means for closing same, an inductive coil of a stepping solenoid of a telephone exchange circuit system, gating means comprising a gating pulse source having first and second poles, biasing means comprising a biasing voltage source having first and second terminals, means for closing said protected switch comprising a pulsed-continuous-current solenoid, a gating pulse source of a telephone exchange and having first and second poles, said pulsed-continuous-current solenoid having a controlling mechanical linkage with said protected switch, means connecting said first pole of said gating pulse source to a first terminal of said pulsed-continuous-current solenoid, and means connecting a second terminal of said solenoid to said second pole of said gating pulse source, a protective two transistor electronic switching means comprising a first transistor having first and second power electrodes, and a control transistor in controlling circuit with said first transistor, said first pole of said continuous current power source being connected to a first side of said coil, said coil having a second side connected to said first power electrode of said first transistor, means connecting said second power electrode of said first transistor to said second pole of said power source, a blocking diode having first and second electrodes, means connecting said first terminal of said bias voltage source to said first electrode of said blocking diode, a voltage dividing resistor network having first and second resistors each having first and second non-polarized sides, said first resistor being of much higher resistance than said second resistor, said first side of said first resistor being connected to said second electrode of said blocking diode, a conductor connecting said second side of said first resistor to the first side of said second resistor, means connecting said conductor to the base of said control transistor, said second side of said second resistor being connected to said first contact of said protected switch, means connecting said second contact of said protected switch to said second terminal of said bias voltage source, a metal oxide varistor having first and second poles, means connecting said first pole of said varistor to said first power electrode of said protective switching means, means connecting said second pole of said varistor to said second power electrode of said protective switching means, whereby arcing is non-existant of said contacts of said protected switching means, and whereby said circuit is sensitive to operate on very low current whereby it is versatile to operate over a broad range of coil resistances.