US 3577209 A
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Description (OCR text may contain errors)
United States Patent lnventor Charles ll. Perkins Richmond, Va.
Feb. 28, 1969 May 4, 1971 I Robertshaw Controls Company Richmond, Va.
ELECTRIC IGNITION SYSTEM 4 Claims, 2 Drawing Figs. us. Cl 317198, 323/68, 338/10, 338/22, 43l/66, 431/75 F23g 7/10 Field oiSearch 317/98,79; 219/264, 240; 431/66, 42, 46, 43, 74, 75; 123/145; 338/10, 22-23; 323/68-69 References Cited UN lT ED STATES PATENTS Appl. No. Filed Patented Assignee Primary Examiner-Vol0dymyr Y. Mayewsky Attorney-Christen, Sabol & OBrien ABSTRACT: An electric ignition system utilizing an electric ignitor having a positive temperature coefficient of resistance 3,025,455 3/ 1962 Jonsson 323/68 and a negative temperature coefficient resistor coupled in sel/1970 Perl 431/66 ries therewith to provide a constant resistance series network.
THERMISTOR 9 o o- Hg 20 .8 26 f f '6 IGNITOR PATENTEU HAY 41971 ,4 I6 IGNITOR Q) FIG! ncmmmzd TEMPERATURE INVENTOR CHARLES H. PERKINS ATTORNEYS BACKGROUND OF THE INVENTION The present invention relates to electrical ignition systems and, more particularly, to'current regulating electrical ignition systems wherein the cold energization current inrush of electrical ignitors is limited to a practical level.
It is conventional to utilize a pilot flame to ignite a burner; however, such flames are subject to the disadvantage of frequent outage due to drafts, dust and lint obstruction of the pilot fuel line, and the like. Electric ignitors are more desirable than pilot flame ignitors due to their reliability and immunity from the conditions enumerated above, but they too have proved to be unsatisfactory under certain conditions of operation. One of the major problems encountered by designers in utilizing-electric ignitors is their extremely low cold resistance.
The low resistance of the ignitor upon initial energization produces a current inrush which is many times the steady-state current at ignition temperatures. In order to reduce this high current flow and to prevent damage to the ignitors, complex flow voltage starting control circuits and expensive current regulating type transformers have been employed in the past.
SUMMARY-OFTHE INVENTION It is, therefore, an object of the, present invention to construct a current regulating electric ignition system having all the advantages of similarly employed prior art systems but utilizing fewer components than the number of components essential to such prior art systems.
Another object of this invention is to simplify a current regulating electric ignition system by utilizing only two components.
This invention has a further object in the utilization of a negative temperature coefficient resistor in series withan electric ignitor to limit the initial current drawn by the ignitor to a practical level. 1
An advantage of this invention is the provision of a simple and reliable electric ignition system which permits the use of electric ignitors having a negative temperature coefficient of resistance.
The present invention is summarized in that an electric ignition system for fuel burners includes an electric ignitor having a varying resistance and being connected to a power source for energization thereby, and a resistor having a varying resistance coupled in series with the electric ignitor in such a manner that the varying resistance of the resistor compensates for the varying resistance of the electric ignitor.
These and other objects and advantages of the present invention will .become more fully apparent from the following detailed description of a preferred embodiment of the invention when considered in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a circuit utilizing the preferred embodiment of the invention; and
DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 which illustrates the preferred embodiment of the invention, shows a burner having a pair of spaced pilot ports 12 and 14. The burner 10 is positioned within igniting proximity of an electric ignitor 16 containing a heating element 18 which is constructed of a material having a positive temperature coefficient of resistance. Heating element 18 is, in turn, connected in series with a thermistor 20 constructed of a material having a negative temperature coefficient of resistance. In this particular example, the heating element 18 was constructed of molybdenum disilicide material and the thermistor 20 was constructed of boron material.
. The series combination of ignitor I6 and thermistor 20 is connected across the secondary winding of a nonregulating transformer 22 which has its primary winding coupled through a switch 24 to an electrical power source represented by power leads 26. Transformer 22, switch 24, and leads 26 represent a conventional electric ignitor power supply system and are illustrated in the drawings in simplified form solely for the sake ofclarity.
As is illustrated in FIG. I, the switch 24 may be operated manually or automatically in accordance with the particular I requirements of the installation. In the usual environment, the operation of the switch 24 coincides with the on-olf operation of the burner 10 as by a thermostatically operated control 'device (not shownIwhich responds to the temperature of the area being heated by the burner I0; thus, wheneverthere is a demand for heat, the burner 10 would be supplied with fuel gas by a control valve (not shown) and the switch 24 would be closed. The closing of switch 24 permits current to flow from the power source through leads 26 to the primary winding of the transformer. Since transformer 22 is of the nonregulating type, the current induced in the secondary winding is not limited and can assume any value up to the maximum current rating of the transformer. As will become evident as the disclosure proceeds, were it not for the unique circuitry of the present invention, a more expensive current regulating transformer would be required instead of the nonregulating type presently employed.
The nonlimited current induced in the secondary winding of the transformer is fed to the series network consisting of thermistor 20 and ignitor 16 to cause the temperature of the ignitor to increase to a temperature sufficient to ignite the fuel escaping from ports 12 and 14. Since the heating element 18 of the ignitor I is constructed of a material which exhibits an extremely low resistance when cold, it would draw an excessive amount of current upon initial energization if it were connected alone directly across the secondary of the transformer. This initial current inrush, however, would then gradually diminish as the temperature of the ignitor increased due to its positive temperature coefficient of resistance. The current vs temperature characteristics of such a system are shown by dashed curve 30 of FIG. 2. By placing the negative temperature coefficient thermistor in series with the ignitor, however,
the high cold resistance of the thermistor compensates for the low resistance of the heater element to prevent the undesirable initial current surge. Furthermore, as the temperature of the system is increased, the resistance, of the thermistor decreases and, after a certain period of time, avalanches to zero so that the high resistance of the heater element at higher temperatures is not increased above its steady state value at operating temperatures. The current vs temperature characteristics of the embodiment of the invention shown in FIG. I are illustrated by curve 40 of FIG. 2. Curves 30 and 40, when compared, show the improved current inrush characteristics obtained by the present invention.
The net result of the series circuit combination is the provision of a simple and reliable current regulating electric ignition system which permits the use of electric heating elements such as molybdenum disilicide which were heretofore impractical due to the expensive and complex current control circuits formerly required;
Inasmuch as the present invention is subject to many variations, modifications and changes in detail, it is intended that all matter contained'in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting senses lclaim:
I. An electric ignition system for burner apparatus compris ing: I I I an electric ignitor having resistance varying with temperature in apredetermined relation,
a power source connected to said ignitor for energizing the same,
a resistor having a resistance varying with temperature in inverse relation to said predetermined relation, coupled in series with said ignitor and said source, means connecting said resistor in series with said ignitor and said source whereby current through said ignitor is maintained substantially constant with respect to temperature.
said ignitor having a positive temperature coefficient of resistance and said resistor having a negative temperature coefficient of resistance.
2. The electric ignition system for burner apparatus as recited in claim 1 wherein said ignitor is made of molybdenum disilicide and said resistor is a boron thermistor.
3. in a current regulating electric ignition system, the combination comprising:
a source ofelectricity,
an electric ignitor having a positive temperature coefficient of resistance and presenting a low resistance at low temperatures and a high resistance at igniting temperatures,
a resistor connected inseries with said electric ignitor and 7