Patent GB-1323441-A (GB)
Title: Converter for changing alternating current into direct current
IPC: H02M7/06
Filed: 1970-06-12
Abstract: 1323441 Supply systems for discharge lamps MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 12 June 1970 [20 June 1969 (2) 29 Aug 1969] 28673/70 Heading H2H [Also in Division G3] An A.C. to D.C. converter for supplying the lamps Lal, La2 of a copying machine comprises a transformer T1 whose primary winding is supplied from an A.C. source 18 through a bidirectional triode thyristor TH and whose secondary winding charges a capacitor C1 through a rectifier 4, an oscillator 10 supplying firing pulses to the thyristor TH, and a gate circuit 9 to which are fed three signals respectively representative of (1) a voltage phasedisplaced relative to that of the source 18; (2) a voltage representative of that of the capacitor C1; and (3) a voltage representative of the comparison between the capacitor voltage and a predetermined reference voltage, to which it is to be charged, the arrangement being such that signal (1) retards the firing angle of thyristor TH to prevent excess current when capacitor C1 is discharged, signal (2) gradually advances the firing angle as the capactior is charged, and signal (3) maintains the capacitor voltage at the reference value until it is discharged through the lamps Lal, La2. Signal (1) is supplied by a secondary winding of a transformer T2 through a capacitor C3 and rectifier D8, the primary winding being supplied from the source 18. Signal (2) is obtained from a variable resistor VR4 forming part of a voltage divider R1, VR1 ... VR4. A tapping on VR1 supplies a signal via resistor R2 to the base of a transistor Q1 for comparison with the reference voltage of a Zener diode ZD, the transistor Q1 controlling another transistor Q2 whose collector supplies the signal (3). The oscillator 10 comprises RC circuit R4, C2, a uni-junction transistor Q6 and a pulse transformer T3, oscillations being prevented when a switch SW2 is in position a causing transistors Q3, Q4 of the gate circuit 9 to be cut off and the transistor Q5 to be conductive. Movement of switch SW2 to terminal b reverses the latter condition, the base potential of transistor Q4 determining the commencement of oscillations and varying with signal (1) from C3, D8, which signal is about 90 degrees out of phase with the voltage of source 18 to fix the firing angle of the thyristor TH at this value for the first half cycle Fig. 3 (not shown). This angle is progressively advanced in subsequent half cycles as the signal from VR4 increases from zero to change the base potential of transistor Q4. When the signal from VR1 exceeds the sum of the breakdown voltage of Zener diode ZD and the volt drops of members Q1, D4, transistors Q1, Q2 conduct to change the gate circuit 9 to its oscillation-preventing condition. The capacitor C1 is discharged through the lamps Lal, La2 by changeover of a switch SWC to position b, which action produces the following sequence of operation: (1) discharge of a capactior C6 to fire a silicon controlled recifier SCR2; (2) discharge of a capacitor C5 through a pulse transformer T4 to apply a starting pulse to the trigger coils of the lamps; (3) conduction of the lamps to allow discharge of capacitor C1 therethrough. To prevent the voltage of capacitor C1 exceeding an upper limit due to loss of the signal from VR1 (caused for example by poor contact or resistor failure) and the consequent failure of the comparator 8 to regulate at the predetermined value, an overcharge protection circuit 14 and members VR2, D3 are provided. When the upper limit is reached, the diode D3 applies the voltage at the junction of VR2, VR3 to a transistor Q7 which conducts and makes conductive another transistor Q8 to fire a silicon controlled rectifier SCR1. The latter enrgises a relay Ry which opens a switch SW4 to de-energize the transformer T1. Simultaneously current passes through members Q8, D10, R8 to fire the rectifier SCR2 and so discharge capacitor C1 through the lamps La1, La2. Contact or resistor failure in VR4 produces current flow through members D6, D5 and R5 to maintain the action of the signal (1), although the charging time is extended. In a modification Fig. 4 (not shown) members 9, 10 employ a breakdown diode and a smoothing circuit arranged so that the voltage of capacitor C1 determines the amount of smoothing, which is progressively increased with the increase in voltage to advance the firing angle of the thyristor TH. Other modifications described: (1) replacement of TH by a pair of anti-parallel connected silicon controlled recifiers, Fig. 5a (not shown); (2) replacement of uni-junction transistor Q6 by a breakdown diode; replacement of TH by a bidirectional diode thyristor, Fig. 5b (not shown); (3) the connection of members VR1, VR3, VR3 in the emitter circuit of a transistor and the use of a Schmitt trigger circuit in the comparator 8, Fig. 5c (not shown); (4) replacement of members SCR1 and Q8 by a reverse blocking triode thyristor, Fig. 5d (not shown); (5) omission of the transistor Q5 and the use of transistor Q4 to control capacitor C2, Fig. 5e (not shown); (6) the use of an astable multivibrator as the oscillator 10, Fig. 5f (not shown).