Thick film resistor for use in a vacuum and a high pressure discharge lamp having such a resistor

An unsaturated high pressure sodium discharge lamp for operation on a constant wattage type ballast, said lamp comprising:

• an evacuated outer lamp envelope;
• a discharge vessel within said outer envelope including a pair of discharge electrodes between which a discharge is maintained during lamp operation, said discharge vessel having a filling comprising mercury, sodium, and an inert gas, said mercury and sodium being completely vaporized during lamp operation, said discharge vessel having a nominal rated life, and an operating current varying between a lower and an upper limit during lamp operation over said rated life when operated on a constant wattage type ballast;
• means comprising a lamp frame for mechanically supporting said discharge vessel within said lamp envelope and electrically connecting said discharge vessel to a source of electric potential outside of said lamp envelope;
• a starting circuit within said lamp envelope for inducing ionization within said discharge vessel during lamp starting to initiate an arc discharge between said discharge electrodes, said starting circuit comprising a first resistor means connected electrically in parallel with said discharge vessel during lamp starting;
• a second resistor means connected electrically in series with said discharge vessel for preventing flicker of the discharge arc when said lamp is operated on a constant wattage type ballast;
• and
• an integral thick film resistor including a substrate and first and second tungsten thick film resistor elements deposited on said substrate which comprise said first and second resistor means, respectively, metallic resistor terminals connected to a corresponding tungsten resistor element, a metallic solder securing each of said terminals to said substrate and a cover of material of low vapor pressure covering said solder, said substrate having a surface emissivity of greater than about 0.9,
• said substrate having a surface area and said cover of material on said solder having a thickness selected such that during operation of said first and second resistive elements with said discharge vessel operating current within said upper and lower limits, said substrate remains within a temperature such that evaporation of said solder is substantially eliminated and substantially does not darken the inner surface of said lamp envelope over said nominal rated life, and such that said substrate has a temperature such that said substrate does not crack over said nominal rated life.

A high pressure discharge lamp having an evacuated outer lamp envelope, a discharge vessel disposed within said lamp envelope and energizable for emitting light, and a resistor within said envelope, characterized in that:

• said resistor is a thick film resistor comprising a substrate and a thick film tungsten resistive element disposed on said substrate;
• said substrate has a surface emissivity of greater than about 0.9 and consists essentially of 91% by weight of Al.sub.2 O.sub.3, 5% SiO.sub.2, 2% MgO, 1% CaO and 1% T.sub.i O.sub.2,
• metallic connection terminals are electrically connected to said resistive element and mechanically fixed to said substrate by a metallic solder,
• a coating of nickel with a thickness of between about 4 and 6 .mu.m covers said solder, and
• the surface area of said substrate and the ohmic resistance of said resistive element are selected such that said resistor has a temperature during lamp operation of less than about 600.degree. C.

A thick film resistor for operation in an evacuated environment, said resistor comprising:

• a substrate and a thick film tungsten resistive element disposed on said substrate,
• said substrate having a surface emissivity of greater than about 0.9 and consisting essentially of 91% by weight of Al.sub.2 O.sub.3, 5% SiO.sub.2, 2% MgO, 1% CaO and 1% T.sub.i O.sub.2,
• metallic connection terminals electrically connected to said resistive element and mechanically fixed to said substrate by a metallic solder, and
• a coating of nickel with a thickness of between about 4 and 6 .mu.m covering said solder,
• the surface area of said substrate and the ohmic resistance of said resistive element being selected such that said resistor has a temperature during operation of less than about 600.degree. C. when operated in a vacuum of less than 10.sup.-2 Torr.