Patent Publication Number: US-6211618-B1

Title: Arc discharge lamp with bimetal strip for fast passive lamp failure

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an arc discharge lamp of the type including a base having an insulator therein, a pair of terminals fixed in said insulator, a light transmissive arc tube having an end, and a glass stem sealed in the end, the stem including a stem press and a thin walled portion attached to the arc tube. A pair of leads connected to respective terminals extend through the press seal into the arc tube, where an electrode is connected between the leads. A conductor fixed to a lead wire in the arc tube extends toward a thin walled portion of the stem to assure fast passive lamp failure. 
     When an electrode in the form of a filament connected between the lead wires of an arc discharge lamp fails, typically when an electron emitter thereon for sustaining a discharge between opposite ends of the lamp is depleted, the arc discharge operates directly off the lead wires. This causes overheating of the electrode structure and elevated arc tube (bulb wall) temperatures. This overheating may cause the electrode structure to fall against the arc tube. It is also possible that the lamp might simply flicker, or the ballast might overheat and fail. 
     The failure problem can be exacerbated when a discharge lamp is used with a ballast which is not designed optimally for use with the lamp. For example, a type F32T8 fluorescent tube works best, from a standpoint of lifetime, lumen maintenance, and dimming capabilities, with a “rapid start” type ballast. However “instant start” type ballasts have become increasing popular because of their lower cost and reduced power consumption, and the standard fluorescent tubes are fitted in fixtures having the instant start ballasts. This has been found to decrease the lifetime of the lamp by as much as 25%. 
     The failure process of F32T8 lamps on a high frequency instant start ballast at end of life is as follows. When the electrode and emission material thereon are depleted, the source of electrons which sustains the discharge throughout normal life is also depleted. With an Instant Start HF ballast enough open circuit voltage is available to ignite an arc discharge and operate from the lead wires which held the filament. However the lead wires have a much higher work function than the electrode and emitter, so it takes far more energy to extract electrons. This extra energy heats the metal parts and the surrounding glass to a very high temperature. This causes the glass in the stem press to soften, which allows the supporting structure of the shield to sag, which allows the shield to become shorted to a lead wire. The shield then becomes the cold cathode, which also has a very high work function and heats the surrounding glass. 
     U.S. Pat. No. 3,265,917 discloses an arc discharge lamp having a conductor welded to an electrode and extending toward the thin walled portion of the stem. It was found that proper placement of a conductor would cause arcing between the end of the conductor and the thin walled portion of the stem, causing it to melt and admit air which rapidly extinguished the arc. However the positioning of the conductor must be very precise; the end should actually be touching the glass in order to strike an arc which burns through the thin-walled portion. Such precise positioning can be problematic from a manufacturing standpoint. 
     SUMMARY OF THE INVENTION 
     According to the invention, the conductor is a bimetal strip having a first end welded to one of the lead wires and a second end proximate the thin-walled portion of the stem. The bimetal strip is arranged so that the second end bends toward the thin-walled portion when it is heated by the arc discharge which occurs on the lead wires when the electrode has failed. 
     Use of a bimetal strip assures that the end of the conductor will contact the thin-walled portion of the stem to cause arc-through and passive failure of the lamp. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side view of the end of a fluorescent lamp according to the invention; and 
     FIG. 2 is an end view of the end of a fluorescent lamp according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures, the end of an arc discharge lamp of the type commonly referred to as a fluorescent tube includes a stamped and formed aluminum base  10  having a phenolic resin insulator  12  fitted therein; a pair of terminals  14  are fixed in the insulator  12  and pass through the base  10 . 
     An arc discharge vessel in the form of a glass tube  16  having an end  18  is fixed in the base  10  with potting compound  20 . A glass stem includes a stem press  22  and a thin-walled portion  24  which is sealed to the end of the lamp vessel in the base. A hole  26  in the stem press  22  connects with an exhaust tube  28  which is used to draw vacuum in the lamp (1-4 Torr) during manufacture. 
     A pair of leads  30  are connected to respective terminals  14  and embedded in the stem press  22  which serves as a seal. Inner portions of the lead wires  30  support an electrode  34  in the form of a double coil coated with an emitter. A center lead  36  embedded in the stem press  22  supports a shield  38  which surrounds the electrode  34 . 
     A conductor in the form of a bimetal strip  40  has a first end  42  welded to one of the leads inside the lamp vessel and extends to a second end  44  proximate to the thin-walled portion  24 . When the electrode fails and the arc discharge occurs on the inner ends of the leads  30 , heat causes the bimetal strip to bow so that the second end  44  touches the thin walled portion, thus striking an arc which burns through the thin-walled portion and admits air which extinguishes the arc. Passive failure is thus achieved without the inner lead sagging against the shield, and without danger of fracturing the glass tube  16 . 
     The foregoing is exemplary and not intended to limit the scope of the claims which follow.