Regulation of hot restrike pulse intensity and repetition

Included in a ballast circuit arrangement for a gas discharge lamp is a ballast transformer arrangement receptive of an input power signal and providing an output, ballast voltage. Further included is a pulse transformer having a secondary winding in serial circuit with the lamp for impressing a high voltage, hot restrike starting pulse across the lamp. A bypass capacitor is coupled to the ballast transformer for being charged by the ballast voltage. A hot restrike starting circuit for pulsing a primary winding of the pulse transformer comprises a serially connected starting capacitor and resistor coupled across the bypass capacitor; and a circuit for discharging the starting capacitor including, in serial circuit, the primary winding of the pulse transformer and a current switch having a control electrode responsive to a control signal. A trigger circuit for supplying such control signal comprises a serially connected trigger capacitor and resistor coupled across the starting capacitor; a circuit for discharging the trigger capacitor including a first voltage-breakover switch and producing the mentioned control signal when the first voltage-breakover switch becomes conductive; and a second voltage-breakover switch in serial circuit with the trigger resistor, between the trigger and starting capacitors. The foregoing arrangement provides multiple, consistently high, hot restrike pulses for restarting the lamp.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to the following, applications that are 
commonly owned by the present assignee: "System for Starting a High 
Intensity Discharge Lamp," Ser. No. 08/306,342, filed concurrently 
herewith and issued as U.S. Pat. No. 5,444,334 on Aug. 22, 1995; and 
"Boosting of Lamp-Driving Voltage During Hot Restrike," Ser. No. 
08/306,800, filed concurrently herewith, and issued as U.S. Pat. No. 
5,449,980 on Sep. 12, 1995. The disclosures of the foregoing applications 
are herein incorporated by reference. 
FIELD OF THE INVENTION 
The present invention relates to an arrangement for rapidly restarting a 
high intensity discharge (HID) lamp after it has turned off and, while 
still hot. More particularly, the invention is directed to an improvement 
for reliably providing multiple high voltage, hot restrike pulses during a 
half cycle of lamp-driving voltage, and for regulating the intensity of 
the pulses so as to consistently be high. 
BACKGROUND OF THE INVENTION 
High intensity discharge (HID) lamps are typically used where large areas 
require illumination, such as in factories, parking lots and sports 
arenas. In some applications, such as illuminating a sports arena during a 
sporting event, after a momentary power failure that terminates 
illumination by the lamps, it is naturally desired that the lamps rapidly 
restart to allow the sporting event to continue. However, a hot HID lamp 
typically requires a high current at an elevated voltage to cause the lamp 
to drop in voltage to where its power supply, or ballast, circuit can 
sustain lamp operation. 
The above cross-referenced application entitled "System for Starting a High 
Intensity Discharge Lamp," Ser. No. 08/306,342, is directed to an improved 
hot restrike circuit for HID lamps that performs well over a long life. 
With reference to terminology employed herein, the hot restrike circuit of 
the co-pending application includes a starting circuit in which a starting 
capacitor is charged through a resistor, and discharged through the 
primary winding of a pulse transformer and a current switch when it 
becomes conductive (i.e. turns on). A secondary winding of the pulse 
transformer provides a high voltage, hot restrike pulse that is applied 
across the lamp to initiate lamp starting. Multiple high voltage, hot 
restrike pulses per half cycle of lamp-driving voltage can be provided to 
assure the high current at an elevated voltage needed to initiate lamp 
turn-on. 
The current switch disclosed in the co-pending application has a control 
electrode that is controlled by a starting aid, which may be conventional 
per se. The specific current switch disclosed in the co-pending 
application is a three-electrode spark gap device that has a main spark 
gap formed between a pair of main electrodes, and a triggering spark gap 
formed between one of the main electrodes and a trigger (or control) 
electrode. 
The starting aid, or "trigger" circuit as used herein, which provides the 
control signal for the current switch of the starting circuit, may 
typically include a trigger capacitor that is charged through a resistor. 
The trigger capacitor is then discharged through the primary winding of a 
pulse transformer, which has a secondary winding that generates a trigger 
pulse as the control signal to trigger into conduction the current switch 
of the starting circuit. 
The use of a conventional starting aid in the hot restrike circuit of the 
co-pending application has enabled rapid restarting of an HID lamp of the 
metal halide variety. However, the present inventor has discovered that 
even further improvements in hot restrike capability can be achieved using 
the principles of the present invention. One improvement is to regulate 
the intensity of hot restrike pulses so that they are consistently at a 
high level. As such, the hot restrike pulses are more effective at 
delivering high current to the lamp. A further improvement is to increase 
the reliability of obtaining multiple hot restrike pulses during a half 
cycle of lamp-driving voltage, which also contributes to the effectiveness 
of the hot restrike pulses. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the invention to provide, for a gas 
discharge lamp, a ballast circuit arrangement having a hot restrike 
capability, wherein the intensity of hot restrike pulses is regulated to 
be at a consistently high level. 
A further object of the invention is to provide, for a gas discharge lamp, 
a ballast circuit arrangement having a hot restrike capability with the 
foregoing regulation-of-pulse-intensity advantage, and wherein an increase 
is realized in the reliability of obtaining multiple hot restrike pulses 
during a half cycle of lamp-driving voltage so as to increase the 
effectiveness of the hot restrike capability. 
In accordance with a preferred embodiment of the invention, there is 
provided a ballast circuit arrangement for a gas discharge lamp, including 
a ballast transformer arrangement receptive of an input power signal and 
providing an output, ballast voltage. Further included is a pulse 
transformer having a secondary winding in serial circuit with the lamp for 
impressing a high voltage, hot restrike starting pulse across the lamp. A 
bypass capacitor is coupled to the ballast transformer for being charged 
by the ballast voltage. A hot restrike starting circuit for pulsing a 
primary winding of the pulse transformer comprises a serially connected 
starting capacitor and resistor coupled across the bypass capacitor; and a 
circuit for discharging the starting capacitor including, in serial 
circuit, the primary winding of the pulse transformer and a current switch 
having a control electrode responsive to a control signal. A trigger 
circuit for supplying such control signal comprises a serially connected 
trigger capacitor and resistor coupled across the starting capacitor; a 
circuit for discharging the trigger capacitor including a first 
voltage-breakover switch and producing the mentioned control signal when 
the first voltage-breakover switch becomes conductive; and a second 
voltage-breakover switch in serial circuit with the trigger resistor, 
between the trigger and starting capacitors. The second voltage-breakover 
switch allows the trigger capacitor to become charged only when the 
voltage on the starting capacitor exceeds a threshold voltage, thereby 
providing consistently high hot restrike pulses, and, further, reducing 
its current conduction after the trigger capacitor is discharged to below 
the holding current of the first voltage-breakover switch for a sufficient 
duration to allow the first switch to reset during the same half-cycle of 
the ballast voltage, whereby the starting circuit provides multiple hot 
restrike pulses in the same half-cycle. 
Preferably, the current switch of the hot restrike circuit comprises a 
three-electrode spark gap device having a main spark gap formed between a 
pair of main electrodes, and a triggering spark gap formed between one of 
the main electrodes and a trigger electrode that is responsive to the 
mentioned control signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a ballast circuit arrangement 10 for powering a high intensity 
discharge (HID) lamp 12. A primary winding 14A of a ballast transformer 14 
receives an a.c. power signal from source 16, and produces an output 
voltage on secondary winding 14B. Transformer 14 is known as an regulating 
transformer, and has a secondary winding 14B that is tapped into primary 
winding 14A at 17. A ballast capacitor 18 produces a desired phase angle 
between current and voltage supplied by source 16, and, in combination 
with ballast transformer 14, limits current to lamp 12. The type of 
ballast transformer used is not critical to the invention. 
As will be described in detail below, starting circuit 40 and trigger 
circuit 60 cooperate to produce a pulse of current through a pulse 
transformer primary winding 20A. During this time, relays 90 and 92 
(described below) between a circuit common 94 and conductor 96 are both 
closed. As a result, respective, additive high voltage (hot restrike) 
pulses are induced in secondary windings 20B and 20C of such pulse 
transformer. For a 2000-watt, 250-volt lamp, for instance, the high 
voltage would likely be in excess of 10 kilovolts for each pulse 
transformer. The high voltage pulses from windings 20B and 20C are 
impressed across lamp 12 as "hot restrike" pulses for initiating 
restarting of the lamp while the lamp is still hot. The use of the two 
secondary windings 20B and 20C has the advantage of reducing the peak lamp 
voltage relative to circuit common 94, since the windings create additive, 
opposite polarity voltages about circuit common 94. Only one secondary 
winding 20B or 20C could be used if desired, however. 
A bypass capacitor 22 is coupled to secondary winding 14B of ballast 
transformer 14 so as to be charged by ballast voltage V.sub.B. Bypass 
capacitor 22 prevents the high voltage, hot restrike pulses from damaging 
transformer 14. 
Referring again to starting circuit 40, a starting capacitor 42 is charged 
from bypass capacitor 22 by current flowing through a resistor 43. A 
discharge circuit for capacitor 42 includes primary winding 20A of the 
above-described pulse transformer for producing hot restrike pulses. 
Capacitor 42 becomes discharged when a conductive state is established in 
current switch 24 between its main current-carrying electrodes 24A and 
24B. This occurs when control electrode 24C of switch 24 receives an 
appropriate control signal from trigger circuit 60. With current switch 24 
embodied as a three-electrode spark gap device as shown, a pulse of 
current supplied by trigger circuit 60 causes a spark discharge from 
control electrode 24C, through main electrode 24B, to main electrode 24A. 
This makes switch 24 conductive so that capacitor 42 discharges to circuit 
common 94 via pulse transformer primary winding 20A. 
With current switch 24 being embodied as a three-electrode spark gap 
device, switch electrodes 24A-24C preferably comprise elongated conductive 
members that are substantially parallel to each other. Further details of 
such a three-electrode spark gap device are disclosed in the above 
cross-referenced application Ser. No. 08/306,342. As described below, 
however, other forms of switching devices that switch in response to a 
control signal on a control electrode are suitably used in the present 
invention. 
To produce a control signal for switch 24, trigger circuit 60 incorporates 
a trigger capacitor 61 that is charged through a resistor 62 from starting 
capacitor 42. In serial circuit with resistor 62 is a voltage-breakover 
switch 63, which, as described below, significantly contributes to the 
advantages of the invention. When the voltage on capacitor 61 exceeds the 
breakover voltage of voltage-breakover switch 64, that switch becomes 
conductive and capacitor 61 discharges through such switch and primary 
winding 65A of a pulse transformer 65. Resistor 66, meanwhile, limits 
current to switch 64 to protect it from overheating. 
When primary winding 65A of pulse transformer 65 receives the mentioned 
discharge from capacitor 61, a considerably higher voltage pulse is 
induced on secondary winding 65B. That pulse on secondary winding 65B is 
coupled to trigger (or control) electrode 24C of current switch 24, via a 
capacitor 67. Capacitor 67 blocks voltage at the operating frequency of 
ballast voltage V.sub.B from reaching secondary winding 65B, although a 
resistor could instead serve such purpose. 
HIGH INTENSITY HOT RESTRIKE PULSES 
Voltage-breakover switch 63, in the charging path for trigger capacitor 1, 
breaks over (i.e. becomes conductive) only when the voltage on starting 
capacitor 42 exceeds the breakover voltage of that switch 63. As a result, 
trigger capacitor 61 discharges through pulse transformer primary winding 
65A, causing starting capacitor 42 to discharge through pulse transformer 
primary winding 20A only when the voltage on capacitor 42 is consistently 
high. In turn, the hot restrike pulses produced on pulse transformer 
secondary windings 20B and 20C is consistently high, and is thus more 
effective at delivering high current to the hot lamp for restarting the 
lamp. This is illustrated in FIG. 2. 
The upper and lower curves in FIG. 2 show voltages V.sub.42 and V.sub.61 
across starting capacitor 42 and trigger capacitor 61, respectively, for a 
half cycle of ballast voltage V.sub.B that appears on bypass capacitor 22. 
Ballast voltage V.sub.B becomes positive after its zero-crossing point 
t.sub.1, and continues to rise until time t.sub.3. Meanwhile, starting 
capacitor 42 becomes charged from ballast voltage V.sub.B via resistor 43, 
and its voltage (V.sub.42) remains somewhat lower than ballast voltage 
V.sub.B. At time t.sub.2, the voltage V.sub.42 on starting capacitor 42 
reaches the breakover voltage of voltage-breakover switch 63 (FIG. 1), as 
indicated by a threshold voltage V.sub.TH. With switch 63 now conductive, 
trigger capacitor 61 starts charging from starting capacitor 42 and 
continues to charge until its voltage V.sub.61 exceeds the breakover 
voltage of voltage-breakover switch 64. At this point, time t.sub.3, 
switch 64 becomes conductive and capacitor 61 discharges through pulse 
transformer 65. This turns on current switch 24 and causes starting 
capacitor 42 to discharge through pulse transformer primary winding 20A. 
Respective hot restrike pulses on pulse transformer secondary windings 20B 
and 20C are thereby produced, as described above. 
The foregoing process occurring between times t.sub.2 and t.sub.3 repeats 
between times t.sub.4 and t.sub.5, t.sub.6 and t.sub.T, and t.sub.8 and 
t.sub.9. The peaks of voltage V.sub.42 on capacitor 42, as shown at points 
100, are consistently high as desired, to achieve a high intensity of hot 
restrike pulses. During the negative half cycles of ballast voltage, the 
waveforms for the voltages shown in FIG. 2 for a positive half cycle are 
repeated, but in the negative direction. 
FIG. 3 provides a contrast with FIG. 2 where a voltage-breakover switch 63 
(FIG. 1) is not used in trigger circuit 60. Instead, the value of resistor 
62 is increased to set an appropriate time constant for charging capacitor 
61. At time t.sub.2 in FIG. 3, trigger capacitor 61 starts charging from 
starting capacitor 42, as shown by voltage V.sub.61 starting to rise. At 
time 13, capacitor 61 becomes charged to the breakover voltage of switch 
64 (FIG. 1), whereupon switch 64 turns on to allow capacitor 61 to 
discharge through pulse transformer 65. This, in turn, causes current 
switch 24 to become conductive, causing capacitor 42 to discharge through 
pulse transformer primary winding 20A, producing hot restrike pulses on 
secondary windings 20B and 20C. 
Capacitor 61 charges between time t.sub.3 and time t.sub.4, and discharges 
(or "fires") at time t.sub.4 ; however, this firing is insufficient to 
produce an adequate control signal on trigger electrode 24C of current 
switch 24. Although not illustrated in FIG. 2, such misfiring of capacitor 
61 can occur even though voltage-breakover switch 63 is employed. A way to 
reduce such misfiring, which involves increasing the breakover voltage of 
trigger switch 64, is disclosed in the above-referenced application Ser. 
No. 08/306,800. 
The foregoing process occurring between times t.sub.2 and t.sub.3, where 
starting capacitor 42 becomes discharged, repeats between times t.sub.4 
-t.sub.5, t.sub.5 -t.sub.6, t.sub.6 -t.sub.7, and t.sub.7 -t.sub.8. At 
time t.sub.9 another misfiring of trigger capacitor 61 occurs, where (as 
described above) current switch 24 is not turned on. Then at time 
t.sub.10, in the absence of trigger capacitor 61 firing, starting 
capacitor 42 fires. This is due to a somewhat rare, spontaneous voltage 
breakdown between main electrodes 24A and 24B of switch 24, which may 
occur due to close spacing between such electrodes and a recent history of 
repeated sparking between such electrodes. 
In FIG. 3, the peaks of voltage V.sub.42 on capacitor 42, as shown at 
points 110, are not consistently high as shown in FIG. 2, resulting in a 
lax regulation of intensity of hot restrike pulses. This is because, 
without breakover switch 63 in trigger circuit 60, trigger capacitor 61 
starts charging when its voltage is surpassed by the voltage V.sub.42 on 
starting capacitor 42. Consequently, capacitor 42 discharges through pulse 
transformer primary winding 20A even at relatively low values of voltage 
V.sub.42, as shown by the left-most of points 110 in FIG. 3, for instance. 
During the negative half cycles of ballast voltage, the waveforms for the 
voltages shown in FIG. 3 for a positive half cycle are repeated, but in 
the negative direction. 
MULTIPLE PULSING 
The use of a voltage-breakover switch 63 in trigger circuit 60 (FIG. 1), 
such as a SIDAC, provides for consistently high hot restrike pulses as 
described above. However, it has been found by the present inventor that 
the type of device or devices implementing voltage-breakover switch 63 has 
important consequences. After trigger capacitor 61 has discharged, it 
typically will have a harmonic voltage (or "ring") on it that tends to 
keep switch 64 conducting. It is important, however, for switch 64 to 
rapidly reset, i.e. to again block current until turned on by sufficient 
voltage on capacitor 61. This allows trigger capacitor 61 to reliably 
recharge and be discharged during a half-cycle of ballast voltage, so that 
starting circuit 40 provides multiple hot restrike pulses in the same 
half-cycle. For the specific component values set forth below, it is 
preferred that at least 4 hot restrike pulses per half cycle are produced. 
When a SIDAC, or several series-connected SIDACS, for instance, are used 
for switch 64, the switch has a low holding current, which decreases even 
more at increasing temperatures. To turn off such a SIDAC requires that 
the current supplied to it, i.e., via switch 63, decrease to below its 
holding current for a sufficient duration of time to allow the SIDAC to 
reset (i.e. block current). For the specific component values set forth 
below, at least about 10 microseconds, and preferably above about 50 
microseconds, is preferred. This can be accomplished through suitable 
selection of switch 63. 
Voltage-breakover switch 63 may, for instance, comprise a transient diode, 
whose electrical function is modelled as back-to-back Zener diodes. After 
the voltage across a transient diode decreases below its breakover 
voltage, its current rapidly reduces to well below the holding current of 
a SIDAC (or other semiconductor) switch 64. Alternatively, such 
back-to-back Zener diodes may be used, and the selection of other suitable 
component(s) to implement switch 63 will be routine to those of ordinary 
skill in the art in view of the present specification. 
It is true that FIG. 3 illustrates multiple pulses during a half-cycle of 
ballast voltage although voltage-breakover switch 63 is not used, and is 
thus not available to "pinch" off the current to switch 64; however, such 
grouping of multiple pulses for the resulting circuit was not reproduced 
reliably. Thus, many half cycles occurred with the production of only a 
single pulse per half cycle. 
FIG. 4 shows an alternative current switch 24' that can be used instead of 
the three-electrode spark gap device shown in FIG. 1, at least for lower 
lamp wattage. Specifically, the circuit of FIG. 4, between nodes 200, 202 
and 204, can replace the corresponding circuit in FIG. 1 between the 
same-numbered nodes. As can be seen from FIG. 4, pulse transformer 65 
(FIG. 1) of trigger circuit 60 is not used in the circuit shown. The 
current switch 24' in FIG. 4, by way of example, may be an SCR, as 
illustrated, a triac, or, when using the invention of above-referenced 
application 08/306,800, for instance, by a transistor such as a bipolar 
transistor, an insulated-gate transistor, or a power field-effect 
transistor. The so-modified circuit of FIG. 1 operates in the same general 
fashion as described above, except, of course, for the absence of pulse 
transformer 65. 
Relays 90 and 92 of FIG. 1 are now considered, relay 90 being a timer 
relay, and relay 92 being a lamp-current relay. 
Timer relay 90, which is normally open, is responsive to a.c. line voltage, 
e.g. from power source 16. When power is first supplied to the ballast 
circuit arrangement 10 of FIG. 1, a first timer function causes relay 90 
to close and then to subject the hot restrike circuitry to a duty cycle of 
about 1:10 so as to minimize stresses on such circuitry. For instance, 
relay 90 may close within about 50 milliseconds of a.c. power being 
applied, and, to complete a duty cycle, remain closed for about 200 
milliseconds, and open for about 2 seconds. If the lamp has not started 
within, for instance, 20 of the mentioned duty cycles, a second timer 
function opens the relay to shut off power to the hot restrike circuitry; 
it is also desired at this time that power to the circuitry (not shown) 
implementing the foregoing timer functions be shut off in such a manner 
that the timer resets. Implementing timer relay 90 will be routine to 
those of ordinary skill in the art based on the present specification. 
Lamp-current relay 92, which is normally closed, senses current in lamp 12. 
It can be implemented, e.g., with a standard current relay whose 
current-sensing winding (not shown) is placed in conductor line 250 
leading to the lamp. 
In a specific example of implementing the ballast circuit arrangement of 
FIG. 1, the following component values may be used for a 250-volt, 
2000-watt metal halide lamp, wherein polarities of transformer windings 
are indicated by dots in FIG. 1: Ballast transformer 14, an auto-regulator 
ballast providing 8.5 amps lamp current to a 250-volt lamp; ballast 
capacitor 18, 50 microfarads; ballast voltage V.sub.B, 800 volts peak; 
bypass capacitor 22, 1.0 microfarads; starting capacitor 42, 1.0 
microfarads; resistor 43, 0.1 k ohms; pulse transformer 20A-20C comprising 
two coils and core assemblies with the primary winding 20A comprising 1 
turn on each coil, and secondary windings 20B and 20C each respectively 
comprising 48 turns on its associated coil; main electrodes 24A and 24B of 
switch 24 each comprising tungsten rods of 3/16" diameter, separated from 
each other by a gap of 0.09 inches, and having a breakdown rating between 
such electrodes from 1.0 to 2.0 kilovolts; trigger electrode 24C of switch 
24 comprising a tungsten rod of 3/16" diameter, separated by a gap of 
0.017 inches from adjacent electrode 24B, with a breakdown rating between 
electrodes 24B and 24C between 5 and 7 kilovolts; a turns ratio between 
primary (pulse) winding 65A and secondary (pulse) winding 65B of 56.8:1; 
voltage-breakover switch 64, one or more serially connected SIDACS having 
a total breakover voltage of 220 volts, such as available under Part No. 
KIV24 from Shidengen Electric Mfg. Co. Ltd. of Tokyo, Japan; resistor 66, 
5 ohms; trigger capacitor 61, 0.1 microfarads; resistor 62, 1.0 k ohms; 
and voltage-breakover switch 63, one or more serially connected transient 
diodes, such as Part No. PGKSERIES from Motorola of Phoenix, Arizona, and 
having a total breakover voltage of 400 volts. 
From the foregoing, it will appreciated that the invention provides, for a 
gas discharge lamp, a ballast circuit arrangement having a hot restrike 
capability, wherein the intensity of hot restrike pulses is regulated to 
be at a consistently high level, and wherein an increase is realized in 
the reliability of obtaining multiple hot restrike pulses during a half 
cycle of lamp-driving voltage so as to increase the effectiveness of the 
hot restrike capability. 
While the invention has been described with respect to specific embodiments 
by way of illustration, many modifications and changes will occur to those 
skilled in the art. It is, therefore, to be understood that the appended 
claims are intended to cover all such modifications and changes as fall 
within the true scope and spirit of the invention.