Ballast circuits for discharge lamps

[57] 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 threephase 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.

Claims, 8 Drawing Figures

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PATENTED JAN 2 21874

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PHASE A.C.

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SHEET 3 OF 4

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TIME

3,787,751

BALLAST CIRCUITS FOR DISCHARGE LAMPS

BACKGROUND OF THE INVENTION

This invention 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 /and at 2/. 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 an alternating 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:

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

FIG. 1 shows three input terminals 10 for connection to a three-phase alternating current supply. The terminals 10 are connected through fuses 12 to respective capacitors 14, and the capacitors 14 are connected to three input terminals of a three-phase bridge rectifier 16. The rectifier 16 comprises six diode elements 18 and has two output terminals 20. In this case two lamps 22 are connected in series, together with a surgelimiting 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 95 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 VT volts, is sufficient to strike two normal 400 watt high pressure mercury vapour lamps in series.

FIG. 4 shows a similar ballast 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

3,787,751 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 (YaJ/C) should be approximately equal to the inductor reactance (2 ir/L). This results in a larger capacitor than is normally used for power factor correction.

The open-circuit prestrike voltage V₄₂ across the lamp is seen to consist of the sum of the open-circuit voltages V_4e and V₄₈ across the diodes 46 and 48 respectively. The forms of the voltages V₄₈, V₄₈ and V₄₂ are shown in FIG. 8. The peak open-circuit prestrike voltage (V₄₂) produced by the circuit of FIG. 5 with a 240 volts r.m.s. input is 240- V~2 volts, Which is sufficient to strike a high pressure mercury vapour discharge lamp.