Extended Life, Double Coil Incandescent Lamp

Abstract

An electric incandescent lamp has a refractory metal wire coil 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 wire coil.

Description

THE INVENTION

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 greater than 12 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.

FIG. 1 in the drawing is an elevational view, partly in section, of an electric incandescent lamp in accordance with this invention.

FIG. 2 is an expanded view of the switch and wire coil of FIG. 1.

The lamp comprises a glass bulb 1 usually having a gaseous filling therein, such as argon and nitrogen. The bottom of bulb 1 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. Also supported in stem press 4 is a dummy support wire 11 which extends upwardly into bulb 1. A coiled tungsten filament 5 is supported between the upper end of support wire 11 and lead-in wire 3. Fastened to the lower portion of support wire 11 is another support wire 12 to which are attached one end of refractory wire coil 13 and one end of U-shaped bimetal switch 14. The other end of coil 13 is connected to lead-in wire 9 while the other end of switch 14, when closed, makes electrical contact with rod 15 fastened to lead-in wire 9. Switch 14 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 coil 13, which are in series with each other. Thus, even though the resistance of filament 5 is quite low, the inrush current is limited by coil 13. In one example of a 100 watt, 120 volt lamp in accordance with this invention, filament 5 consisted of a coiled coil of 2.55 mil tungsten wire having a total wire length of 575 mm and a body length of 22 mm. Coil 13 consisted of a coiled coil of 2.45 mil tungsten wire having a total wire length of 610 mm and a body length of 22 mm. The cold resistance of coil 13 was 11 ohms while the cold and hot resistances of filament 5 were 10.3 and 153 ohms, respectively. Thus, coil 13 reduced the inrush current from over 12 amperes to about 8 amperes or less. 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 14, was gradually increasing. When the temperature of switch 14 reached about 150.degree.C, switch 14 deflected sufficiently to establish contact with rod 15 of lead-in wire 9, thereby electrically shorting out coil 13 and permitting full normal operating current to flow through filament 5. Switch 14 should be located within bulb 1 in a position such that it is heated to its operative temperature within a few seconds after the lamp is energized. However, switch 14 should not be so close to filament 5 that it is subjected to unnecessarily high temperatures that could render switch 14 inoperative before the lamp has reached the end of its normal life. Preferably, coil 13 is disposed proximate switch 14 so that the heat from coil 13 will activate the switch.

When the operating lamp is deenergized, switch 14 must cool to about 150.degree.C in order to open and thereby place resistor 13 in series with filament 5 again. Thus, if the lamp is immediately switched on again before switch 14 has cooled sufficiently, the high inrush current is not prevented. However, such immediate on-switching 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.

Claims

We claim:

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 extending 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 connected between said two lead-in wires; an inrush current limiting refractory metal wire coil in series with said filament and a thermal switch in parallel with said wire coil, said switch operative to electrically short out said wire coil upon attainment thereof of a predetermined elevated temperature and to maintain said wire coil shorted out during lamp operation.

2. The lamp of claim 1 including means to heat said switch to said predetermined elevated temperature.

3. The lamp of claim 1 wherein said switch is normally open when the incandescent lamp is unenergized.

4. The lamp of claim 3 including means to maintain said switch closed after activation thereof.

5. The lamp of claim 1 wherein said wire coil is proximate said switch in order to heat said switch to its operative temperature within a few seconds after lamp energization.