US3787751A

US3787751A - Ballast circuits for discharge lamps

Info

Publication number: US3787751A
Application number: US00279445A
Authority: US
Inventors: V Farrow
Original assignee: Thorn Electrical Industries Ltd
Current assignee: Thorn Electrical Industries Ltd
Priority date: 1972-08-10
Filing date: 1972-08-10
Publication date: 1974-01-22
Anticipated expiration: 1991-01-22

Abstract

A ballast circuit for a discharge lamp is arranged to supply a uni-directional voltage to the lamp which is made up of at least two current components which differ in phase. Flicker at the supply frequency f and at 2f is thus much reduced. A preferred ballast for use with a three-phase supply employs ballast capacitors in each of the three supply lines connected to a three-phase bridge rectifier, the output of which is applied to the lamp. A preferred ballast for use with a single phase supply includes a ballast capacitor and a ballast inductor connected through respective bridge rectifiers which have some components in common.

Description

United States Patent Farrow Jan. 22, 1974 [54] BALLAST CIRCUITS FOR DISCHARGE 3,249,809 5/1966 Steiner! 315/010. 5 3,676,735 7/1972 Roche 315/207 LAMPS [75] Inventor: gictfr grancis Farrow, London, Primary Examiner Herman Karl Saalbach ng an Assistant ExaminerJames B. Mullins [73] Assignee: Thorn Electrical Industries Limited, Attorney, Agent, or F inn-Robert OCOImeII;

London, England Dike, Bronstein, Roberts & Cushman PP No.3 279,445 A ballast circuit for a discharge lamp is arranged to supply a uni-directional voltage to the lamp which is 52 us. (:1 315/137, 315/205, 315/207, made "P of at least two Current cmnponems which 315/227 R, 315/244, 315/283, 315/D1G. 5 fer in phase. Flicker at the supply frequency fand at 51 Int. Cl. H05b 41/16 f is thus much A Preferred ballast for use 58 Field 61 Search... 315/137, 139, 141, 205, 207, w a three-Phase pp y p y ballast capacitors 315/244" 7 R, 7 283, DIG 5 In each of the three supply lines connected to a threephase bridge rectifier, the output of which is applied 5 References Cited to the lamp. A preferred ballast for use with a single UNITED STATES PATENTS phase supply includes a ballast capacitor and a ballast 5 inductor connected through respective bridge rectifiggY/gggg 315/l37 ers which have some components in common- 3,054,884 9/1962 Manz et al. 315/207 X 2 Claims, 8 Drawing Figures 5 F US E I I O O I F E 3 1 PHASE I I A. C. F 1.

14 1 BRIDGE 10 12 RECTIFIER l l IF PATENTEU SHEET 1 (1F 4 FUSES BRIDGE RECTIFIER 1IIL 14 F/G. 7f

LA MP5 LINE CURRENT SHEET 2 OF 4 ounykwmmvr A1 V V 3 PHASE A.C. Q R Q Q 3 PHASE 3 M INDUCTOR 7 *K30 [D LAMP L BRIDGE RECTIFIER PATENTEU P in 3, 787 751 SHEET 3 OF 4 SINGLE PHASE AC.

1 BALLAST CIRCUITS FOR DISCHARGE LAMPS BACKGROUND OF THE INVENTION Thisinvention relates to ballast circuits for discharge lamps.

Gas discharge lamps are conventionally operated from alternating current sources by using a ballast circuit. A basic type of ballast circuit includes an inductor in series with the lamp to absorb the difference between the supply voltage and the lamp voltage; because the inductor is reactive it consumes a relatively. small, though significant, amount of power. A capacitor may be connected across the series combination of the inductor and lamp to correct the power factor presented to the supply.

Modifications of this basic circuit include the use of auto-transformers and auto-transformers with built-in leakage reactance. However, capacitors have not been used in place of the inductor because they give rise to undesirable current waveforms, the rapid charging and discharging being sufficient to damage the lamp and also produce marked flicker. Thus, where a leading power factor is required inductors and capacitors are used in series, with the capacitive reactance being greater than the inductive reactance. The inductive reactance has the effect of improving the lamp current waveform while the overall ballast effect is capacitive.

All these ballasts produce flicker in the light output at the mains supply frequency f and at 2f. The psychological implications of this flicker are not fully understood, but it is known that the flicker can cause some considerable distress.

We have appreciated that one way of eliminating flicker is to operate the lamp on a direct current supply. We have therefore been concerned to provide circuits which will enable a lamp to be run on an effectively direct current from an alternating current source, while still providing a ballast effect and not being unnecessarily complex, and in such a way that unacceptable flicker is at least substantially reduced.

SUMMARY OF THE INVENTION According to this invention there is provided a ballast circuit for a discharge lamp, comprising input terminals for receiving analternating voltage, output terminals for applying a uni-directional voltage to a discharge lamp, and means connected between the input and output terminals for applying to the output terminals at least two current components which differ in phase.

Another disadvantage of known ballasts is that they require a bulky iron-cored inductor, which is expensive and which results in an overall efficiency of only about 90 percent, measured as a percentage of the power input from the source which actually reaches the lamp. An added advantage of one ballast circuit embodying this invention is that it does not require the use of an inductor.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example and with reference to the accompanying drawings, in which: I

FIG. 1 is a circuit diagram of a ballast circuit for operating a discharge lamp on direct current from a threephase alternating current source, the ballast circuit not including any inductors;

FIG. 2 is a waveform diagram showing the line current and phase voltage for one phase of the supply in FIG. 1;

FIG. 3 is a waveform diagram showing the output current from the ballast circuit of FIG. 1;

FIG. 4 is a circuit diagram of another ballast circuit for operating a discharge lamp on direct current from a three-phase alternating current source;

FIG. 5 is a circuit diagram of a ballast circuit for operating a discharge lamp on direct current from a single-phase alternating current source;

FIG. 6 is a waveform diagram showing the input current to the ballast circuit of FIG. 5;

FIG. 7 is a waveform diagram showing the output current from the ballast circuit of FIG. 5; and

FIG. 8 is a waveform diagram showing certain voltages in the circuit of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS limiting resistor 24, across the terminals 20.

The alternating currents through each of the capacitors 14 are identical in form but phase displaced relative to each other by 120. The line current relative to phase voltage for one capacitor is shown in FIG. 2. The output current from the bridge rectifier 16 is shown in FIG. 3 and is seen to consist of a uni-directional current with a ripple the frequency of which is six times the supply frequency. The ripple amplitude is relatively small, the ratio A:B being typically about 1:3.

At this ripple frequency the afterglow of the lamp phosphors is significant, and this gives a further reduction in light output ripple. Thus this ballast circuit provides a very substantial reduction in ripple without the use of a complex ballast circuit. Surprisingly, it is found to operate well while using capacitors alone without inductors, and the efficiency is thereby increased to upwards of percent. The ballast circuit provides a leading power factor which can be used to help correct a lagging load in factory installations.

If the supply voltage is 415 volts,the prestrike voltage generated. namely 415 X\/ 2 volts, is sufficient to strike two normal 400 watt high pressure mercury vapour lamps in series.

FIG. 4 shows a similarballast circuit in which the three capacitors 14 are replaced by a three-phase inductor 30. In this case the reactance presented to the supply is inductive, and the circuit will provide a lagging power factor which can be used in conjunction with the capacitor ballast circuit of FIG. 1 to at least partially correct the power factor. The three-phase inductor 30 could be replaced by three individual inductors connected in the three lines. In other respects the ballast circuit of FIG. 4 is similar to that of FIG. 1, and includes a bridge rectifier the output of which is connected to a discharge lamp.

Another ballast circuit is shown in FIG. 5. Two input terminals 40 are connected to a discharge lamp 42 through a double bridge circuit 44. The circuit 44 may be considered as two superimposed bridge circuits, with half of each bridge being common. The common elements are

diodes

46 and 48 which are connected in opposite senses between one of the terminals 40 and respective output terminals 41 which are connected to the lamp 42. To the other terminal 40 are connected both a capacitor 49 and an inductor 50. Four

further diodes

52, 54, 56 and 58 are then connected as follows: diode 52 between one end of the lamp 42 and the capacitor 49, diode 54 between the same end of the lamp and the inductor 50, diode 56 between the other end of the lamp and the capacitor, and diode 58 between the said other end of the lamp and the inductor. The cathode of.

diodes

46, 52 and 54 are all connected together and to one end of the lamp, and the anodes of

diodes

48, 56 and 58 are all connected together and to the other end of the lamp.

Thus the lamp is supplied through both the capacitor 49 and the inductor 50 with current which leads and lags the voltage respectively, and the lamp 42 thus receives two current components of differing phase. The effect is to produce an input line current as shown in FIG. 6, and an output current as shown in FIG. 7, in which alternate peaks correspond to component currents through the capacitor 49 and inductor 50 respectively. The fundamental of the line current is in phase with the supply voltage. It will be seen that the output current includes a large d.c. component and a ripple of predominantly four times the supply frequency.

To reduce the twice-line-frequency ripple component to a minimum, the capacitor reactance /21fC) should be approximately equal to the inductor reactance (2 rrfL). This results in a larger capacitor than is normally used for power factor correction.

The open-circuit prestrike voltage V across the lamp is seen to consist of the sum of the open-circuit voltages V and V across the

diodes

46 and 48 respectively. The forms of the voltages V V and V are shown in FIG. 8. The peak open-circuit prestrike voltage (V produced by the circuit of FIG. 5 with a 240 volts r.m.s. input is 240' 2 volts, which is sufficient to strike a high pressure mercury vapour discharge lamp.

I claim:

1. A ballast circuit for a discharge lamp, comprising:

first and second input terminals for receiving an alternating voltage;

first and second output terminals for applying a unidirectional voltage to a discharge lamp;

two oppositely-poled diodes connected between said first input terminal and said first and second output terminals respectively;

a capacitor and an inductor both connected to said second input terminal;

two oppositely-poled diodes connected between said capacitor and said first and second output terminals respectively; and

two oppositely-poled diodes connected between said inductor and said first and second output terminals respectively.

2. In combination with a discharge lamp, a ballast circuit comprising:

three input terminals for connection to a three-phase alternating current supply;

two output terminals for applying a uni-directional voltage to said discharge lamp;

three ballast capacitors each connected to a respective input terminal; and

a three-phase bridge rectifier connected between said capacitors and said output terminals, whereby said voltage applied to said discharge lamp has a ripple frequency which is six times the frequency of the alternating current received by said input terminals.

Claims

1. A ballast circuit for a discharge lamp, comprising: first and second input terminals for receiving an alternating voltage; first and second output terminals for applying a uni-directional voltage to a discharge lamp; two oppositely-poled diodes connected between said first input terminal and said first and second output terminals respectively; a capacitor and an inductor both connected to said second input terminal; two oppositely-poled diodes connected between said capacitor and said first and second output terminals respectively; and two oppositely-poled diodes connected between said inductor and said first and second output terminals respectively.

2. In combination with a discharge lamp, a ballast circuit comprising: three input terminals for connection to a three-phase alternating current supply; two output terminals for applying a uni-directional voltage to said discharge lamp; three ballast capacitors each connected to a respective input terminal; and a three-phase bridge rectifier connected between said capacitors and said output terminals, whereby said voltage applied to said discharge lamp has a ripple frequency which is six times the frequency of the alternating current received by said input terminals.