GB2140229A - Discharge lamp start and supply circuit - Google Patents

Abstract

Discharge lamps and in particular low pressure sodium (SOX) lamps have been started by circuits having a two terminal electronic starter and a series connected choke for ballast stabilisation. Particularly with higher power lamps and if supply voltage drops the available voltage may be insufficient to ensure lamp reignition throughout the run-up period. It is also possible for the lamp to settle in a low power high voltage state, never attaining full operating power. To correct this state in a circuit in which the lamp 1 in parallel with the ignitor 2 is connected in series with a ballast 3a, 3b across the mains supply with a power factor correction capacitor 6 the circuit is adapted to increase the lamp voltage at which peak power is obtained. To that end a tapping 7 is provided on the choke and the power factor connection capacitor 6 is connected between the tapping and the opposing lamp terminal. A high frequency blocking choke 8, or a tuned filter network, may be provided to prevent ignition pulses being fed into the mains supply. <IMAGE>

Description

SPECIFICATION Lamp start and supply circuit The present invention relates to a starting and operating circuit for discharge lamps such as low pressure sodium vapour discharge lamps. It is particularly applicable to the higher power low pressure sodium vapour lamps of the type known as SOX lamps.

The electrical characteristic8 of such lamps are specified in relevant standards, in Britain the British Standard (BS) 3767. The lamps require high cold start voltages and run-up periods of about 1 5 minutes. During the run-up period the lamp voltage is generally higher than the operating voltage and exhibits a peak several minutes after starting due to sodium vapourisation. This is followed by a decline to the operating voltage. Generally the current falls as the voltage rises and vice versa.

It has been the practice to operate the lamps from leakage-reactance autotransformers which provide a high voltage for starting and also provide the stabilisation required to limit lamp current and power to the correct values. Such circuits using leakage resistance autotransformers have a number of disadvantages including a low operating power factor requiring a large capacitor for correction.

More simple and efficient circuits have thus been in increasing use, for such lamps, the circuits having a two terminal electronic ignitor device to start the lamp and a series connected choke coil to provide ballast stabilisation. A smaller capacitor is required for power factor correction.

Such a circuit is shown in Figure 1. The lamp 1 has connected in parallel therewith the igniter circuit 2. Circuits for such ignitors are well known and the exact form is not critical. its function is to generate high frequency pulses or oscillations of high voltage to start the discharge in the lamp. A typical device will generate 700-1000 volts peak and will be arranged to stop osciliating when the lamp has started.

A choke ballast 3 is connected in series with the parallel lamp and ignitor across mains input terminals 4 and 5. A power factor correction capacitor 6 is also connected across the input terminals in parallel with the other components and during normal operation, when the lamp has fully run-up, corrects the lagging power factor to a value greater than 0.85.

The reignition pulse amplitude is largely dependent on the instantaneous mains voltage available (in Britain 240V RMS + tolerance) at the end of each lamp voltage half cycle Ithe supply voltage leads the lamp voltage by about 600).

Thus during run-up the supply voltage must be sufficient to supply the increasing lamp reignition voltage requirements or the lamp may not reignite.

We have found that in a circuit according to Figure 1 the voltage available may be insufficient to ensure lamp reignition throughout the run-up period.

This leads to a problem, in particular with SOX lamps rated at more than 35 watts (240 volts) in that after a short period of operation the reignition and RMS lamp voltages increase so much that the voltage available from the supply can no longer sustain the discharge voltage and the lamp may extinguish. This effect is more pronounced if the supply voltage is below 240 volts AC.

A further problem with the higherpower lamps is that during run-up the lamp voltage may carry lamp power above a peak in its characteristic. In this event the lamp cools, the lamp voltage falls and power is raised again on reapproaching the peak in the power characteristic. The effect of this is a low frequency damped oscillation with the lamp settling in a low power high voltage state and it may never run-up to its full operating power.

The problem may be seen from Figure 2 which iliustrates the load voltage as a function of load power and as a function of load current. It can be seen that: as the current tends to zero the voltage available for reignition is limited to the 240 volt supply voltage; the current decreases rapidly with increasing lamp voltage and may become insufficient to sustain the discharge; and if the lamp voltage rises above 1 50 volts at which the power peaks, the lamp may not run up to full power.

It is an object of this invention to provide a start and supply circuit for low pressure sodium vapour discharge lamps and other discharge lamps exhibiting similar problems for which the effects of the disadvantages discussed herein before are at least reduced.

According to the invention there is provided a starting and operating circuit for a discharge lamp, the circuit having mains supply terminals, a ballast and an ignitor circuit connected in series across the mains supply terminals, means for connecting the said lamp in parallel with the ignitor, to be ignited by pulses or oscillations therefrom, and a power factor correction capacitor, wherein means is provided for increasing the lamp voltage at which peak power is obtained.

The means for increasing the lamp voltage preferably comprises the provision of a tapping on the ballast and the connection of the power factor capacitor between said tapping and the mains supply terminal connected to the side of the ignitoropposing said ballast.

Means may be provided for increasing the shunt impedance of the circuit, comprising mains supply, ballast and power factor correction capacitor, to the ignitor pulses or oscillations.

A suitable means for increasing the shunt impedance is a filter, for example a choke filter or a filter network connected between the choke ballast and the igniter.

The invention is particularly applicable to SOX lamps of 35 watt or greater power rating.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 has the significance explained hereinbefore, Figure 2 has the significance explained hereinbefore, Figure 3 is a lamp start and supply circuit in accordance with this invention, and Figure 4 is a graph of load voltage as a function of load power and load current for the circuit of Figure 2 together with the curves of the graph of Figure 2.

There is shown in Figure 3 the modified circuit provided by this invention. In this Figure those parts found also in Figure 1 are identified by the same reference numerals. The circuit would particularly suit a 90 watt 240 v SOX lamp. The most significant difference is that the choke ballast 3 is effectively divided into parts 3a and 3b by being provided with a tapping 7. The power factor correction capacitor 6 is then connected between the tapping 7 and power supply terminal 5.

The modification provides an element of semiresonance as is usual in the well known semiresonant starter, thus raising the open circuit voltage available for re-ignition, relatively raising the current for lamp voltages above about 120 volts and raising the lamp voltage at which the power peak occurs to 200 volts. This can be seen in Figure 4 which shows in solid line the load voltage as a function of load power and as a function of load current for the circuit of Figure 3 and also shows the curves of Figure 2 in broken iine for comparison.

The effect, which can be seen in Figure 4, is then that a lamp with a voltage of about 1 50 volts will run at the same power with the Figure 3 circuit as with that of Figure 1. However if the voltage increases the power will continue to increase where with the Figure 1 circuit it would decrease. Thus a lamp having a higher voltage can not only run-up without extinguishing but can also operate at correct power.

There is a minor disadvantage with the circuit as so far explained in that the shunt impedance, to high frequency of the circuit of mains/ballast/power-factor-corrections-capacitor, is reduced so that the pulses or oscillations of the ignitor can be fed into the mains supply. They may then not have sufficient amplitude to start the lamp. To overcome this the circuit of Figure 3 includes a small high-frequency blocking choke filter 8 between the ballast 3 and the parallel circuit of lamp 1 and ignitor 2. The choke filter 8 may be replaced with a filter network having a plurality of components tuned to reject the frequencies generated by the ignitor. In either case the filter may conveniently be built into the ignitor housing.

It should be noted that the connection of a power factor correction capacitor to a ballast tapping is known for fluorescent lamp circuits to enhance the starting voltage. Fluorescent lamps do not encounter the special run-up problems due to rising lamp voltage as low pressure sodium vapour discharge lamps; they require starting devices or ignitors which provide preheat current to their electrodes before starting and include, for example, a switch to conduct such preheat current. The problems which the present invention set out to solve and the concept cf the solution are therefore significantly different from the superficially similar circuits of fluorescent lamps.

The invention is applicable to other types of discharge lamps (wherein the term discharge lamp is not intended to include fluorescent lamps) for which these or similar problems arise.

Claims (9)

1. A starting and operating circuit for a discharge lamp, the circuit having mains supply terminals, a ballast and an ignitor circuit connected in series across the mains supply terminals, means for connecting the said lamp in parallel with the ignitor, to be ignited by pulses or oscillations therefrom, and a power factor correction capacitor, wherein means is provided for increasing the lamp voltage at which peak power is obtained.

2. A circuit according to Claim 1 in which the means for increasing the lamp voltage comprises the provision of a tapping on the ballast and the connection of the power factor capacitor between said tapping and the mains supply terminal connected to the side of the ignitor opposing said ballast.

3. A circuit according to either of the preceding Claims including means for increasing the shunt impedance to the ignitor pulses or oscillations of the circuit comprising mains supply, ballast and power factor correction capacitor.

4. A circuit according to Claim 3 in which the means for increasing the shunt impedance is a high frequency blocking filter.

5. A circuit according to Claim 4 in which the filter is connected between the choke ballast and the ignitor.

6. A circuit according to Claim 4 in which the filter is a filter network connected between the choke ballast and the ignitor.

7. A circuit according to any of Claims 4-6 in which the filter and ignitor are incorporated in a single housing.

8. A starting and operating circuit for a discharge lamp, the circuit being substantially as herein described with reference to Figures 3 and 4 of the accompanying drawings.

9. In combination a low pressure sodium lamp and a starter circuit according to any of the preceding Claims.

1 0. A combination according to Claim 10 in which the lamp is a 90 watt 240 volt SOX lamp.