Abstract:
A method to eliminate filament hot shock in lamp filaments, particularly in compact filament light sources such as high efficiency infrared reflective coated halogen lamps, during installation or during energization comprising a voltage reduction circuit that reduces the voltage applied to the lamp filaments for a predetermined period of time and a timing circuit that is activated each time the lamp is energized and controls the predetermined period of time during which the voltage reduction circuit reduces voltage applied to the lamp filaments. Optionally, a one time latch circuit may be included that enables the timing circuit upon energization and disables it after the voltage reduction circuit has operated continuously for the predetermined period of time, and forever thereafter.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a lamp having compact filaments and, more particularly, to the hot shock failure mode known to occur in this type of lamp. 
     BACKGROUND OF THE INVENTION 
     With the introduction of high efficiency infrared reflective coated halogen lamps, the filament size has become more compact and, therefore, more susceptible to shock. Hot shock is a failure mode known to lamp makers where two or more primary or secondary turns of an incandescent filament touch, adhere to one another, and short out a portion of the active filament, resulting in an early burnout of the filament. Over the years, data has been collected from actual customer applications that indicate hot shock damage in this type of lamp occurs during the initial installation with the power on. As customers with hundreds of lamps complained of early hot shock failures, new lamps of the same design were installed with the power off, and the hot shock failures were greatly reduced. In a very few cases, the problem persisted due to vibration caused by construction in the area. 
     Presently, the primary solution to the hot shock problem requires that the filament be designed with increased spacing between turns of the filament and/or the addition of higher levels of nitrogen. In either case, the lamp efficacy is reduced, and in many cases, the only solution is to install the lamps with the power turned off. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an apparatus and a method to eliminate filament hot shock in lamp filaments, particularly in compact filament light sources such as high efficiency infrared reflective coated halogen lamps, during installation or during energization wherein the method comprises a voltage reduction circuit that reduces the voltage applied to the lamp filaments for a predetermined period of time and a timing circuit that is activated each time the lamp is energized and controls the predetermined period of time during which the voltage reduction circuit reduces voltage applied to the lamp filaments. As part of the apparatus, a one time latch circuit is optionally included to enable the timing circuit upon energization and disable it once the voltage reduction circuit has operated continuously for the predetermined period of time, and forever thereafter. 
     Use of the invention means that it is no longer necessary to increase is the spacing between turns of the filament or to add higher levels of nitrogen to the lamp tube to minimize hot shock. Both of the aforementioned remedies reduce the efficacy of the lamp, a drawback that is eliminated by the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of a typical high efficiency infrared reflective coated halogen lamp filament tube; 
     FIG. 2 is a drawing of a typical high efficiency infrared reflective coated halogen lamp installation; 
     FIG. 3 is a filament voltage graph for the lamp; 
     FIG. 4 is a drawing, partially in block form, of an improved high efficiency infrared reflective coated halogen lamp installation; 
     FIG. 5 is a filament voltage graph for the lamp of FIG. 4; 
     FIG. 6 is a drawing of a typical high efficiency infrared reflective coated halogen lamp installation, in a second embodiment of the invention; 
     FIG. 7 is one embodiment of a delay circuit of the present invention and, 
     FIG. 8 is a second embodiment of the delay circuit of the present invention with a one-time latch. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a typical high efficiency infrared reflective coated halogen lamp filament tube  10  to which the invention may be suitably applied. In one embodiment, a filament tube of this type will typically have a filament  12  approximately 10 millimeters in length consisting of 8 to 25 secondary turns. The wall  14  of the filament tube  10  will typically be coated so that the wall is translucent to electromagnetic radiation  16  in the visible spectrum, however, it will reflect radiation  18  in the infrared region. The reflected infrared radiation  18  helps heat the filament  12 , thereby reducing power requirements for the lamp. The close spacing between turns of the filament will occasionally allow neighboring turns of the filament to contact each other if the tube is undergoing vibration such as it would encounter during installation into an energized socket. Even a momentary contact at normal operating temperatures of the filament  12  will allow adjacent turns of the filament  12  to weld permanently together, effectively shorting one or more turns of the filament, resulting in higher current flow, larger power consumption, overheating of the filament  12  and consequent early failure of the filament  12 . This type of failure is referred to as a hot shock failure mode known to lamp makers where two or more primary or secondary turns of an incandescent filament  12  touch, adhere to one another, and short out the active filament, resulting in an early burnout of the filament  12 . It is of course to be understood that the present invention may also be implemented in other sizes and types of lamps which may suffer from hot shock failure due to the closeness of the filament turns. 
     FIG. 2 shows a high efficiency infrared reflective coated halogen lamp  20  that utilizes the filament tube of FIG. 1 during installation into an energized socket  22 . As shown in FIG. 3, a voltage  24  applied to the filament  12  substantially instantaneously goes to full line voltage  26  upon insertion of the lamp  20  into the socket  22 . Vibration as lamp  20  is tightened, after the filament  12  has been energized, can cause adjacent turns of the filament  12  to touch, resulting in the aforementioned hot shock failure mode. 
     Referring now to FIG. 4, with continuing reference to FIGS. 2 and 3, a first embodiment of the invention is illustrated. The elements of FIG. 4 are similar to those of FIG. 2, with the exception being the addition of a delay circuit  28  to the lamp  20 . With the addition of the delay circuit  28 , the filament voltage  24  no longer goes substantially instantaneously to full line voltage  26 . 
     Instead, the filament voltage  24  is reduced to approximately 40 volts for the first 30 seconds as shown in FIG. 5, and only then is allowed to approach full line voltage. The delay circuit includes a voltage reduction circuit  29  that reduces the voltage applied to the lamp filaments for a predetermined period of time and a timing circuit  30  that is activated each time the lamp is energized. The timing circuit  30  controls the predetermined period of time during which the voltage reduction circuit  29  reduces voltage applied to the lamp filament  12 . 
     To determine an appropriate optimal voltage reduction of the delay circuit  28 , twenty (20) lamps of the same design (60PAR/HIR 120V) were tested, a sample of 5 at 120, 80, 60 and 40 volts. Each lamp was lit by a constant current supply at those voltages. The lamps were then swung on a pendulum arm against a stop. The distance was increased until a voltage drop was measured indicating hot shock. The results showed that the average distance required to hot shock increased with a reduction in voltage. At 40 volts, the distance required to hot shock the lamp deformed the filament and caused instant burnout. Forty (40) volts was consequently chosen as the optimal voltage reduction amount for the delay circuit  28  for the described lamps ( 12 ). At 40 volts, the filament  12  is hot enough to provide enough illumination to confirm lamp operation for the installer, but not hot enough to allow hot shock failure mode. A lower voltage is insufficient to adequately illuminate the lamp, however, reduced voltages up to 80 volts are also acceptable. 
     Delay circuit  28  described above results in a short delay before full illumination of the lamp  20 . This delay is not a significant drawback since this type of lamp is typically used in a commercial environment where other lamps are fully illuminated at the time. For example, a retail store may have one burned out bulb, out of many, that requires replacing, and this is usually done while the lighting circuit remains energized. If this delay is undesirable, the delay circuit  28  can optionally further include a one time latching circuit  32  that permanently disables the timing circuit after the voltage reduction circuit has operated, at least once, continuously for the full predetermined time period. In this way, the lamp  20  will have reduced voltage applied to the filament  12  during initial installation but will substantially instantaneously apply full line voltage  26  to the filament  12  each time the lamp is turned on thereafter. 
     Referring now to FIG. 6, a second embodiment of the invention is illustrated. In this embodiment, the delay circuit  28  is incorporated into an adaptor  34 . The delay circuit  28  incorporated into the adaptor  34  includes the same voltage reduction circuit  29 , timing circuit  30  and optional one time latching circuit  32  as previously described. The adaptor  34  is installed in the same socket  22  as the standard lamp  20  would have been installed without the invention. Operation of the second embodiment is, otherwise similar to that of the first embodiment. 
     Referring now to FIG. 7, an exemplary embodiment of a delay circuit suitable for adaptation to the present invention is illustrated. The circuit comprises a rectifier circuit  10 , a timer circuit  12  and a voltage reduction circuit  14  connected in parallel with timer circuit  12 . The rectifier circuit  10  is connected between input terminals  16  and  18  and includes a rectifier diode  20  whose anode is connected to input terminal  16  and whose cathode is connected to filter capacitor  22  with the remaining lead of capacitor  22  being connected to input terminal  18 . 
     The purpose of the rectifier circuit  10  is to provide an approximately DC voltage at the junction of rectifier diode  20  and filter capacitor  22  for timing circuit  12 . 
     Timing circuit  12  includes a first timing resistor  24 , a timing capacitor  26  and a second timing resistor  28  serially connected in the order listed between the junction of rectifier diode  20  with filter capacitor  22  and input terminal  18 . Timing circuit  12  further includes switch  30 , relay  32  and current limiting resistor  34 , with relay  32  comprising energizing coil  36  and normally closed contacts  38 . In this embodiment, switch  30  comprises a MOSFET transistor including gate  40 , source  42  and drain  44  with gate  40  connected to the junction of timing capacitor  26  and second timing resistor  28 , and with source  42  connected to input terminal  18 . Current limiting resistor  34  is first connected to the junction of rectifier diode  20  and filter capacitor  22  with the remaining lead connected to energizing coil  36  whose remaining lead is connected to drain  44  of switch  30 . Voltage reduction circuit  14  comprises resistor  46 , capacitor  48 , diac  50  and thyristor  52 . Resistor  46  and capacitor  48  are serially connected in the order listed between input terminal  16  and output terminal  54 . Relay contacts  38  are also connected between input terminal  16  and output terminal  54 . Thyristor  52  includes main terminal MT1 ( 58 ), main terminal MT2 ( 60 ) and gate terminal  62  with terminal  58  connected to output terminal  54  and terminal  60  connected to input terminal  16 . Diac  50  is connected is between gate terminal  62  and the junction of resistor  46  with capacitor  48 . 
     To briefly describe the operation of the circuit of FIG. 7, when the circuit is initially energized, capacitor  22  quickly charges to nearly 170 volts, assuming an input voltage between terminals  16  and  18  of 120 volts RMS. Current now flows through first timing resistor  24 , timing capacitor  26  and second timing resistor  28 , charging timing capacitor  26 . This charging current creates a voltage drop sufficiently large across second timing resistor  28  to turn switch  30  on which, in turn, causes current to flow through energizing coil  36 , opening contacts  38 . With contacts  38  open, current flowing between input terminal  16  and output terminal  54  must now pass through voltage reducing circuit  14 . Voltage reducing circuit  14  is a typical light dimming circuit wherein thyristor  52  does not turn on until sufficient charge has accumulated on capacitor  48  to overcome the breakdown voltage of diac  50 . Diac  50 , resistor  46  and capacitor  48  are selected such that the RMS output voltage is reduced to the desired value, 40 volts RMS in this exemplary case. Timing resistors  24  and  28 , and timing capacitor  26  are selected such that the voltage drop across timing resistor  28  is insufficient to keep switch  30  turned on after approximately  30  seconds at which time current stops flowing through energizing coil  36  allowing contacts  38  to close and, in turn, allowing the input voltage on terminal  16  to pass unrestricted to output terminal  54 . Timing resistors  24  and  28  are further selected such that the voltage rating of gate  40  is not exceeded. Input terminal  18  and output terminal  56  are interconnected and are intended to be at ground potential. Relay  32  has normally closed contacts  38  so that, in the event of a failure in rectifier circuit  10  or timing circuit  12 , full input voltage at terminal  16  will be supplied to output terminal  54 . Capacitor  22  can be sized sufficiently large to provide a short term memory so that momentary interruptions of the input voltage will not incur another time delay before full input voltage is reapplied to output terminals  54  and  56 . 
     FIG. 8 illustrates an exemplary embodiment similar to FIG. 7 with the addition of components to disable the timing circuit and voltage reduction circuit after completion of one full voltage reduction cycle. The circuit in FIG. 8 is identical to that in FIG. 7 with the following exceptions. Time delay fuse  64  and serially connected resistor  66  are inserted between the cathode of rectifier diode  20  and capacitor  22 . The junction of resistors  24  and  34  is reconnected to the junction of fuse  64  and resistor  66 . Resistor  68  and zener diode  70  are serially connected in the order listed between drain  44  and input ground terminal  18 . Switch  72  is inserted with gate  74  connected to the junction of resistor  68  with zener diode  70 , source  76  connected to input terminal  18 , and drain  78  connected to the junction of capacitor  22  with resistor  66 . The circuit of FIG. 8 operates essentially identically to that of FIG. 7, however, when switch  30  opens after approximately  30  seconds, switch  72  closes, drawing enough current to burn out fuse  64  thereby disabling the timing circuit. Future energizations of this circuit will not incur a time delay. 
     Exemplary component values for the circuits of FIGS. 7 and 8 are as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Rectifier diode 20 
                 1 A, 200 V 
               
               
                   
                 Filter capacitor 22 
                 0.1 μF 
               
               
                   
                 First timing resistor 24 
                 220 megΩ 
               
               
                   
                 Timing capacitor 26 
                 0.06 μF 
               
               
                   
                 Second timing resistor 28 
                 22 megΩ 
               
               
                   
                 Switch 30 
                 MOSFET, 200 V, V G  = 2 V 
               
               
                   
                 Relay 32 
                 1 A, V COIL  = 24 V at 5 mA 
               
               
                   
                 Resistor 34 
                 20 kΩ 
               
               
                   
                 Resistor 46 
                 330 kΩ 
               
               
                   
                 Capacitor 48 
                 0.062 μF 
               
               
                   
                 Diac 50 
                 32 V 
               
               
                   
                 Triac 52 
                 1 A, 200 V 
               
               
                   
                 Fuse 64 
                 0.1 A Time delay 
               
               
                   
                 Resistor 66 
                 220Ω 
               
               
                   
                 Resistor 68 
                 100 kΩ 
               
               
                   
                 Zener diode 70 
                 12 V 
               
               
                   
                 Switch 72 
                 MOSFET, 200 V, V G  = 2 V 
               
               
                   
                   
               
             
          
         
       
     
     There are many other timing and voltage reduction circuits known in the art that are suitable for use in the present invention. Accordingly, the present invention envisions the inclusion of any of these circuits in the embodiments according to FIGS. 7 and 8. 
     Prior art solutions to the hot shock failure mode required the filament to be designed with increased spacing between filament turns and/or the addition of higher levels of nitrogen. In both cases, the lamp efficacy was reduced. In many cases the only solution was to install lamps with the power off. The methods disclosed above avoid the aforementioned drawbacks incurred by increasing the spacing between filament turns. 
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.