Circuit for controlling an electromagnet

Claim 2

A circuit for controlling an electromagnet having a winding with first and second terminals, said circuit comprising: a main current control element in series with the electromagnet winding across a supply, said main current control element acting initially as a switch to allow "pull-in" current to pass through the winding from said first terminal to said second terminal, and subsequently as a current limiter to limit the current to a lower "hold" level; a capacitor; and current path means for connecting said capacitor across the winding, for conducting during changeover from "pull-in" to "hold", permitting current flow through said capacitor from said second terminal to said first terminal and providing a reverse charge on said capacitor, and for conducting again at switch-off, discharging said capacitor through the winding, reversing the winding current and providing a magnetizing force in opposition to that of eddy currents sustained in the electromagnet after said main current control element is switched off.

Claim 3

A circuit as claimed in claim 1, wherein said main current control element comprises a main transistor, and said circuit further comprising a control transistor operatively connected to and controlling said main transistor and sensitive to the current in said winding.

Claim 4

A circuit as claimed in claim 1 or 2, wherein said current path means comprises first and second parallel current paths, one conductive during changeover and the other conductive at switch-off.

Claim 5

A circuit as claimed in claim 2, wherein said control transistor has an emitter and a base, and said circuit further comprising: a current sensing resistor in series with the winding and connected to the emitter of the control transistor; and means connected to the base of the control transistor for applying a first voltage signal to the base of the control transistor during "pull-in" and a lower second voltage signal thereto during "hold".

Claim 6

A circuit for controlling an electromagnet having a winding with first and second terminals, said circuit comprising: a main current control element in series with the electromagnet winding across a supply, said main current control element acting initially as a switch to allow "pull-in" current to pass through the winding from said first terminal to said second terminal, and subsequently as a current limiter to limit the current to a lower "hold" level, said main current control element comprising a main transistor; a capacitor; current path means for connecting said capacitor across the winding, for conducting during changeover from "pull-in" to "hold", permitting current flow through said capacitor from said second terminal to said first terminal and providing a reverse charge on said capacitor, and for conducting again at switch-off, discharging said capacitor through the winding, reversing the winding current and providing a magnetizing force in opposition to that of eddy currents sustained in the electromagnet after said main current control element is switched off; a control transistor, having an emitter and a base and having a collector operatively connected to said main transistor, for controlling said main transistor and being sensitive to the current in said winding; a current sensing resistor in series with the winding and connected to the emitter of the control transistor; and means, connected to the base of the control transistor, for applying a first voltage signal to the base of the control transistor during "pull-in" and a lower second voltage signal thereto during "hold".

Claim 7

A circuit for controlling an electromagnet having a winding with first and second terminals, said circuit comprising: a main current control element in series with the electromagnet winding across a supply, said main current control element acting initially as a switch to allow "pull-in" current to pass through the winding from said first terminal to said second terminal, and subsequently as a current limiter to limit the current to a lower "hold" level, said main current control element comprising a main transistor; a capacitor; current path means for connecting said capacitor across the winding, for conducting during changeover from "pull-in" to "hold", permitting current flow through said capacitor from said second terminal to said first terminal and providing a reverse charge on said capacitor, and for conducting again at switch-off, discharging said capacitor through the winding, reversing the winding current and providing a magnetizing force in opposition to that of eddy currents sustained in the electromagnet after said main current control element is switched off; a control transistor, having an emitter and a base and having a collector operatively connected to said main transistor, for controlling said main transistor and being sensitive to the current in said winding; a current sensing resistor in series with the winding and connected to the emitter of the control transistor; and means, connected to the base of the control transistor, for applying a first voltage signal to the base of the control transistor during "pull-in" and a lower second voltage signal thereto during "hold".

Claim 8

A circuit as claimed in claim 5, wherein said current path means comprises first and second parallel current paths one conductive during changeover and the other conductive at switch-off.

Claim 9

A circuit as claimed in claim 6, wherein said current path means further comprises: a first diode connected in series with said capacitor and arranged to permit current flow in one direction during the change from "pull-in" to "hold"; and a first transistor and a second diode connected in series with said capacitor and permitting current flow in the opposite direction at switch-off.

Claim 10

A circuit as claimed in claim 7, further comprising a second transistor in series with said first diode and being conductive during said change.