Patent GB-706944-A (GB)
Title: Improvements in or relating to x-ray apparatus
IPC: H05G1/40
Filed: 1951-11-20
Abstract: 706,944. X-ray apparatus. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Nov. 20, 1951 [Dec. 4, 1950], No. 27201/51. Class 98(1) [Also in Group XXXV] The supply to an X-ray tube 11 is switched by an electronic contactor 21 comprising a pair of inversely parallel connected discharge tubes 23, 25, and an electronic timer circuit 89 controls the conduction of such tubes. The contactor 21 is serially connected with a power input transformer 17 across the primary winding 19 of an H.T. transformer 15, the secondary windings 29, 31 of which supply an X-ray tube anode supply bridge rectifier circuit 13 feeding D.C. across the X-ray tube. The bridge 13 comprises four rectifiers of which two 47, 51 are conductive only on closure of a switch 294 to complete a filament heating circuit thereof, thereby to provide either half or full wave rectification. Conduction of the contactor 21 is controlled by two further inversely parallel connected tubes 91, 93, conduction of which former energizes a first control transformer 113, a secondary winding 271 of which, feeding through a phase shifter 273 and a third control transformer 265 produces positive pulses on the grids of the tubes 23, 25 to overcome a negative bias imposed thereon by networks 253, 255. On closure of a supply circuit-breaker 20 a timer start tube 99 conducts to charge timing capacitors 165, 187 and a time delay relay 209 is energized to close its contacts 207. On closure of manual contacts 211 the coil 281 of a relay 193 is energized and contacts 189 thereof open to stop charging of the capacitor 187, and anode potential is applied to a rectifier 103 and the contactor control tube 91 which, depending upon the instant of switching on, are rendered conductive. Conduction of tube 91 also energizes a first primary winding 119 of a second control transformer 115 to place an anode voltage on a timer start rectifier 101 which then conducts to charge its associated time constant capacitor 233 to place a negative potential on the control grid of the tube 99 and thereby render it non-conductive. The main timing capacitor 165 then discharges for a period determined by a network 161 until a negative bias is removed from the grid of a timer stop tube 95 which then conducts and places a voltage on its anode resistor 203 to render a first contactor control tube cut off rectifier 105 conductive which charges a time constant capacitor 241 to place a negative bias on the tube 91 and render it non-conductive, to stop the exposure. The other tube 93 is similarly stopped. Timing of an X-ray tube exposure within a range of 1/120th to 15 seconds is effected by the time constant network 161 comprising a resistor network 163 and the capacitor 165, and a synchronous motor driven timer 283. The period of short exposures, 1/60th-1¥ seconds, is selected by engaging a selector 169 with appropriate tappings 177 on a resistor 167, and for longer exposures a long range switch 287 is closed, the selector 169 is engaged on contact 179 and the timer 283 is set for the appropriate period. The shortest period, 1/120th second, is selected by setting the selector on tap 175 for the shortest short range period 1/60th second (corresponding to one cycle of the supply) and opening switch 294 to give half wave rectification in the X-ray tube supply. In the event of this timer stop circuit not operating, a safety time stop circuit is provided comprising a variable resistor 185, the capacitor 187, the relay 193 and a tube 97, in which the capacitor 187 and tube 97 operate in a similar manner to the main timing stop components 165, 95.