Patent GB-1279477-A (GB)
Title: Alternating current power supply circuit
IPC: G05F1/455
Filed: 1969-12-08
Abstract: 1279477 Automatic voltage control INTERNATIONAL BUSINESS MACHINES CORP 8 Dec 1969 [30 Dec 1968] 59768/69 Heading G3R The R.M.S. voltage across a lamp 38 is maintained at a desired value by variation of the firing angle of a controlled semi-conductor switch 48 whose trigger signals are provided by a regulating circuit 45, which is arranged to supply an increased level of power to the lamp for a predetermined time period following energization and which includes means to automatically compensate the reference signal to maintain R.M.S. voltage regulation at large firing angles. As shown in Fig.3, a signal on line 54 representative of the voltage across the lamp is applied to a rectifying network T1, T2, T3, R3, R4, D9 which produces a D.C. signal I F  for combination with reference signals I 1  and I R . The latter is a steady state signal indicative of the desired value of lamp voltage and provided by a circuit comprising members Z1, D3, T10, R9, R28 supplied via transformer/rectifier 40, D7, D8 from the A. C. supply 42 for the lamp. The signal I 1  is provided by a circuit comprising members C1, T9, R7, R8 arranged so that when the lamp is initially energized the capacitor C1 is discharged to allow conduction of transistor T9 and production of the signal I 1  which is progressively reduced to zero by charging of the capacitor C1. Thus the voltage applied to the lamp is progressively reduced from a high initial value to the steady state value in a time period which is determined by the capacitor charging time constant and which is chosen to match the increase in thermal energy stored in apparatus heated by the lamp, this apparatus forming part of a conventional xerographic copying machine, Fig. 1 (not shown). In order to limit or suppress the signal I 1  when the lamp is energized soon after the previous period of energization, the capacitor discharge time constant (through resistors R8, R12) is chosen to match the heat loss characteristic of the apparatus, so that the value of I 1  depends on the time interval between successive energizations. The signals I 1 , I R , I F  are added at the summing point 56 for integration by a capacitor C2 whose voltage is compared with that of the Zener diode Z1 in a differential amplifier T11, T12, the arrangement being such that the voltage across a capacitor C4 is a linearly increasing ramp whose slope varies in accordance with the voltage on the capacitor C2. Capacitor C4 is automatically discharged in synchronism with each half cycle of the supply 42 by a circuit including transistors T4, T5 and diodes D5, D6, D10, -- D12 supplied via lines 46, 47 from the transformer 40. The ramp voltage from capacitor C4 is applied to a Schmitt trigger T7, T8 which produces firing signals for the switch 48 via the transistor T13 and pulse transformer 59. Variation of the signal I F  and/or I 1  thus varies the voltage of C2, the slope of the ramp voltage and hence the firing angle of the switch 48. A resistor R6 reduces the signal I R  at a linear rate with increasing supply voltage, to compensate for the increasing ratio of R.M.S. voltage to average voltage (represented by the voltage across the capacitor C2) under these conditions.