Duty cycle control apparatus

Claim 1

A duty cycle control apparatus for controlling, in response to an applied control signal, the mean voltage applied to an inductive load from a direct current source, comprising input terminals for connection to the source, output terminals for connection to the load, a semiconductor controllable rectifier connected in a current path between the input and output terminals, a control circuit for rendering the semiconductor controllable rectifier alternately conducting and nonconducting in accordance with a variable mark-to-space ratio of ''''on'''' periods of current flow from the source to the load to ''''off'''' periods of no current flow from the source to the load, a unidirectionally conductive freewheeling current path around the load for enabling load current to flow between said ''''on'''' periods, and current-sensing means for producing a currentlimiting signal indicative of mean current flowing from the Source through the semiconductor rectifier, said control circuit including means responsive to said current limiting signal for varying said mark-to-space ratio during the continuous operation of said apparatus in accordance with said signal when said signal exceeds a predetermined value and for maintaining the operation of the apparatus at a corresponding mark-to-space ratio until said signal no longer exceeds said predetermined value such as to impose a restriction upon the available maximum mark-to-space ratio so as to tend to maintain the mean current of the controllable rectifier within predetermined limits without to the same extent limiting the current in the load.