Circuit for operating warning lights

Electric lamp and discharge devices: systems – Special application – Vehicle

Reexamination Certificate

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Details

C315S193000, C315S362000, C307S010600, C307S010800

Reexamination Certificate

active

06507154

ABSTRACT:

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'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 emerge

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