Abstract:
The present invention pertains to an electrical circuit for operating warning lights that are commonly used on emergency and service vehicles. The electric circuit enables the use of pairs of 21 volt DC halogen lamps on a light bar installed on an emergency vehicle equipped with a 42 volt electrical system.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention pertains to an electrical circuit for operating warning lights that are commonly used on emergency and service vehicles. The warning lights incorporate halogen lamps and a rotating reflector. The reflector rotates or oscillates about the halogen lamp to give the appearance of a flashing light. The electrical circuit of the present invention provides a circuit for operating standard halogen lamps having a rating of 18-21 volt DC and a power rating of between 25-50 watts on vehicles having a 36-42 volt electrical system. 
     (2) Description of the Related Art 
     Since the invention of the rotating beacon in the 1940s, the vast majority of emergency and service vehicles have been protected by warning lights that use an incandescent lamp. In these systems, the lamp is given the appearance of flashing by a rotating reflector positioned in the warning light. The reflector is rotated around the lamp and produces a rotating, reflected arc of coverage and a level of warning within a specified zone around the emergency vehicle. 
     Although some emergency vehicles use other warning light systems, such as light emitting diodes (LEDs) and gaseous discharge tubes (strobe systems), the use of a halogen lamp and a rotating reflector is preferred and used on a majority of warning light systems because of its initial low cost, high efficiency, easy maintenance, and long life. With the advent of reasonably priced halogen incandescent lamps, the halogen cycle lamp, with its higher efficiency, has become a light source of choice, and these other systems have yet to match any of the advantages of the halogen lamp system. 
     Typically, standard halogen lamps are rated for operation between 12 and 24 volts and for operation between 25 and 50 watts. Current operational characteristics of the halogen lamps make them very adaptable for use on automobiles having electrical systems operating at between 12 and 14 volts. However, automobile manufacturers have recently announced the introduction of automobiles that will use a 42 volt electrical system. Thus, the change in the automotive voltage from 12 volts DC to 42 volts DC precludes the transfer of existing warning light systems to new emergency vehicles, as is commonly done today by many municipalities. 
     Additionally, the change in the electrical systems of automobiles to 42 volts presents other problems. At this time, no lamp manufacturer is able to manufacture a halogen lamp in the 25 to 50 watt range that can operate at 42 volts in automotive service. In automotive systems that use halogen lamps, there is a minimum diameter of tungsten filament that is required to maintain the halogen cycle. In order to develop a halogen lamp that operates at voltages that have been increased three-fold from 14 to 42 volts, for a given minimum practical filament diameter, the filament length must also increase three-fold. This consequently causes the wattage of the lamp to increase three-fold. While it is theoretically possible to replace three 50 watt elements with one 150 watt element, in reality, it is difficult to get the distribution of light (arc of coverage) that is desired in order to provide adequate levels of warning. 
     For instance, a typical light bar for an emergency vehicle is equipped with a system of halogen lamps that may be selectively illuminated to provide three different levels of warning. Each level of warning provides a different arc of coverage around the vehicle, and, generally speaking, two 50 watt lamps are provided in different sets at different positions on the light bar to generate the required arc of coverage and level of warning. In a low level warning condition, two 50 watt lamps are provided on the light bar to only cover the rear of the emergency vehicle. For an increased level of warning, the two 50 watt lamps are provided for rear only coverage and four 50 watt lamps are provided for additional coverage on the front, sides and rear of the vehicle. In a further increased level of warning, the two 50 watt lamps are provided for rear only coverage, the four 50 watt lamps are provided for additional coverage to the front, sides, and rear of the vehicle, and two additional 50 watt lamps are provided for added coverage to the front. As discussed earlier, increasing the voltage of the vehicle three-fold to 42 volts would also require increasing the wattage of the warning lamps three-fold to 150 watts. However, it is difficult to use a combination of 150 watt lamps, with each 150 watt lamp replacing three 50 watt lamps, and obtain this type of coverage. Moreover, the loss of a single 150 watt lamp causes a catastrophic loss of protection, in effect being equivalent to losing three 50 watt lamps. Thus, it is desired to continue the use of standard halogen lamps on current light bars and to adapt the light bar to operate with a 42 volt electrical system. 
     Halogen lamps in the 25 to 50 watt range have been successfully manufactured for operation in automotive service with voltages up to 24 volts. Although placing two 21 volt halogen lamps in series produces the required 42 volt voltage drop, placing two 21 volt lamps in series for operation at 42 volts presents problems that are unacceptable for operation in a light bar application. When two 21 volt lamps are placed in series, if one lamp fails, both are extinguished. Obviously, this presents an unsatisfactory condition in an apparatus designed to provide visible warning. 
     Additionally, connecting two halogen lamps in series produces a high rate of start-up failure for the lamps. The cold-resistance of the lamp filament of a typical halogen lamp is very small and varies substantially from lamp to lamp even between new lamps produced from the same manufacturer having identical ratings and capacity. For example, the cold resistance of the filament of a 14 volt, 50 watt halogen lamp may vary from 0.3 ohms to 0.7 ohms, although when lighted and heated, the filaments of identical lamps have virtually the same resistance of about 3.5 ohms. When a mix of new and old lamps from different manufacturers are measured, the cold resistance of the lamp filaments can vary even more. Taking into account the variations in the socket contact resistances and wire lengths in the light bar, the effective cold resistance of each lamp in the light bar greatly varies. This variation in cold resistance makes operating the halogen lamps in series problematic. 
     The inventor has observed that the filament life in a halogen lamp is a function of the voltage applied, the ambient temperature of the lamp&#39;s glass envelope, and the heat sinking effect provided by the socket structure. The inventor has seen that at room temperature and voltages above 17 volts, the time to failure of the typical halogen lamp is very short. In a series connected arrangement, when the circuit is energized, each lamp will have a different initial voltage drop thereacross due to their different resistances. As the filament heats to incandescence, the voltages across the lamps will equalize. However, when the lamps are connected in series and the circuit is initially energized, the mismatch in cold resistances of the filaments often creates a high voltage drop across one of the two lamps. This increases the failure rate of the lamp. In order to limit failure when lamps are connected in series, the lamp with the lower resistance filament must heat quickly to gain resistance so as to prevent failure of the higher resistance filament of the other lamp with which it is connected in series. Again, this condition is unacceptable in an apparatus designed to provide visible warning. 
     Therefore, what is needed is a light bar that allows the use of standard halogen lamps having a voltage rating of between 18 and 21 volts and a power rating of between 25 to 50 watts on an emergency vehicle equipped with a 36-42 volt DC electrical system. Such a light bar would be provided with an electrical circuit to allow use of standard halogen lamps in the standard light bar when it is installed on a new emergency vehicle equipped with the 36-42 volt electrical operating system. Such a circuit would allow the use of halogen lamps having different cold resistances. Such a circuit would protect mismatched lamps from failure during start up. Such a circuit would maintain the required level of warning in case of failure of a lamp. Such a circuit would be easily retrofitted onto existing light bars to permit the light bar to be installed on new automobiles equipped with the 36-42 volt DC electrical system. 
     SUMMARY OF THE INVENTION 
     The electrical circuit of the present invention enables current users of halogen lamp warning lights to use a standard halogen lamp having a voltage rating of between 18 and 21 volts DC in a light bar. The electrical circuit is installed in light bars that are to be used on emergency vehicles equipped with the 36-42 volt electrical systems. The circuit basically enables the series connection of 18-21 volt lamps of the light bar, thereby adapting the light bar for use on a vehicle with a 36-42 volt electrical system. The electrical circuit of the present invention prevents start up failure of the halogen lamps and maintains the required level of warning in case of failure of a lamp. 
     The circuit of the present invention provides a first lamp paired with a first resistor, and a second lamp paired with a second resistor. The circuit also includes means for switching the circuit between a start up circuit where the series connected first lamp and first resistor are connected in parallel with the series connected second lamp and second resistor, and an operation circuit where the first lamp is disconnected from the first resistor and the second lamp is disconnected from the second resistor and the first lamp is connected in series with the second lamp. The switching means employs a time delay switch that is operable to switch the circuit from the start up circuit to the operational circuit after the first and second lamps reach incandescence. Thus, in the start-up circuit, the circuit allows the lamps to become illuminated through their respective resistors and achieve a uniform and relatively high illuminated resistance before the lamps are switched into a series connection in the operation circuit. 
     In another aspect of the invention, the switching means switches the circuit from the operation circuit to the start up circuit when the series connection between the first and second lamps is opened by either a lamp failure or a lamp being removed from the circuit. Thus, the electric circuit of the present invention maintains one of the operational lamps illuminated by reconnecting the operational lamp through its resistor in the start-up circuit. In this way, the desired level of warning may be maintained in the event of a failure of one of the two lamps. Preferably, the switching means allows the circuit to be operated as required with the one operational lamp until the other failed lamp is replaced. 
     The electric circuit of the present invention uses a resistor having a relatively high cold resistance in series with each lamp to bring the lamp to incandescence. The circuit of the present invention connects two lamps in series only after each lamp has reached incandescence, thus preventing start up failure of the lamps from excessive voltage at initial circuit energization. The circuit of the present invention detects the failure of either of the lamps operating in series and instantly reconnects the lamps to their respective start up resistors, thus allowing the operational lamp to continue to function with minimum degradation of the required level of warning. The circuit of the present invention may be operated and repeatedly turned on and off with the operational lamp continuing to operate at full brightness before the failed lamp is replaced. Thus, the electrical circuit of the present invention allows current 14 volt light bars to be retrofitted to emergency vehicles equipped with the 36-42 volts DC operating system with the  14  volt lamps replaced with pairs of 18-21 volt halogen lamps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and features of the invention are revealed in the following detailed description of the preferred embodiment of the invention and in the drawings, wherein: 
     FIG. 1 is an electric circuit diagram showing an alignment of the electrical circuit of the present invention at start-up; 
     FIG. 2 is an electric circuit diagram showing an alignment of the electrical circuit of the present invention in normal operation; 
     FIG. 3 is an electric circuit diagram showing the alignment of the electric circuit of FIG. 2 after a failure of a first lamp; 
     FIG. 4 is an electric circuit diagram showing the alignment of the electric circuit of FIG. 2 after a failure of a second lamp; 
     FIG. 5 is an electric circuit diagram showing an alignment of the electrical circuit of the present invention, incorporating a dual filament lamp, at start-up; 
     FIG. 6 is an electric circuit diagram showing an alignment of the present invention, incorporating a dual filament lamp, in normal operation; 
     FIG. 7 is an electric circuit diagram showing an alignment of the electrical circuit of the present invention, incorporating a dual filament lamp shown in FIG. 6, after a failure of the first lamp filament; and 
     FIG. 8 is an electric circuit diagram showing an alignment of the electrical circuit of the present invention, incorporating a dual filament lamp shown in FIG. 6, after a failure of the second lamp filament. 
    
    
     Corresponding reference numbers indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-4 show schematic representations of the circuit of the invention during different operative modes of the circuit. The basic concept of the circuit is the revelation that, regardless of the filament resistance at room temperature of a halogen lamp, any halogen lamp of a given wattage and voltage can be safely brought to incandescence if placed in series with an equivalent, properly matched resistor. A properly matched resistor being a resistor that has the same nominal resistance as that of the halogen lamp when heated at incandescence and has a relatively high resistance at room temperature. Furthermore, when any two halogen lamps having the same nominal wattage and voltage are brought to incandescence, they can then be disconnected from the start up resistor matched with the lamp and the two lamps can then be placed in series without failure of either of the lamps. Because all lamps will eventually fail in use, when either of the two lamps of the circuit are operating in series and one eventually does fail, the circuit automatically and immediately reconnects both lamps to their matched start up resistors and the operative lamp will continue to function at substantially full brightness. By eliminating the two significant problems of lamp operation in series, the circuit of the invention makes the series operation of pairs of 18-36 volt lamps practical with a 36-42 volt electrical supply source. 
     Referring to FIG. 1, the circuit  10  of the invention is provided with a first junction or input junction  12  that is adapted to be connected to a power source, in the illustrative embodiment a 42 volt DC power system. Connection of the first junction  12  with the DC power source is selectively controlled by a first manual switch  14  connected in series between the power source and the first junction. The circuit also includes a second junction or an output junction  16 . The second junction  16  is adapted to be connected to a source of ground G. In the illustrative embodiment, the ground G is a ground of a vehicle having a 42 volt DC power system, for example the chassis of the vehicle. Extending between the first, input junction  14  and the second, output junction  16  of the circuit are several different conductive paths that are ultimately connected in series and in parallel to each other in the operation of the circuit to be explained. 
     The first conductive path  20  extends from the first, input junction  12  to a first lamp  22 , preferably a 21 volt DC halogen lamp. From the first lamp  22 , the first conduct path continues to a means for switching such as the first switching device  24  represented by dashed lines in the drawing figures. The first switching device  24  is represented in the drawing figures as a double pole, double throw relay. Additional switching devices of the circuit yet to be described are also represented in the drawing figures as electromechanical relays. However, the electromechanical relays shown and to be described are employed in the illustrative embodiment of the circuit to simplify its description. It should be understood that the same circuit of the invention could be made to function using other equivalent means for switching, for example, low resistance, solid state switches such as field effect transistors (FET&#39;s). The first conductive path  20  continues from the first switching device to a first resistor  26 . In the preferred embodiment, the first resistor  26  has a relatively high resistance at room temperature that substantially matches the resistance of the first lamp  22  when heated at incandescence. From the first resistor  26 , the first conductive path  20  continues to its termination at the second output junction  16 . Thus, the first conductive path connects in series the first input junction  12 , the first lamp  22 , the first switching device  24 , the first resistor  26  and the second output junction  16 . 
     A second conductive path  30  also begins at the first, input junction  12  and extends to a second resistor  32 . From the second resistor  32 , the second conductive path  30  extends to the first switching device  24 . From the first switching device  24 , the second conductive path  30  continues to a second lamp  34  and then continues on to its termination at the second, output junction  16 . Thus, the second conductive path  30  provides a series circuit between the first input junction  12 , the second resistor  32 , the first switching device  24 , the second lamp  34  and the second output junction  16 . In the preferred embodiment, the second lamp  34  is the same as the first lamp  22  in that it is also a 21 volt DC lamp and has the same resistance when heated at incandescence. In addition, the second resistor  32  is the same as the first resistor  26  having the same resistance as the first resistor. 
     A third conductive path  38  also begins at the first input junction  12 . The third conductive path  38  extends from the first input junction  12  to a time delay switch  40 . The time delay switch  40  is operable to remain open for a predetermined period of time when supplied with a predetermined voltage before closing. In the preferred embodiment, the period of time delay is approximately 5 seconds and the minimum voltage applied to the time delay switch  40  causing it to close is 42 volts DC. From the time delay switch  40 , the third conductive path  38  continues to a second switching device  42  also represented by dashed lines in the drawing figures. In the drawing figures and in the description to follow, the second switching device, like the first switching device  24  is an electromechanical relay. However, as explained earlier, equivalent types of switching devices may be employed other than the electromechanical relays shown in the illustrative embodiment of the circuit of the invention. In the illustrative embodiment, the second switching device  42  is a single pole, double throw relay. From the second switching device  42 , the third conductive path  38  continues to the first switching device  24 . From the first switching device  24 , the third conductive path  38  continues to a third switching device  44  that is the same as the second switching device  42 . From the third switching device  44 , the third conductive path  38  continues to its termination at the second, output junction  16 . Thus, the third conductive path  38  provides a series circuit between the first, input junction  12 , the time delay switch  40 , the second switching device  42 , the first switching device  24 , the third switching device  44  and the second, output junction  16 . 
     A fourth conductive path  48  also begins at the first, input junction  12 . From the first input junction  12 , the fourth conductive path  48  extends to the second switching device  42 . From the second switching device  42 , the fourth conductive path  48  continues to the third switching device  44  and then terminates at the second output junction  16 . Thus, the fourth conductive path  48  provides a series connection from the first, input junction  12 , through the second switching device  42 , the third switching device  44  to the second, output junction  16 . 
     The circuit also includes a fifth conductive path  52  that begins at a junction  54  with the fourth conductive path  48  between the second switching device  42  and the third switching device  44 . From the junction  54  with the fourth conductive path  48 , the fifth conductive path  52  continues to the first switching device  24 . 
     The first start-up stage of operation of the circuit  10  is shown in FIG.  1 . In FIG. 1, the manual switch  14  has been moved from its open position shown in dashed lines, to its closed position shown in a solid line. This connects the 42 volt DC power source to the first, input junction  12 . When initially connected with power, the time delay switch  40  of the circuit is open preventing power from reaching the coil  58  of the relay employed as an example of the first switching device  24 . This allows the pair of normally closed contacts  60  of the first switching device  24  to remain closed, connecting the first lamp  22  in series with the first resistor  26  and the second lamp  34  in series with the second resistor  32 . The relays of the second switching device  42  and third switching device  44  have their respective coils  64 ,  66  connected in series. In the preferred embodiment, each coil  64 , 66  is a 42 volt coil that each open their contact at about 75% of the coil ratings and hold their contacts open at 50% or less of the coil ratings. The capacitors  62  across each of the coils  58 ,  64 ,  66  of the relays dampen any oscillations and provide added circuit stability. 
     With the manual switch  14  closed, the circuits through the normally closed contacts  60  of the first switching device  24  are energized and the first lamp  22  is connected in series with the first resistor  26  between the first, input junction  12  and second, output junction  16  and the second lamp  34  is connected in series with the second resistor  32  between the first, input junction  12  and second, output junction  16 . Thus, the first conductive path  20  and the second conductive path  30  are each series circuits including a lamp and resistor, and the two series circuits are connected in parallel between the first, input junction  12  and the second, output junction  16 . This applies 42 volts to the first lamp  22  in series with the first resistor  26  and to the second lamp  34  in series with the second resistor  32 . Both of the lamps  22 , 34  come to incandescence within a few seconds and stabilize. Because the draw on amperage is greater with the two parallel circuits, each having a lamp  22 , 34  operating independently through its respectively paired resistor  26 , 32 , than with a single circuit with two lamps operating together in series, the first lamp  22  and second lamp  34  connected in parallel come to incandescence but not full brightness. This soft start of the two lamps provides time for the lamps to stabilize before being connected in series. 
     As shown in FIG. 2, after an interval of about 5 seconds following closing of the manual switch  14 , the time delay switch  40  also closes, holds, and applies 42 volts to the coil  58  of the first switching device  24 . This voltage applied to the coil of the first switching device  24  causes its contacts  60  to open, disconnecting the first lamp  22  from the first resistor  26  and disconnecting the second lamp  34  from the second resistor  32 . The voltage applied to the coil  58  of the first switching device  24  also causes the first lamp  22  to be connected in series with the second lamp  34  as shown in the operational circuit of FIG.  2 . With the first lamp  22  and second lamp  34  connected in series across the first switching device  24 , 42 volts are applied across the series connected lamps and they come to full brightness. The lamps continue to operate at full brightness until the manual switch  14  is opened. When the manual switch  14  is opened, the voltage source is removed from the circuit and the circuit is deenergized, the first lamp  22  and the second lamp  34  are extinguished and the time delay switch  42  is opened and reset. 
     As explained earlier, each of the coils  64 , 66  of the respective second switching device  42  and third switching device  44  are 42 volt coils that cause the contacts of the switching devices to open at about 75% of the coil rating and hold the contacts open at about 50% of the coil ratings. During normal operation of the circuit described above and as illustrated in FIG. 2, with the switching device coils  64 , 66  being connected in series, and with the series connected coils also being connected in parallel with the series connected first lamp  22  and second lamp  34 , about half of the circuit voltage of 21 volts is applied across the two coils. This voltage is inadequate to cause either of the contacts of the second switching device  42  or third switching device  44  to open. 
     If the filament in the first lamp  22  fails, the midpoint  70  of the series connection between the first lamp  22  and the second lamp  34  remains connected to the source of ground G through the filament in the second lamp  34  and the voltage at the midpoint  70  goes to 0 volts. When the series circuit of the two lamps  22 ,  34  is opened by the failure of the first lamp  22 , the voltage across the coil  64  of the second switching device  42  then rises to 42 volts and the contact  72  of the second switching device is opened and holds as illustrated in FIG.  3 . When the contact  72  of the second switching device  42  is opened, it breaks the circuit to the coil  58  of the first switching device  24  which then causes its series connection between the first lamp  22  and second lamp  34  to be broken and closes the first switch contacts  60  reconnecting the first lamp  22  in its series connection with the first resistor  26  and reconnecting the second lamp  34  and its series connection with the second resistor  32 . The first lamp  22 , which has burned out, remains out, but the second lamp  34  is instantly relighted as it is re-energized by reconnection through its paired resistor  32 . The change over between the series connection between the two lamps  22 , 34  and the paired connections of the lamps with their associated resistors is so rapid that it is imperceptible to the human eye and the second lamp  34  appears to be continuously lit throughout the change over. 
     Opening the manual switch  14  extinguishes the second lamp  34  and closing the manual switch  14  re-energizes the circuit  10  which causes the second lamp  34  to again illuminate in the same manner as described above. Replacing the first lamp  22  restores the circuit  10  to normal operation. Should the second lamp  34  fail before the first lamp  22  is replaced, the circuit continues to function, but of course there is no light from either lamp. 
     In a similar manner as shown in FIG. 4, if during normal operation of the circuit the second lamp  34  should burn out, the voltage at the midpoint  70  of the series connection between the two lamps increases to 42 volts which then causes the coil  66  of the third switching device  44  to open its contact  74 . This again breaks the series connection between the first lamp  22  and the second lamp  34  and causes the switch contacts  60  of the first switching device  24  to close. This connects the first lamp  22  to its paired, first resistor  26  and the first lamp continues to operate. 
     Although the drawing figures contained herein show the first and second halogen lamp as being two separate lamp elements, it should be noted that the first and second lamps may be of the type where the lamp filament for each lamp is contained in the same lamp envelope or capsule. Typically, lamps of this type have a double contact, bayonet base that is wired so that one end of each of the filaments is interconnected and have a common termination to the shell. The other ends of each filament are then separately terminated to one of the base contacts. In a circuit using such a style of halogen lamp, the switching means would be configured as shown in FIG. 5 with auxiliary contacts or other switching means to switch the ground connection, as necessary to align the filaments in lamp  25  in parallel with the associated start-up resistor in the start-up circuit or in series together in the operation circuit. Preferably, each filament would be designed to operate at about 25 watts for a total of 50 watts when both filaments are at incandescence. In such style of lamp, in the event of failure of either filament, the other filament would continue to light and provide continued protection at half power. This arrangement would be particularly suited for a light bar having a configuration where there is only one lamp in the circuit to provide the required level of warning, for instance, a single yellow element used in the rear of the light bar. 
     Referring to FIG. 5, a further embodiment of the circuit  10  of the invention is provided with a first junction or input junction  12  that is adapted to be connected to a power source, in the illustrative embodiment a 42 volt DC power system. Connection of the first junction  12  with the DC power source is selectively controlled by a first manual switch  14  connected in series between the power source and the first junction. The circuit also includes a second junction or an output junction  16 . The second junction  16  is adapted to be connected to a source of ground G. In the exemplary embodiment, the ground G is a ground of a vehicle having a 42 volt DC power system, for example the chassis of the vehicle. Extending between the first, input junction  14  and the second, output junction  16  of the circuit are several different conductive paths that are ultimately connected in series and in parallel to each other in the operation of the circuit to be explained. 
     The first conductive path  20  extends from the first, input junction  12  to a first resistor  26 . From the first resistor  26  the first conductive path continues to a means for switching such as the contacts  63  contained in the first switching device  29  represented by dashed lines in the drawing figures. The first switching device  29  is represented in the drawing figures as a three pole, double throw relay. Additional switching devices of the circuit yet to be described are also represented in the drawing figures as electromechanical relays. However, the electromechanical relays shown and to be described are employed in the illustrative embodiment of the circuit to simplify its description. It should be understood that the same circuit of the invention could be made to function using other equivalent means for switching, for example, low resistance, solid state switches such as field effect transistors (FET&#39;s). The first conductive path  20  continues from the first switching device to the first lamp filament  28 . From the first lamp filament  28  the first conductive path  20  continues to a second set of contacts  65  contained in the first switching device  29 . From the first switching device  29 , the first conductive path  20  continues to its termination at the second, output junction  16 . Thus, the first conductive path connects in series the first, input junction  12 , the first resistor  26 , the first lamp filament  28 , the first switching device  29 , and the second, output junction  16 . 
     A second conductive path  30  also begins at the first, input junction  12  and extends to a second resistor  32 . From the second resistor  32 , the second conductive path  30  extends to the third set of contacts  67  in the first switching device  29 . From the first switching device  29 , the second conductive path  30  continues to a second lamp filament  27  which merges with the first conductive path  20 , and then continues on to its termination at the second, output junction  16 . Thus, the second conductive path  30  provides a series circuit between the first, input junction  12 , the second resistor  32 , the first switching device  29 , the second lamp filament  27 , and the second, output junction  16 . In the preferred embodiment, the second lamp filament  27  is the same as the first lamp filament  28  in that it is also a 21 volt DC filament and has the same resistance when heated at incandescence. In addition, the second resistor  32  is the same as the first resistor  26  having the same resistance as the first resistor. 
     A third conductive path  38  also begins at the first, input junction  12 . The third conductive path  38  extends from the first, input junction  12  to a time delay switch  40 . The time delay switch  40  is operable to remain open for a predetermined period of time when supplied with a predetermined voltage before closing. In the preferred embodiment, the period of time delay is approximately 5 seconds and the minimum voltage applied to the time delay switch  40  causing it to close is 36-42 volts DC. From the time delay switch  40 , the third conductive path  38  continues to a second switching device  42  also represented by dashed lines in the drawing figures. In the drawing figures and in the description to follow, the second switching device, like the first switching device  29 , is an electromechanical relay. However, as explained earlier, equivalent types of switching devices may be employed other than the electromechanical relays shown in the illustrative embodiment of the circuit of the invention. In the illustrative embodiment, the second switching device  42  is a single pole, double throw relay. From the second switching device  42 , the third conductive path  38  continues to the first switching device  29 . From the first switching device  29 , the third conductive path  38  continues to a third switching device  44  that is the same as the second switching device  42 . From the third switching device  44 , the third conductive path  38  continues to its termination at the second, output junction  16 . Thus, the third conductive path  38  provides a series circuit between the first, input junction  12 , the time delay switch  40 , the second switching device  42 , the first switching device  29 , the third switching device  44  and the second, output junction  16 . 
     A fourth conductive path  48  also begins at the first, input junction  12 . From the first, input junction  12 , the fourth conductive path  48  extends to the second switching device  42 . From the second switching device  42 , the fourth conductive path  48  continues to the third switching device  44  and then terminates at the second, output junction  16 . Thus, the fourth conductive path  48  provides a series connection from the first, input junction  12 , through the second switching device  42 , the third switching device  44  to the second, output junction  16 . 
     The circuit also includes a fifth conductive path  52  that begins at a junction  54  with the fourth conductive path  48  between the second switching device  42  and the third switching device  44 . From the junction  54  with the fourth conductive path  48 , the fifth conductive path  52  continues to the first switching device  29 . 
     The first start-up stage of operation of the circuit  10  is shown in FIG.  5 . In FIG. 5, the manual switch  14  has been moved from its open position shown in dashed lines, to its closed position shown in a solid line. This connects the 42 volt DC power source to the first, input junction  12 . When initially connected with power, the time delay switch  40  of the circuit is open, preventing power from reaching the coil  58  of the relay employed as an example of the first switching device  29 . This allows the pair of normally closed contacts  63 ,  67  of the first switching device  29  to remain closed, connecting the first resistor  26  in series with the first lamp filament  28  and the second lamp filament  27  in series with the second resistor  32 . The relays of the second switching device  42  and third switching device  44  have their respective coils  64 ,  66  connected in series. In the preferred embodiment, each coil  64 ,  66  is a 42 volt coil that each open their contact at about 75% of the coil ratings and hold their contacts open at 50% or less of the coil ratings. The capacitors  62  across each of the coils  58 ,  64 ,  66  of the relays dampen any oscillations and provide added circuit stability. 
     With the manual switch  14  closed, the circuits through the normally closed contacts  63  of the first switching device  29  are energized and the first lamp filament  28  is connected in series with the first resistor  26  between the first, input junction  12  and second, output junction  16  and the second lamp filament  27  is connected in series with the second resistor  32  between the first, input junction  12  and second, output junction  16 . Thus, the first conductive path  20  and the second conductive path  30  are each series circuits including a lamp and resistor, and the two series circuits are connected in parallel between the first, input junction  12  and the second, output junction  16 . This applies 42 volts to the first lamp filament  28  in series with the first resistor  26  and to the second lamp filament  27  in series with the second resistor  32 . Both of the lamp filaments  27  and  28  come to incandescence within a few seconds and stabilize. Because the draw on amperage is greater with the two parallel circuits, each having a lamp filament  27  and  28  operating independently through its respective paired resistor  26 ,  32 , then with a single circuit with two lamp filaments operating together in series, the first lamp filament  28  and second lamp filament  27  connected in parallel come to incandescence but not full brightness. This soft start of the two lamp filaments provides time for the lamps to stabilize before being connected in series. 
     After an interval of about 5 seconds following closing of the manual switch  14  as shown in FIG. 6, the time delay switch  40  also closes, holds, and applies 42 volts to the coil  58  of the first switching device  29 . This voltage applied to the coil of the first switching device  29  causes its contacts  63 ,  67 , and  65  to change over. The change over disconnects the first lamp filament  28  from the first resistor  26  and the second lamp filament  27  from the second resistor  32 . The change over also connects first lamp filament  28  to junction  16 , and the ground G and connects the second lamp filament  27  to input junction  12  and the source of 42 volts. The change over also disconnects the junction  53  of the first and second lamp filaments  28 ,  27  from output junction  16  and connects the junction  53  of the filaments to point  54 . 
     With the first lamp filament  28  and the second lamp filament  27  connected in series and 42 volts applied across the series connected lamp filaments, the filaments come to full brightness. The lamps continue to operate at full brightness until the manual switch  14  is opened. When the manual switch  14  is opened, the voltage source is removed from the circuit and the circuit is deenergized, the first lamp filament  28  and the second lamp filament  27  are extinguished and the time delay switch  42  is opened and reset. 
     As explained earlier, each of the coils  64 ,  66  of the respective second switching device  42  and third switching device  44  are 42 volt coils that cause the contacts of the switching devices to open at about 75% of the coil rating and hold the contacts open at about 50% of the coil ratings. During normal operation of the circuit described above and as illustrated in FIG. 5, with the switching device coils  64 ,  66  being connected in series, and with the series connected coils also being connected in parallel with the series connected first lamp filament  28  and second lamp filament  27 , about half of the circuit voltage of 21 volts is applied across the two coils. This voltage is inadequate to cause either of the contacts of the second switching device  42  or third switching device  44  to open. 
     If the first lamp filament  28  fails as is shown in FIG. 7, point  53  remains connected to input  14  through lamp filament  27  and goes to 42 volts. Point  54  which is connected to point  53  by contact set  65  in the first switch means  29  also goes to 42 volts and in turn closes and holds point set  74  in switching device  44  and breaks the circuit to switching means  29  and returns contacts  63 ,  67 , and  65  to the open position. Lamp filament  27  is reconnected to 42 volts through resistor  32  and to ground through point set  65  and is illuminated. 
     In an analogous fashion, if lamp filament  27  fails as is shown in FIG. 8, point  53  and  54  go to ground and switching means  42  closes and holds and in turn returns switching means  29  to the open position. Lamp filament  28  is then reconnected to ground through contact set  65  and through contact set  63  to 42 volts through resistor  26  and is illuminated. 
     Opening and closing switch  14  permits normal operation of the remaining, intact filament. Replacing the lamp restores the normal operation of the circuit. 
     Although the circuit has been described herein using mechanical relays and switches, it is also possible to create the circuit of the present invention using solid state electronics and replacing the switching mechanisms with various types of transistors. Accordingly, the circuit of the present invention may be modified using these types of devices and incorporating other electronic devices to provide the required operational conditions for the solid state electronic devices to be used. 
     Various other changes to the preferred embodiments of the invention described above may be envisioned by those of ordinary skill in the art. However, those changes and modifications should be considered as part of the invention which is limited only by the scope of the claims appended hereto and their legal equivalents.