Method of operating a high-pressure metal vapor discharge lamp and circuit arrangement for carrying out this method

A method of operating a high-pressure metal vapor discharge lamp at a periodic operating voltage of more than 500 Hz wherein the operating voltage is suddenly varied in phase at intervals, and preferably also wobbled in frequency, in order to avoid acoustic resonances in the lamp.

This invention relates to a method of operating a high-pressure metal 
vapour discharge lamp at a periodic operating voltage of more than 500 Hz. 
A periodic operating voltage is, for example, to be understood to mean a 
sinusiodal, triangular or rectangular voltage. 
In the operation of high-pressure metal vapour discharge lamps, such as 
high-pressure sodium vapour discharge lamps, mercury vapour discharge 
lamps and--metal--halide discharge lamps, above the AC line frequency, arc 
instabilities occur in the lamps in given frequency ranges. These 
instabilities are caused by acoustic resonance of the discharge arc in the 
discharge tube of the lamp. In some lamp types, arc instabilities occur in 
very wide frequency ranges of the operating voltage. Therefore, it has 
been assumed so far that a stable operation of these lamps is possible 
only in relatively narrow frequency bands. Frequency bands--of a width 
from 3 to 5 kHz--free from instabilities between 20 and 50 kHz for 
miniature metal vapour discharge lamps have been described, for example, 
in U.S. Pat. No. 4,170,746. 
Moreover it is known from DE-OS No. 2,335,589 to modulate the 
high-frequency supply voltage (20 kHz, for example) with a low-frequency 
signal in order to avoid acoustic resonances causing arc instabilities in 
low-pressure and high-pressure mercury vapour discharge lamps. However, in 
the last mentioned DE-OS it is admitted that even a high degree of 
frequency modulation is not sufficient to suppress resonance effects 
completely. 
According to the U.S. Pat. No. 3,890,537 acoustic resonances in mercury 
vapour discharge lamps operated with a pulsatory current are avoided in 
that the chopper frequency is wobbled automatically. There are resonance 
frequencies, however, at which wobbling of the chopper frequency fails. 
In particular in small high-pressure metal vapour discharge lamps, it has 
been found that wobbling of the operating voltage alone has practically no 
effect. 
Therefore, an object of the invention is to provide a method of operating 
high-pressure metal vapour discharge lamps above 500 Hz in which 
practically no acoustic resonances and arc instabilities caused thereby 
occur in the lamps over a very large frequency range of, for example, 500 
Hz to 200 kHz. 
In a method of the kind mentioned in the preamble, this is achieved 
according to the invention in that the operating voltage is constantly 
subjected to sudden phase variations. 
The constant phase variations disturb the excitation of acoustic 
resonances, which cause the arc instabilities. 
Preferably, the operating voltage is subjected to phase jumps in the order 
of 50.degree. to 130.degree., in particular of 90.degree.. 
The phase variation may take place periodically. However, side bands then 
occur in the frequency spectrum of the operating voltage, which may in 
turn lead to instabilities. The amplitude of these side bands may be 
reduced, however, in that the phase variation is effected statistically, 
i.e. at irregular distances. For excitation, instabilities require a given 
threshold value. Stastistic phase variation causes them to fall below this 
threshold value. 
Preferably the phase variation takes place after each 1/2 to 20 periods of 
the operating voltage. 
As has been stated above, with periodic phase variations, frequency bands 
with arc instabilities can occur, in particular in miniature metal halide 
discharge lamps, in the frequency range of the operating voltage, which is 
free from instabilities with statistic phase variation. 
In an advantageous further embodiment of the method according to the 
invention of operating small high-pressure metal vapour discharge lamps 
for frequencies of more than 5 kHz, in which the periodic operating 
voltage is subjected periodically to sudden phase variations and yet in a 
large frequency range of, for example, 20 to 60 kHz practically no 
acoustic resonances and arc instabilities caused thereby occur in the 
lamps, if the operating voltage is wobbled in frequency at the same time. 
Surprisingly it has been found that due to wobbling of the operating 
voltage frequency, which has per se substantially no effect with 
simultaneous periodic phase variations, the frequency range free from 
acoustic instabilities as compared with the stability range with a 
periodic phase variation alone is enlarged. 
The expression "small high-pressure metal vapour discharge lamps" is to be 
understood to mean lamps having a discharge tube volume of up to at the 
most 2 cm.sup.3. 
In a preferred further embodiment of the method according to the invention, 
the operating voltage comprises a direct voltage component. The resultant 
pulsatory direct current operation can be achieved by an electronic 
ballast unit more simply than with a pure alternating current operation. 
In small lamps, in particular miniature metal halide discharge lamps, a 
pulsatory direct current operation does not give rise to problems because 
in this case, in contrast with high-pressure discharge lamps having a 
large electrode distance, the axial demixing of the substances by 
cataphoresis substantially does not play a part. 
In the case of the pulsatory direct current operation, the operating 
voltage can exhibit phase jumps of 100.degree. to 250.degree., preferably 
180.degree.. 
The invention further relates to a circuit arrangement for carrying out the 
method described. This circuit arrangement according to the invention is 
characterized in that a periodic signal of a signal generator is supplied 
on the one hand directly and on the other hand through a phase shifter to 
an analogue multiplexer and then through a power amplifier to the 
high-pressure metal vapour discharge lamp as the operating voltage, the 
analogue multiplexer connecting by means of a digital pulse generator 
either periodically or statistically alternately directly or through the 
phase shifter the signal generator to the power amplifier. 
With a simultaneous wobbling of the operating voltage, this circuit 
arrangement is preferably characterized in that the analogue multiplexer 
connects periodically the signal generator to the power amplifier and the 
signal generator is controlled by a voltage generator of constant 
amplitude (U.sub.w) and frequency (f.sub.w) so that its output signal is 
varied in frequency about a central frequency f.sub.o with a frequency 
sweep of .+-..DELTA.f. 
In order to superimpose a direct voltage on the operating voltage, 
according to a further embodiment of the circuit arrangement in accordance 
with the invention, a direct voltage source is connected between the power 
amplifier and the lamp.

In the circuit arrangement of FIG. 1, a signal generator 1 produces a 
sinusoidal output voltage of constant frequency f, which from the junction 
2 is supplied on the one hand through a lead 3 directly and on the other 
hand through a phase shifter 4 to an analogue multiplexer 5. The output 6 
of the analogue multiplexer 5 is connected to a linear power amplifier 7, 
to which is connected the series combination of a high-pressure metal 
vapour discharge lamp 8 and a ballast impedance 9. 
The analogue multiplexer 5 acts as a switch having two switching contacts 
S1 and S2. Through leads 10 and 11 the analogue multiplexer 5 is 
controlled by a digital pulse generator 12. An inverter 13 is inserted 
into the lead 11. When the output of the digital pulse generator 12 is 
switched to a LOW signal (L), L also is present at the lead 10, whereas 
the lead 11 is switched through the inverter 13 to a HIGH signal (H). In 
this condition the analogue multiplexer 5 closes its switch S1, so that 
the voltage of the signal generator 1 is directly applied to the output 6. 
When the output of the digital pulse generator 12 is switched to H, H is 
also present at the lead 10, whereas the lead 11 is set to L through the 
inverter 13. In this condition the switch S2 of the analogue multiplexer 5 
is closed so that the phase-shifted voltage of the signal generator 1 is 
applied to its output 6. The phase variation of the output voltage of the 
analogue multiplexer 5 with respect to the output voltage of the signal 
generator 1 may be effected by means of the phase shifter 4 by phase jumps 
of 50.degree. to 130.degree., preferably of 90.degree.. The desired phase 
variation may be adjusted in the phase shifter 4. 
When the digital pulse generator 12 supplies a periodic pulse sequence (a), 
the voltage via lead 3 or the voltage subjected to phase variation of the 
signal generator 1 alternately appears at the output 6 of the analogue 
multiplexer 5. The frequency of the phase variation of the output voltage 
of the analogue multiplexer 5 is given by the fundamental frequency 
f.sub.p of the periodic pulse sequence (a) of the digital pulse genertor 
12. A phase variation at the output 6 of the analogue multiplexer 5 thus 
occurs each time after f/f.sub.p periods of the signal generator voltage 
so that the time sequence of the phase variations can be adjusted by 
choice of this ratio. When the H/L change-over of the digital pulse 
generator 12 takes place irregularly, i.e. statistically distributed 
(pulse sequency b), a likewise irregular phase variation of the voltage at 
the output 6 of the analogue multiplexer 5 is obtained. 
The output voltage of the analogue multiplexer 5 thus obtained, which is 
subjected to periodic or statistic phase variations in the form of phase 
jumps is then supplied to the lamp 8 as an operating voltage through the 
linear power amplifier 7. 
In a practical embodiment, the lamp 8 was a 300 W-mercury high-pressure 
discharge lamp. The signal generator 1 produced a voltage of about 3 kHz. 
This frequency lies in an instability range of the lamp concerned. FIGS. 2 
and 3 show the variations in time of the lamp voltage in case of a 
90.degree. phase jump after each 1.5 respectively 5.5 periods of the 
sinusoidal output voltage of the signal generator 1. (The separate phase 
jumps are characterized by small arrows below the voltage variations). By 
these phase variations a stable operation of the lamp can be achieved. 
Also with phase jumps of more or less than 90.degree., for example, 
between 50.degree. and 130.degree., arc instabilities in the lamp can be 
avoided. 
In the circuit arrangement of FIG. 4, as compared with the circuit 
arrangement of FIG. 1, an additional voltage generator 14 is provided 
which produces a periodic, for example sinusoidal, voltage having an 
amplitude U.sub.w and a frequency f.sub.w. This is the input of the 
voltage controlled signal generator 1. The latter in turn produces a 
periodic, in the present case sinusoidal, output voltage, the frequency of 
which is varied about an adjustable central frequency f.sub.o with the 
wobble frequency f.sub.w between f.sub.o -.DELTA.f and f.sub.o +.DELTA.f, 
the frequency sweep .DELTA.f being determined by the amplitude U.sub.w of 
the wobble voltage. This output voltage of the signal generator 1, which 
is permanently varied in frequency, is then applied from the junction 2 on 
the one hand directly through the lead 3 and on the other hand through the 
phase shifter 4 to the analogue multiplexer 5. The output 6 of the 
analogue multiplexer 5 is again connected to the linear power amplifier 7, 
to which the series-combination of a small high-pressure metal vapour 
discharge lamp 8, the ballast impedance 9 and a direct voltage source 15 
is connected. 
When the switch S2 of the analogue multiplexer 5 is closed, the 
phase-shifted and wobbled voltage of the signal generator 1 is applied to 
the output 6. Since the digital pulse generator 12 supplies a periodic 
pulse sequence, alternately the directly wobbled voltage or the wobbled 
voltage varied in phase of the signal generator 1 occurs at the output 6 
of the analogue multiplexer. The frequency of the phase variation of the 
output voltage of the analogue multiplexer 5 is given by the fundamental 
frequency f.sub.p of the periodic pulse sequence of the digital pulse 
generator 12. A phase variation at the output 6 of the analogue 
multiplexer 5 thus occurs each time after approximately f.sub.o /f.sub.p 
periods of the signal generator voltage so that by the choice of this 
ratio the time sequence of the phase variations can be adjusted. The 
output voltage of the analogue multiplexer 5 with periodic phase 
variations in the form of phase jumps thus obtained is then applied 
through the linear power amplifier 7 to the lamp 8 as the operating 
voltage. 
In the circuit arrangement of FIG. 4, an additional direct voltage source 
15 (for example, 100 V) is connected between the power amplifier 7 and the 
lamp 8. As a result, the lamp 8 is operated with a direct current on which 
the operating alternating current supplied by the power amplifier 7 is 
superimposed. In the case of pulsatory direct current operation, the 
frequencies of the voltage modulation and of the power modulation 
correspond to each other. In order to attain a phase jump in power of 
180.degree., the phase jump of the operating voltage therefore also has to 
be 180.degree., in contrast with 90.degree. with a pure alternating 
voltage operation of the lamp. The admissible range for the phase jumps 
amounts to approximately 100.degree. to 250.degree.. 
In a practical embodiment, the lamp 8 was an elliptical 45 W metal halide 
high-pressure discharge lamp having a bulb diameter of approximately 7 mm 
and a bulb volume of approximately 0.5 cm.sup.3. The voltage-controlled 
signal generator 1 produced a voltage with a central frequency f.sub.o in 
the frequency range of from 20 to 50 kHz. The frequency sweep .DELTA.f 
amounted to 500 Hz to 15 kHz and preferably 5 kHz. The wobble frequency 
f.sub.w of the voltage generator 14 was between 30 Hz and 15 kHz and 
preferably 100 Hz. 
FIG. 5 shows the variation as a function of time of the lamp voltage 
U.sub.L in the case of a phase jump of 180.degree. of the wobbled 
sinusoidal output voltage of the signal generator 1 (the separate phase 
jumps are indicated by arrows above the voltage variation). The voltage 
variation may also be such that it does not touch the zero line. With the 
voltage variation shown, a frequency band free from instabilities of 
approximately 25 kHz in width was obtained. On the contrary, the frequency 
band free from instabilities with operation of the lamp at a pure 
sinusoidal operating voltage was only approximately 3 kHz. 
The wide stability frequency band permits of obtaining high tolerances when 
fixing the frequencies of the electronic ballast units and also when 
maintaining the lamp dimensions. 
With the use of the method in electronic high-frequency ballast units, in 
the circuit arrangement according to FIGS. 1 or 4, the circuit part 
composed of the power amplifier 7, the ballast impedance 9 and, as the 
case may be, the direct voltage source 15 is replaced by the power 
electronics of the corresponding ballast unit, which is then controlled by 
the output signal of the analogue multiplexer 5. 
With the method described above, not only longitudinal, but also azimuthal 
and radial acoustic resonances are avoided in high-pressure metal vapour 
discharge lamps. 
The lamps need not comprise a cylindrical envelope, but may alternatively 
have, for example, a spherical or ellipsoidal form.