Ignition circuit for high pressure arc discharge lamps

Apparatus for igniting and operating a high pressure arc discharge lamp includes a pulse generating circuit for generating high voltage pulses for starting the lamp. The pulse generating circuit is comprised of a step-up transformer, a start capacitor, a voltage sensitive bidirectional switch (e.g., a sidac) and an impedance which forms a charge circuit for the capacitor. The capacitor discharges via the switch and a part of the transformer to generate a high voltage pulse which is coupled to the lamp electrodes. An auxiliary capacitor and a serially connected inductor are coupled in parallel with the pulse generating circuit so as to clamp the open circuit voltage at a high level upon generation of the ignition pulse thereby to maintain a level of lamp current sufficient to sustain the discharge arc. This makes the lamp ignition more reliable and extends the lamp life.

BACKGROUND OF THE INVENTION 
This invention relates to a circuit for starting and operating a high 
pressure arc discharge lamp, and more particularly to an ignitor or 
starting circuit which improves the starting characteristic of a "hot" 
dual ended high pressure discharge lamp. 
Two conditions must be fulfilled in the starting process of a gas discharge 
lamp. First, the starting circuit must provide sufficient energy in the 
voltage pulse applied to the lamp electrodes. Second, the circuit must 
allow enough current to follow through in order to bring the electrodes to 
the proper emission temperature. 
If the second condition is not satisfied, there will be an insufficient 
flow of current from the AC power supply source which will result in a 
lowering of the emission temperature of the lamp electrodes which are 
exposed to the high voltage pulses. This condition will cause sputtering 
of the electrodes with a concomitant decrease in the life expectancy of 
the discharge lamp. 
It is generally known that a high voltage pulse of a value several times 
the lamp operating voltage is required to start or ignite certain types of 
high pressure gaseous discharge lamps such as a high-pressure sodium 
discharge lamp. U.S. Pat. No. 3,407,334 (10/22/68) in the name of O. G. 
Attewell, U.S. Pat. No. 3,963,958 (6/15/76) in the name of J. A. Nuckolls 
and U.S. Pat. No. 4,403,173 (9/6/83) in the name of W. Mayer describe 
three such ignitor circuits for high-pressure gaseous discharge lamps. In 
each of these patents a starting capacitor is serially connected with a 
resistor so that a high voltage starting pulse is obtained upon discharge 
of the capacitor into a step-up transformer via a voltage threshold 
device. The transformer has an output winding coupled to the electrodes of 
the high-pressure discharge lamp. After the lamp ignites, the high voltage 
discharge pulses are suppressed because the lower value of the lamp 
operating voltage prevents the starting capacitor from charging up to the 
breakdown level of the voltage threshold device. 
Although the circuits described in the Nuckolls and Attewell patents are 
adequate for the ignition and operation of standard high-pressure sodium 
lamps, they are unreliable for the ignition of certain newer types of dual 
ended electrode halide lamps such as the HQI lamp manufactured by Osram 
GmbH and others. The older standard high-pressure sodium lamps generally 
require a starting voltage pulse in the range of 2.5 KV to 4 KV, whereas 
the HQI type of lamp requires a start voltage pulse in the range of 4 KV 
to 5 KV. 
In these prior art circuits, a substantial dip in the lamp open circuit 
voltage occurs immediately after the generation of the high voltage 
ignition pulse. The dip in the lamp voltage is caused by the discharge of 
the start or surge capacitor when the voltage threshold device "closes". 
This reduction in voltage means less power is available at the lamp 
electrodes to sustain the arc discharge immediately after lamp ignition. 
Another disadvantage of the above prior art pulse ignition circuits is 
that, after a momentary interruption of power to the high pressure 
discharge lamp, reignition of the lamp is not reliable because of the 
relatively low voltage that is available upon reapplication of the AC 
supply voltage. The applied high voltage ignition pulses will eventually 
ignite the lamp, but the low amplitude of the power frequency voltage may 
cause the lamp to hang up in a "low-glow" mode for a considerable period 
of time. This in turn will lead to deterioration of the lamp electrodes. 
The Mayer patent provides a circuit for starting the newer type of higher 
pressure metal vapor discharge lamp under cold or warm conditions by means 
of a pulse superimposition ignition circuit. A starting capacitor and a 
resistor are serially connected to the AC supply voltage. This series 
circuit is also connected to the primary winding of a step-up pulse 
transformer. The transformer secondary winding is coupled to the lamp 
electrodes. A semiconductor voltage threshold element, such as a 
four-layer diode, is coupled to the capacitor so as to allow the capacitor 
to discharge via the transformer primary winding to induce a large number 
of high voltage pulses in the transformer secondary winding in each half 
cycle of the AC supply voltage. Mayer also provides an oscillatory circuit 
composed of an auxiliary capacitor and a serially connected damping 
resistor which form, together with a ballast choke, a series resonant 
circuit coupled to the primary winding of the pulse transformer so as to 
increase the lamp supply voltage during ignition. This ignitor was 
designed for use with reactor ballasts to ignite lamps which required 
higher R.M.S. starting voltages than were available from the 220 V power 
supply. The semiconductor switching element (four-layer diode) disconnects 
the series resonant circuit after lamp ignition so that in normal 
operation the lamp will only receive power from the AC supply voltage 
source. 
SUMMARY OF THE INVENTION 
In view of the foregoing, it is an object of the invention to provide a new 
and improved circuit for starting and operating a high pressure arc 
discharge lamp for use with AC supply voltages as low as 120 volts. 
Another object of the invention is to provide a lamp starting and operating 
circuit that provides a relatively high ratio of the lamp ignition voltage 
to the lamp operating voltage. 
A further object of the invention is to provide a lamp starting and 
operating circuit that is relatively simple in construction but is 
nevertheless very reliable in operation. 
Another object of the invention is to provide an improved lamp starting 
circuit of the capacitor discharge type which maintains an adequate level 
of power frequency voltage and thereby provides a more reliable ignition 
of a high-pressure arc discharge lamp. 
A still further object of the invention is to provide an improved circuit 
for starting and operating a high-pressure arc discharge lamp which 
provides reliable reignition of a high-pressure lamp after a power 
interruption of short duration. 
It is still another object of the invention to provide a starting and 
operating circuit for a high pressure discharge lamp in which conventional 
types of ballasts may be used. 
These and other objects of the invention are accomplished by the addition 
of a further storage capacitor serially connected with a coil to the basic 
capacitor discharge type of starting circuit in a manner so as to clamp 
the open circuit voltage upon ignition of the high pressure gas discharge 
lamp thereby to maintain the lamp voltage at a high enough level to 
provide a sufficient current to sustain the discharge arc.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates the essential elements of the basic prior art capacitive 
discharge starting circuit for a high-pressure arc discharge lamp. The AC 
supply voltage, for example 220 volts at a frequency of 60 Hz, is 
connected to input terminals 1 and 2. Input terminal 1 is connected via a 
series connection of an iron core inductive reactor type ballast 3 to one 
electrode of a high pressure sodium discharge lamp 5. The ballast 3 
provides the customary current limiting impedance. The other input 
terminal 2 is directly connected to the other lamp electrode. This type of 
lamp requires a relatively high voltage pulse in order to be ignited, e.g. 
2.5 KV-4 KV, and thereafter operates on a much lower voltage, such as 
95-105 volts. 
In order to generate high voltage starting pulses for the lamp 5, the 
inductive ballast 3 is connected as a step-up autotransformer having a 
primary winding 4 defined by the winding turns between one end 16 and a 
tap point 10 of the ballast. A pulse discharge capacitor 6 has one 
electrode connected to the junction point 16 between transformer winding 4 
and the one electrode of the discharge lamp. The other electrode of 
capacitor 6 is connected to input terminal 2 via a series circuit 
consisting of a resistor 7 and an inductor 8. A normally open 
bidirectionally conductive voltage sensitive switch 9 is connected to the 
tap point 10 on the pulse autotransformer and to a circuit junction point 
11 between the capacitor 6 and the resistor 7. The switch 9 may be a 
bidirectional semiconductor switching device such as a Triac, a Sidac, or 
a four-layer diode. 
A capacitor 28 is connected across the input terminals 1, 2 and serves to 
improve the power factor. 
It will be appreciated that before the lamp 5 has ignited, it will present 
an open circuit to the autotransformer. Initially, when power is applied 
to the input terminals, the capacitor 6 will begin to charge up via the 
ballast inductor 3, the resistor 7 and the inductor 8. The rate of charge 
of the capacitor will be governed by the time constant of this circuit. 
When the capacitor voltage reaches the predetermined threshold or breakdown 
voltage of the voltage sensitive switch 9, the switch closes to allow the 
capacitor 6 to discharge through the primary winding 4 of the 
autotransformer, i.e. that part of the winding between tap point 10 and 
junction point 16. The primary voltage is stepped up by the transformation 
ratio of the autotransformer to produce a pulse voltage across the entire 
winding of sufficient amplitude to ignite the discharge lamp 5. The 
ignition pulse generated is superimposed upon the 60 Hz AC waveform 
supplied from input terminals 1 and 2 and is arranged to occur near the 
peak of the AC supply voltage waveform. 
When the lamp becomes conductive, the output voltage of the reactor ballast 
will be limited to the operating voltage of the lamp, which is 
considerably lower than the lamp ignition voltage. As a result, the 
capacitor 6 will no longer charge up to a voltage value sufficient to fire 
the solid state switch 9 so that the switch will remain in its open 
circuit condition while the lamp is conductive. This effectively removes 
the starting circuit from the lamp supply system so that further ignition 
pulses are inhibited during the time that the lamp is in operation 
(conductive). 
During operation of the lamp, the reactor 3 operates to provide the lamp 
ballast function as is conventional in discharge lamp circuits. 
It has been found that with the conventional values of the circuit elements 
used in this prior art circuit, upon the discharge of capacitor 6 the peak 
voltage available at the lamp electrodes drops from a value of 
approximately 300 volts to a value of 180 volts. This reduced voltage is 
sufficient to maintain reliable conduction in some high-pressure lamps, 
but is not sufficient to maintain reliable conduction in some other 
high-pressure lamps. This problem is especially severe in the case of the 
restart of a "hot" HQI lamp, i.e. upon the reapplication of power to the 
input terminals after a momentary power interruption during normal 
operation (conduction) of the HQI lamp. 
In the case of reignition of a hot HQI lamp, a reduction of the voltage 
below the 180 volt level upon discharge of the start capacitor 6 will not 
sustain conduction in the lamp. This reduction in voltage means less power 
is available at the lamp electrodes to sustain the arc discharge 
immediately after ignition. 
As the lamp cools down and the start circuit continues to generate 
high-voltage ignition pulses, the lamp will eventually ignite, but the low 
recovery voltage may cause it to hang up in a condition of lower than 
normal light output for a period of time, leading to deterioration of the 
lamp electrodes. This will reduce the life expectancy of the HQI lamp. The 
generation of ignition pulses during the extended time period when the 
lamp is cooling down produces further deterioration of the lamp 
electrodes. Exposure of the lamp electrodes to high voltage ignition 
pulses when the emission temperature of the lamp electrodes is lowered 
produces sputtering of the electrodes and a decrease in the life of the 
lamp. 
Referring now to FIG. 2 of the drawing, which illustrates a preferred 
embodiment of the invention that substantially eliminates the aforesaid 
drawbacks of the prior art circuit of FIG. 1, there is shown a pair of 
input terminals 1 and 2 for connection to a source of low frequency AC 
supply voltage, for example 120 volts at a frequency of 60 Hz. Input 
terminal 1 is connected to a tap point 11 on a high reactance ballast 
autotransformer 12 comprising windings 35, 36, 29 and magnetic shunt 43. 
An extension winding 35 of the autotransformer has a top end terminal 
connected to one terminal of a power factor correction capacitor 13. The 
other terminal of capacitor 13 is connected to a common junction point 
between input terminal 2 and the bottom end terminal of the 
autotransformer winding 36. The power factor correction capacitor 13 thus 
is connected across a pair of end terminals of the ballast 
autotransformer. 
A further tap point 14 on the primary winding of the autotransformer is 
connected to one end of the secondary winding 29 of the autotransformer. 
The windings 15, 22 together form a pulse autotransformer 29 for 
generating a high voltage ignition pulse for the high pressure discharge 
lamp 17. Windings 35, 36, 15 and 22 are magnetically coupled to one 
another. The other end 16 of the pulse autotransformer 29 is connected to 
one electrode of a high pressure gas discharge lamp 17 via an output 
terminal 26. The other electrode of discharge lamp 17 is connected to 
input terminal 2 via output terminal 27. 
A starting capacitor 18 is connected in series with a resistor 19 and a 
first inductor 20 between the circuit point 16 and the input terminal 2. A 
normally open voltage sensitive semiconductor switch 21 having a 
predetermined breakover voltage is connected across the series connection 
consisting of the primary winding 22 of the pulse autotransformer and the 
ignition capacitor 18. The switching device 21 may be a bilateral switch 
such as a Sidac having a threshold voltage of approximately 240 volts. 
This device may also be any other switch suitable for this application. 
The circuit consisting of elements 15-22 is similar to the circuit 
described above in connection with FIG. 1 of the drawing. 
In accordance with the invention, a further capacitor 23 and a small 
inductor 24 are connected in a second series circuit between a junction 
point 25 (between one terminal of switch 21 and one terminal of the 
winding 22) and input terminal 2. The capacitor 23 effectively clamps the 
open circuit voltage of the lamp so as to reduce the voltage dip after 
capacitor 18 discharges via switch 21 and winding 22 to generate the high 
voltage ignition pulse for the discharge lamp. The capacitor 23 maintains 
the lamp voltage at a relatively high voltage level during the discharge 
of ignition capacitor 18. The inductor 24 prevents a short circuit of the 
high frequency ignition pulse generated in the pulse autotransformer 29 
during the discharge of capacitor 18. 
When the input terminals 1, 2 are connected to a source of AC supply 
voltage, the capacitor 18 is initially charged via the windings 15, 22 of 
pulse autotransformer 29 and resistor 19 and inductor 20. At the same 
time, the capacitor 23 is charged via winding 15 and inductor 24. When the 
voltage across capacitor 18 reaches the breakover voltage of bilateral 
switch 21, the capacitor discharges via the switch 21 and winding 22 of 
the pulse autotransformer 29. By step-up autotransformer action, a high 
voltage, e.g. between 4 KV and 5 KV, is generated across the entire 
winding 29. This high voltage pulse is transferred to the terminals of the 
lamp 17 via the stray capacitance (not shown) between winding 15 and the 
common line connecting terminals 2 and 27. The stray capacitance provides 
a low impedance path for high frequency components of the generated pulse 
voltage. At the same time, capacitor 23 clamps the power frequency voltage 
at the lamp terminals to a sufficiently high voltage level to insure 
reliable ignition of the lamp (FIG. 4). 
Once the lamp is in operation, the voltage across its terminals drops to 
the arc voltage of the lamp. This clamps the voltage across capacitor 18 
to a level below the threshold voltage of the switching device 21, thereby 
inhibiting the generation of any high voltage pulses during normal lamp 
operation. 
During the ignition phase, the serially connected inductor 24 presents a 
high impedance path to the discharge of capacitor 18, i.e. it acts as a 
high impedance to the generated pulse so that the required pulse amplitude 
is not attenuated. The high reactance ballast function for the lamp 17 is 
provided by the windings of autotransformer ballast 12, including winding 
29. 
Once the lamp is ignited, its operation is controlled by the high leakage 
reactance of the autotransformer ballast by limiting the lamp current in a 
manner similar to that of the simple reactor serially connected to the 
lamp in FIG. 1. The starting process, however, is improved by providing a 
higher level of power frequency voltage at the lamp terminals at the time 
of pulse generation thereby to sustain the arc. 
The circuit in accordance with the invention also makes it possible to 
restart the lamp in a more reliable manner in the event of a momentary 
interruption of power at input terminals 1 and 2 after the lamp has been 
in operation, i.e. an improved restart operation of a warm high-pressure 
discharge lamp. 
In one preferred embodiment of the invention, capacitor 13 was an 8/.mu.F 
(300 V) capacitor. Capacitor 18 was 0.33/.mu.F (400 V) and capacitor 23 
was 0.47/.mu.F (400 V). Resistor 19 was 3.5 Kohm (20 W). Inductors 20 and 
24 were each 60 mH and the semiconductor switch 21 was a Sidac (235 
V.sub.bo). The lamp 17 was a 70 watt HQI lamp of the type manufactured by 
Osram (GmbH). Comparison tests performed on this lamp showed that in the 
circuit of FIG. 2, but without capacitor 23 and inductor 24, i.e. a prior 
art circuit similar to FIG. 1 and without the present invention, the peak 
open circuit voltage available to ignite the lamp dropped from an open 
circuit level of approximately 320 volts to a recovery level in the order 
of 160 volts, as shown in the waveform in FIG. 3 of the drawing. This 
latter voltage is not adequate to provide satisfactory ignition in the HQI 
lamp. 
In contrast, when the series circuit of capacitor 23 and inductor 24 was 
included in the circuit, as shown in FIG. 2, the recovery voltage was in 
the order of 300 volts with an open circuit voltage of approximately 330 
volts peak. This is illustrated in the waveform of FIG. 4 of the drawing. 
The invention thus maintains a high level of voltage across the lamp 
during the discharge of starting capacitor 18. This voltage is sufficient 
to start a cold or warm HQI lamp upon the application or reapplication of 
power and causes sufficient lamp current to flow to maintain conduction 
and thereby minimize the lamp life problem otherwise caused by electrode 
sputtering. 
FIG. 5 illustrates a modified form of the invention shown in FIG. 2, which 
operates as a so-called constant wattage autotransformer. In this 
embodiment the AC supply voltage at terminals 1 and 2 is connected across 
a winding 38 of a ballast autotransformer. A tap point 14 is connected to 
a winding 15, 22 of the ballast autotransformer via a capacitor 39, which 
improves the regulation characteristics of the ballast circuit over that 
of the high reactance circuit shown in FIG. 2. 
The operation of the ignitor circuit for the constant wattage 
autotransformer ballast configuration is similar to that of the high 
leakage reactance autotransformer circuit in FIG. 2, previously described. 
FIG. 6 employs the invention in conjunction with a constant wattage 
isolation transformer. The secondary winding of the ballast transformer 
now provides the additional function of a step-up pulse autotransformer. A 
capacitor 40 is connected between the lower end terminal of the 
transformer secondary winding and one electrode of the high pressure 
discharge lamp. This capacitor corrects the power factor and also provides 
better current regulation when the lamp is in operation. The entire lamp 
circuit is electrically isolated from the input terminals 1, 2. Once again 
the operation of the ignitor circuit is similar to that of the high 
leakage reactance autotransformer in FIG. 2. 
FIG. 7 illustrates the invention applied to the basic reactor ballast 
circuit of FIG. 1. The power factor correction capacitor 28 connected 
across the input terminals may not be required in certain applications. 
This circuit operates as described above, except that now the series 
circuit of capacitor 23 and inductor 24 clamp the lamp voltage when the 
capacitor 6 discharges via the bilateral voltage dependent semiconductor 
switching device 21. The dip in the open circuit voltage at the lamp 
electrodes during generation of the high voltage ignition pulse in this 
circuit is much smaller than that in the circuit of FIG. 1 and thereby 
provides improved lamp ignition. 
FIG. 8 illustrates the invention employed in an operating circuit for a 
high pressure discharge lamp 17 which utilizes a separate self-contained 
pulse transformer 30. The ballast means 31 may now be of a conventional 
nature such as in FIGS. 2, 5, 6 and 7 and is therefore shown in block form 
in the interest of brevity. Elements which are the same as those shown in 
FIG. 2 bear identical reference numerals. The primary winding 37 of the 
pulse transformer provides a charge path for start capacitor 18 which also 
includes a resistor 19 and an inductor 20 in series therewith. 
The 60 Hz power from input terminals 1 and 2 charges capacitor 18 via the 
series circuit of winding 37, resistor 19 and inductor 20 and via the 
ballast device 31. At the same time, 60 Hz power from the ballast device 
31 charges the capacitor 23 via inductor 24. 
When the capacitor 18 is charged to the threshold voltage of bilateral 
semiconductor switching device 21, it discharges into primary winding 37 
which, via the voltage step-up action provided by the proper ratio of 
turns of the secondary winding 32, produces a high voltage high frequency 
ignition pulse or pulses of sufficient magnitude to ignite the lamp 17. 
The series circuit of capacitor 23 and inductor 24 clamps the voltage at 
the lamp terminals to a sufficiently high level so as to provide the 
improved ignition operation described above in connection with FIG. 2. 
After the lamp ignites and is in operation, the secondary winding 32 and 
the ballast means 31 together limit the lamp current in the usual manner. 
The lower operating voltage across the lamp terminals inhibits further 
operation of the high voltage pulsing mechanism by preventing capacitor 18 
from charging to the breakdown voltage of the switching device 21. 
FIG. 9 illustrates another form of the invention which now uses a 
self-contained pulse autotransformer. In this circuit the pulse ignition 
capacitor 18 is charged via the primary winding 22 of a step-up pulse 
autotransformer 29 having a secondary winding 15. The stray capacitance 33 
is shown between the output lines of the ballast means 31. The stray 
capacitance provides a virtual short circuit path for the high frequency 
components of the generated high voltage pulse. 
The ignition and operation of the lamp are self-evident from the 
description provided above for FIGS. 1, 2 and 8. The secondary winding 15 
assists in the ballast function for the lamp when it is in operation. 
This embodiment of the invention also provides a compact device and affords 
a high degree of flexibility. 
FIG. 10 shows a modified form of the invention also using a self-contained 
pulse autotransformer 29. All of the components making up the high voltage 
pulse generator are connected in parallel with the output terminals 26, 
27. In this circuit the charge path for the pulse ignition capacitor 18 is 
resistor 19 and inductor 20. 
When the capacitor 18 is charged to the threshold voltage of the normally 
open voltage sensitive semiconductor switching device 21, the switching 
device begins to conduct so that the capacitor rapidly discharges via the 
switching device and the primary winding 22 of pulse autotransformer 29. A 
high voltage pulse is then generated by means of the step-up winding turns 
in the autotransformer 29 and is applied across the discharge lamp 17 via 
a capacitor 34. The series circuit of capacitor 23 and inductor 24 
connected between the output terminal 27 and the junction point between 
the switching device 21 and a tap point on the autotransformer again 
operates to maintain a higher power frequency voltage level at the lamp 
electrodes as capacitor 18 discharges into the primary winding of the 
autotransformer. The capacitor 23 was initially charged via the primary 
winding 22 and the series inductor 24. 
Ignition of the lamp reduces the voltage across the lamp terminals to the 
lamp arc voltage and, as before, operates to inhibit the further 
generation of ignition pulses by limiting the voltage to which capacitor 
18 can charge to a level below the threshold level of the voltage 
sensitive switching device 21. During operation of the lamp, the pulse 
autotransformer 29 carries a small current and hence does not contribute 
any power losses to the circuit. A higher efficiency is therefore 
obtainable with this embodiment of the invention than with the embodiments 
utilizing a self-contained transformer or autotransformer. 
The capacitor 34 is connected in series with the pulse autotransformer 29 
in order to limit the power frequency current flow therein, thereby making 
it possible to use a smaller pulse autotransformer. If the capacitance of 
capacitor 34 is relatively small, it will provide sufficient impedance to 
the 60 Hz supply voltage, while acting as a very low impedance to the high 
frequency pulse generated in the pulse autotransformer 29. 
The circuits in accordance with the invention can be made to operate other 
HQI lamps or high-pressure sodium lamps of the same nominal lamp voltage. 
By appropriate choice of the circuit component values, the circuit can be 
designed to operate other discharge lamps that have different pulse 
voltage requirements and different lamp operating characteristics. 
While the invention has been illustrated and described with respect to a 
particular preferred embodiment thereof, it will be understood that 
various modifications of the invention disclosed will become apparent to 
those skilled in the art. Accordingly, the invention is not to be limited 
by the described apparatus, but is intended to cover all modifications and 
equivalents within the scope of the appended claims.