Power control circuit for magnetron

Claim 1

A duty cycle power control circuit for a microwave generating device of the type having an anode, a cathode, and a filament for heating the cathode and characterized by a threshold anode voltage level above which sufficient anode current flows to generate microwave energy and below which anode current is substantially zero, said control circuit comprising: a power transformer comprising a primary winding adapted for energization by an external power supply, a secondary anode winding for supplying an anode voltage to the microwave generating device and a secondary filament winding for supplying a filament-voltage to the microwave generating device, said primary winding comprising a main winding and a serially connected standby winding; duty cycle controlled circuit means for periodically switching between a power on state and a standby state to implement the desired duty cycle for the microwave generating device, said duty cycle controlled circuit means being effective in its standby state to connect said main winding in series with said standby winding across an external power supply and effective in its power on state to disconnect said standby winding and connect said main winding across the external power supply; said power transformer windings being arranged such that when said circuit means is in its standby state the anode voltage applied to the microwave generating device is less than the threshold level and when in its power on state the anode voltage is greater than the threshold level and in both states the filament voltage is sufficient to maintain a filament temperature above a predetermined minimum temperature set sufficiently high to avoid moding in the microwave generating device.