Patent GB-1302542-A (GB)
IPC: H02M3/00
Filed: 1969-04-10
Abstract: 1302542 Control systems for low voltage loads SEVCON ENG Ltd 9 April 1970 [10 April 1969] 18485/69 Heading H2H [Also in Division G3] An arrangement for deriving a low voltage D.C. supply from a higher voltage D.C. source comprises a semi-conductor switching bridge circuit which includes a capacitor, the bridge circuit input being connected to the higher voltage D.C. source and the output to the load, the arrangement being operated to charge the capacitor alternately in opposite directions, such that each time the charging current direction is changed, the energy stored in the capacitor is supplied to the load. Initially thyristors 19, 21 are turned on, the load 7 then being energized from a high voltage battery 3 by way of thyristor 21, capacitor 23, thyristor 19, inductor 9 and fuse 100 thus charging the capacitor 23 with its lower plate positive. When the capacitor 23 is fully charged thyristors 19, 21. turn off and thyristors 15, 17 are triggered on after a suitable delay. The voltage of the battery 3 is now impressed on a circuit including thyristor 15, capacitor 23, thyristor 17, load 7, inductor 9 and fuse 100. The energy in the capacitor 23 is thus transferred to the load and the capacitor charged in the reverse direction. Thyristors 15, 17 turn off when the capacitor is fully charged and the sequence repeated, the voltage and current supplied to the load being a function of the rate of switching of the pairs of thyristors 1. The switching rate is determined by a control circuit which includes a Zener diode 37, unijunction transistor 39 and a potential divider 85, 87, 89. On switching on current flows by way of resistors 32, 33, 34 and Zener diode 37 to charge a capacitor 11 thus establishing a Zener voltage across the bases of the unijuntion transistor 39. A capacitor 45 charges and after a time determined by the time constant of the RC circuit 43, 45 the unijunction transistor 39 fires thus turning on a transistor 51. Turn on of the transistor 51 energizes a transformer 55, the secondary winding 57 of which turns on thyristor 21. Turn on of the transistor 51 also causes turn on of a further transistor 61 thus developing a voltage across a resistor 63 which gates on the thyristor 19. When the capacitor 23 becomes fully charged the thyristors 19, 21 turn off and a voltage is established across a circuit which includes a capacitor 69 and a diode 75. When the capacitor 69 is sufficiently charged a Shockley diode 77 breaks down whereupon the thyristor 15 is turned on. The discharge circuit of capacitor 69 includes the primary winding 71 of transformer 73, the secondary winding 74 then gating on the thyristor 17. The switching rate, and hence the current supplied to the load, is thus determined by the various time constants in the control circuit. The potential divider 85, 87, 89 measures the load voltage and at a point set by the potentiometer 87 a transistor 91 becomes effective to partly shunt the capacitor 45 thus altering its charging rate and the time constant of the circuit 43, 45. Thus this load voltage is controlled to constancy. Should the load voltage become excessive a Zener diode 97 breaks down to turn on a thyristor 95 to rupture the fuse 100. The battery 3 may be the power source of a fork-lift truck and the load 7 the low voltage lighting circuits thereon.