US 3868540 A
An electric incandescent lamp has a resistor in series with the lamp filament in order to limit inrush current to the filament. After a short period of lamp operation, a bimetallic switch shorts out the resistor.
Description (OCR text may contain errors)
INCANDESCENT LAMP HAVING EXTENDED LAMP LIFE Electric incandescent lamps generally comprise a coiled tungsten filament disposed within a glass envelope. The lamp emits light when the filament is heated to incandescence by the passage of electric current therethrough.
The electrical resistance of the tungsten filament is greatly dependent on its temperature. For example, the resistance of the hot filament, during normal operation, can be about ten or more times greater than its resistance at room temperature. One result of this resistance change is that the filament is subjected to a large inrush current when the lamp is first energized. For example, in a 100 watt, 120 volt lamp, the inrush current is as high as 13 amperes; the current then decreases to a steady-state value of 0.833 amperes, after a few milliseconds, as the tungsten filament is heated to its normal operating temperature.
The high inrush current causes localized overheating of the filament and is a major cause of premature lamp failure. It is an object of this invention to prevent high inrush current, thereby significantly increasing the useful life of the lamp.
The single FIGURE in the drawing is an'elevational view, partly in section, of an electric incandescent lamp in accordance with this invention.
The lamp comprises a glass bulb 1 usually having a gaseous filling therein, such as argon and nitrogen. The bottom of bulb l is sealed to flare 2 of the usual stem press glass mount 10. Lead-in wires 3 and 9 extend through and are supported by stem press 4 of glass mount 10. Fastened to the upper end of lead-in wire 9 is a resistor 11 which, in turn is connected to, and supports, a support wire 12. A coiled tungsten filament 5 is supported between the upper end of lead-in wire 3 and the upper end of support wire 12. For this purpose, lead-in wire 9, resistor 11 and support wire 12 are sufficiently rigidly connected to provide adequate support for the upper end of filament 5. In parallel with resistor 11, that is to say, connected between support wire 12 and lead-in wire 9 is a U shaped bimetallic switch 13. Switch 13 is normally open at room temperature, that is, when the lamp is cold or unenergized, as shown in the drawing.
Lead-in wires 9 and 3 extend downward between tipped off exhaust tube 6 and flare 2 and are connected respectively, to the usual screw base 8, which is fastened to the bottom of bulb 1, and to center contact 7 of base 8.
When the lamp is first energized, current flows through filament 5 and resistor 11, which are in series with each other. Thus, even though the resistance of filament 5 is quite low, the inrush current is limited by resistor 11. In one example ofa 100 watt, 120 volt lamp in accordance with this invention, the value of resistor 11 was 50 ohms and the cold and hot resistances of filament 5 were 9.7 and 144 ohms, respectively. Thus, resistor ll reduced the inrush current from about 13 amperes to about 2 amperes. Also, the heating rate of filament 5 was thereby substantially decreased.
As filament 5 was heating up to its normal operating temperature the temperature of the lamp, as well as the temperature of switch 13, were gradually increasing. When the temperature of switch 13 reached about 150C, switch 13 deflected sufficiently to establish contact with lead-in wire 9, thereby electrically shorting out resistor 11 and permitting full normal operating current to flow through filament 5. Switch 13 should be located within bulb l in a position such that it is heated to its operative temperature within a few seconds. However, switch 13 should not be so close to filament 5 that it is subjected to unnecessarily high temperatures that could render switch 13 inoperative before the lamp has reached the end of its normal life.
When the operating lamp is deenergized, switch 13 must cool to about C in order to open and thereby place resistor 11 in series with filament 5 again. Thus, if the lamp is immediately switched on again before switch 13 has cooled sufficiently, the high inrush current is not prevented. However, such immediate onswitching normally occurs only rarely; thus lamp life is generally extended by this invention even if immediate on-switching may occur several times during normal lamp life.
It is not necessary, for purposes of this invention, that resistor 11 and switch 13 be placed within bulb 1. For example, resistor 11 and switch 13 may be placed within a socket adapter that can be screwed into an incandescent lamp lighting fixture, say a table lamp. Then, a normal incandescent lamp can be screwed into the socket adapter. In such a case, resistor 11 will be placed in series with the lamp filament and will still prevent high inrush current. However, switch 13 should be heated to a high enough temperature during normal lamp operation to be activated and short out resistor 11. If the adapter is not in a position, relative to the lamp, to have switch 13 heated to such a temperature, then it would be necessary to install resistor 11 close enough to switch 13 that the FR losses in resistor 11 would heat switch 13 to its activating temperature. Then after resistor 11 is shorted out by activation of switch 13, it would be necessary to have a small resistor, say about 1200 ohms, across the line, that is, in parallel with the filament, to generate enough heat to keep switch 13 closed. The power consumption of such a resistor is small enough, less than one watt, as to not substantially reduce lamp efficacy.
In some cases, it may be desirable to install resistor 11 and bimetal switch 13 directly into the lighting fixture itself, thus permitting the use of usual incandescent lamps and dispensing with the need of the socket adapter. Such an embodiment may still require the need of the above mentioned small resistor to keep switch 13 closed.
1. An incandescent lamp comprising: a gas filled glass bulb sealed at its lower end to the flare of a stem press glass mount; a screw base, having a center contact, attached to the lower end of said bulb; two lead-in wires extended through the stem press of said mount, one of said lead-in wires being electrically connected to said center contact and the other being electrically connected to the rim of said screw base; a coiled tungsten wire filament disposed within said bulb and electrically conected between said two lead-in wires; an inrush current limiting resistor in series with said filament and a thermal switch in parallel with said resistor, said switch operative to electrically short out said resistor upon attainment thereof of a predetermined elevated temperature; and means to heat said switch to said predetermined elevated temperature, said means being coiled tungsten wire filament.