Patent GB-946216-A (GB)
Title: Improvements in thermoprinting apparatus and method
IPC: G03B27/30
Filed: 1959-10-20
Abstract: 946,216. Thermographic contact printing; automatic voltage control. MINNESOTA MINING & MANUFACTURING CO. Oct. 20, 1959 [Oct. 20, 1958], No.35596/59. Headings G2A, G2C and G3R. A method of reproducing a graphic original on heat-sensitive copy paper comprises forming an assembly of the original and the copy paper and subjecting successive areas of the moving assembly to uniform radiation pulses while relating the rate of advance of the assembly and the frequency of the pulses so that the entire assembly is uniformly irradiated. As shown, an original 17 and a heat-sensitive copy sheet 18 are fed together by an endless belt 16 around a transparent rotatable copy cylinder 12 concentrically surrounding a stationary narrow bore flash lamp 35, preferably a xenon-filled lamp as claimed in Specification 946,217, and journalled in floating bearings 24 having slots in which are slidable mounting screws on the machine frame, the cylinder being supported against the drive and idler rollers 13, 14 by the belt which is tensioned by a spring-urged roller 15. A switch arm 74a at the entry to the machine closed by the passage of the original and copy sheet is arranged to close the charging circuit for the capacitor 84 energizing the flash lamp and a capacitor 88 feeding the lamp trigger electrode 42, and to ensure that the discharge occurs when the first section of the original and copy sheet extends between the roller nips on the cylinder, a reflector 34 which also carries the lamp trigger electrode 42 limiting the illumination field on the cylinder to this area. Successive sections of the original are copied on the copy sheet without exposure overlap by repeated flashes of the lamp at times controlled by a double cam 60 geared to the drive roller 13, and when the original and copy sheet pass from beneath the switch arm 74a, the cam ensures that one more flash is effected and that a switch 109 in the charging circuit for the capacitor is opened and a switch 94 in the discharge circuit is closed to short the capacitors 84, 88 and dissipate their residual charge. The speed of the cam is adjustable relative to that of the drive roller 13 by means of a core and wheel transmission. To maintain the lamp emission constant the voltage developed across the capacitor which is charged by a mainsfed half wave rectifier is held constant by a voltage pick-off and comparing circuit which alters the impedance in the charging circuit to compensate for mains voltage variations. A potential divider 112 feeds a chosen fraction of the capacitor voltage to a transistor amplifier 119 which is also fed with a stable voltage from a source such as a mercury battery or standard cell or, as shown, a Zener diode 120, and the amplified error signal resulting from a mismatch due to change in capacitor voltage is fed to a further transistor amplifier 121. The output from this is fed to the control winding 128 of a variable impedance device 124 having a saturable core. Each current pulse charging the capacitor 84 charges a capacitor 131 in parallel with the control winding 129 of a load reactor 130 having an iron core with a rectangular hysteresis loop and with the load winding 127 of the device 124. The result is that increases above the required value in the voltage tapped off by the divider 112 increase the impedance of the load winding 132 in the rectifier circuit due to capacitor 131 discharging through the winding 129 so that charging action of the rectifier is reduced until the charge on the capacitor 84 has again fallen to bring the voltage to the required value, while as long as the voltage is below this value charging by the rectifier circuit will continue. A safety circuit also ensures that on opening the machine cover or the main switch 75 switch 94 closes to short the capacitors 84, 88.