Reduced duty cycle high intensity discharge lamp ignitor

A reduced cycle ignitor circuit for a high intensity discharge lamp incorporating a thermally timed, cycled operation which provides intermittent starting pulses for the high intensity discharge lamp. The ignitor circuit comprises a capacitor and semiconductor connected in series with a ballast transformer. The ignitor circuit further comprises a series connected resistor and thermostat, having thermally operated, normally closed electrical contacts, through which current is conducted to charge the capacitor. The thermostat is thermally coupled to the resistor. During operation, current flows through the series connected resistor and thermostat and charges the capacitor to a breakover threshold voltage level of the semiconductor. At that point, the semiconductor conducts and the energy stored in the capacitor is discharged through the semiconductor and a winding of the ballast which, through auto-transformer action, generates a high voltage pulse to start the high intensity discharge lamp. The resistor heats the thermostat during the charging cycle which reaches a temperature at which the normally closed thermostat contacts open, which disables the ignitor circuit until the resistor and thermostat cool and the thermostat contacts close.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a reduced duty cycle ignitor 
circuit for a high intensity discharge lamp, and more particularly 
pertains to a reduced duty cycle ignitor circuit for a High Intensity 
Discharge (HID) lamp which incorporates a thermally timed cycled operation 
which provides intermittent starting pulses for the HID lamp. The 
intermittent starting pulses should provide reduced stress on the ballast, 
socket and wiring dielectric systems of the HID lamp and result in longer 
electrical lives therefor. 
2. Discussion of the Prior Art 
One drawback to conventional HID ignitor ballast circuits is that when the 
HID lamp fails or is removed from the ballast circuit, the ignitor 
continuously generates high voltage pulses which over time will stress the 
ballast, socket and wiring dielectric systems. This can result in shorter 
electrical lives for those systems, particularly for the ballast system. 
As a response to this problem, several companies have introduced HID lamp 
ignitors which disable the ignitor after a predetermined period of time, 
thereby reducing the stress on the ballast, socket, and wiring dielectric 
systems. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
reduced duty cycle high intensity discharge lamp ignitor. 
A further object of the subject invention is the provision of a reduced 
duty cycle ignitor circuit for a high intensity discharge lamp which 
incorporates a thermally timed cycled operation which provides 
intermittent starting pulses for the HID lamp. This provides reduced 
stress on the ballast, socket and wiring dielectric systems which should 
result in longer electrical lives therefor. 
The basic functional difference between the HID ignitor of the present 
invention and prior art HID ignitors is that the ignitor of the subject 
invention cycles on and off during extended periods of operation. 
The ignitor circuit of the present invention operates in a normal mode for 
several minutes and then ceases operation for several minutes, allowing 
for the dissipation of any ozone that may have been formed by the 
high-voltage pulses. The ignitor then resumes normal operation and this 
cycle is continuously repeated, resulting in less stress on the ballast 
system components than would be experienced with a standard ignitor 
circuit. 
In accordance, with the teachings herein, the present invention provides a 
reduced duty cycle ignitor circuit for a high intensity discharge lamp 
incorporating a thermally timed cycled operation which provides 
intermittent starting pulses for the high intensity discharge lamp. The 
intermittent starting pulses provide reduced stress on the ballast, socket 
and wiring dielectric systems and result in longer electrical lives 
therefor. The ignitor circuit comprises a capacitor and semiconductor 
connected in series with a ballast transformer of a starter circuit for 
the high intensity discharge lamp. The ignitor circuit further comprises a 
series connected resistor and thermostat, having thermally operated, 
normally closed electrical contacts, through which current is conducted to 
charge the capacitor. The thermostat is thermally coupled to the resistor. 
During operation, current flows through the series connected resistor and 
thermostat and charges the capacitor to a breakover threshold voltage 
level of the semiconductor. At that point, the semiconductor conducts and 
the energy stored in the capacitor is discharged through the semiconductor 
and a winding of the ballast transformer, which generates a high voltage 
pulse to start the high intensity discharge lamp. The resistor heats the 
thermostat during the charging cycle which reaches a temperature at which 
the normally closed thermostat contacts open. This disables the ignitor 
circuit until the resistor and thermostat cool and the thermostat contacts 
close, thereby resuming a thermally timed cycled operation of the ignitor 
circuit. 
In greater detail, in a first embodiment the ignitor circuit comprises a 
series connected resistor, thermostat and capacitor, and the semiconductor 
is connected between the thermostat and the capacitor. In a second 
embodiment, the ignitor circuit comprises a series connected thermostat, 
resistor and capacitor, and the semiconductor is connected between the 
resistor and the capacitor. In a third embodiment, the ignitor circuit 
comprises a series connected resistor, thermostat and capacitor, and the 
semiconductor is connected between the resistor and the thermostat. 
In each of the first, second and third embodiments, an additional impedance 
can be placed across the thermostat. By placing the impedance across the 
contacts of the thermostat, the voltage across the opened contacts of the 
thermostat can be reduced, but the current flowing through the resistor 
and capacitor will also be reduced to a level which will prevent ignitor 
operation. 
In a further embodiment for a two lead high intensity discharge lamp 
ignitor, the pulse magnetics comprises an on-board pulse transformer in 
the ignitor circuit. 
In a preferred embodiment, the resistor comprises a hollow core resistor, 
and the thermostat is inserted into the center of the resistor, to improve 
the consistency of the thermal response times of the resistor and 
thermostat combination by reducing the effects of ambient environmental 
variations in temperature and air flow. In an alternate embodiment, the 
thermostat comprises a hollow core thermostat, and the resistor is 
inserted into the center of the thermostat. In another alternate 
embodiment, the thermostat is mechanically attached to the surface of the 
resistor.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to the drawings in detail, FIG. 1 illustrates a first embodiment 
of a reduced duty cycle high intensity discharge lamp ignitor circuit 
pursuant to the teachings of the present invention. The circuit of FIG. 1 
comprises a power supply 12, a capacitor C and semiconductor S connected 
in series across a portion of the ballast winding 14 for a high intensity 
discharge (HID) lamp 16, and a resistor R and thermostat T, through which 
current is conducted to charge the capacitor C. 
During each half-cycle of operation, current flows through the series 
connected Resistor R, Thermostat T, and Capacitor C until the capacitor C 
charges to the breakover threshold voltage level which causes the 
semiconductor S to conduct. As the semiconductor S conducts, the energy 
stored in the capacitor C is discharged through the semiconductor S and a 
portion of the ballast winding. This discharge, by transformer action, 
generates the high voltage pulse to start the HID lamp. 
The difference between the circuit of the present invention and the prior 
art is the inclusion therein of the thermostat T. The thermostat T is 
thermally coupled (placed in contact or in close proximity) to the 
resistor R. As the resistor R heats during the charging and starting 
cycle, it will reach a surface temperature causing the normally closed 
thermostat T contacts to open which will stop current flow through the RC 
combination, thus disabling the ignitor circuit. But as the ignitor 
circuit is no longer drawing current through the resistor R and capacitor 
C, the resistor R cools and the thermostat T closes after several minutes, 
which in turn allows the ignitor circuit to reestablish operation. 
The ignitor circuit of FIG. 2 is a second embodiment of the present 
invention, and illustrates another connection of the thermostat T in the 
ignitor circuit, with T and R being switched, although the operation of 
the circuit of FIG. 2 is substantially identical to that of the circuit of 
FIG. 1. 
The ignitor circuit of FIG. 3 is a third embodiment of the present 
invention, and places the thermostat T in the capacitor leg of the ignitor 
circuit which functions similarly to the circuits of FIGS. 1 and 2. The 
difference is that the semiconductor S will conduct during each half cycle 
of operation and allow for some residual heating of the resistor R. The 
configuration of FIG. 3 could be employed if the cool down time of the 
resistor R, i.e. the disable time of the ignitor circuit, is too short. 
FIG. 4 illustrates a further embodiment of the present invention similar to 
the ignitor circuit of FIG. 1, but with the inclusion of an impedance Z 
placed across the thermostat T contacts. This ignitor circuit can be 
particularly useful if the thermostat T is not rated for the full 
open-circuit voltage of the ballast. By placing this impedance Z across 
the contacts of the thermostat T, the voltage across the opened contacts 
of the thermostat T can be reduced, but the current flowing through the 
resistor R and capacitor C will also be reduced to a level which will 
prevent ignitor operation. The principle of operation of this circuit can 
also be applied to the circuits of FIGS. 2, 3, 5 and 6 as illustrated 
schematically by the inclusion of an impedance Z shown in phantom in those 
circuits. 
FIG. 5 illustrates a further embodiment of the present invention wherein 
the concept is applied to two-lead style H.I.D. ignitors, as are being 
employed commercially by some manufacturers. A two-lead ignitor circuit is 
essentially the same as the above described three-lead ignitor circuits 
with the exception that the transformer action is executed by an on-board 
pulse transformer, and thus a separate ballast transformer is not employed 
for generation of the starting pulses. 
FIG. 6 illustrates a further embodiment of the present invention which 
places a resistor Rp in series with the semiconductor S to limit the pulse 
current through S. 
FIG. 7 illustrates a preferred arrangement for thermally coupling the 
thermostat T and the resistor R, and employs a hollow core resistor R with 
an appropriate inner diameter, such that the thermostat T can be inserted 
into the center of the resistor R. This arrangement improves the 
consistency of the thermal response times of the resistor R/thermostat T 
combination by reducing the effects of ambient environmental variations 
(in temperature, air flow, etc.). In an alternative embodiment the 
thermostat T can be placed on the outside of the resistor R, but the 
variation in time cycles as a function of environmental conditions would 
be more pronounced. 
FIG. 8 is a graph of ignitor operation as a function of time for one 
embodiment of the present invention under the following conditions: 
Luminaire Type: Tri-Bay(Industrial) 
Lamp Wattage and Type: 400W Metal-Halide 
Ballast Type: Electro-Reg 
Ambient Temperature: 65.degree. C. 
Conditions: Lamp operating for more than 24 hours. Lamp then removed from 
circuit to simulate failed lamp condition. 
The graph shows that the ignitor circuit operates for approximately 3 
minutes, and then stops for approximately 7 minutes. The ratio of the 
on/off times will vary as a function of thermostat operating temperatures 
and luminaire ambient temperatures. 
While several embodiments and variations of the present invention for a 
reduced duty cycle high intensity discharge lamp ignitor circuit are 
described in detail herein, it should be apparent that the disclosure and 
teachings of the present invention will suggest many alternative designs 
to those skilled in the art.