OVERHEAD PROJECTOR HAVING A MODULAR INNER ENCLOSURE

Abstract: An embodiment of an overhead projector (10) including an outer enclosure including a projection stage; a plurality of hazardous components; and an inner enclosure (30) located inside of the outer enclosure, wherein the inner enclosure meets both fire and electrical enclosure requirements and the inner enclosure encloses all hazardous components. The inner enclosure is a modular electrical and optical assembly located inside of the outer enclosure including a main shell compartment, the hazardous components - including a transformer (50) having a core and a primary winding placed inside the main shell compartment - a projection lamp (44) electrically coupled to the first winding. The projection lamp has a light source portion and electrical contacts. The projection lamp is optically aligned with respect to the stage.

Description: OVERHEAD PROJECTOR HAVING A MODULAR INNER ENCLOSURE BACKGROUND OF THE INVENTION The present invention relates to an overhead projector having improved electrical and fire-safety characteristics. In particular, the present invention relates to an overhead projector having a novel inner fire-resistant and electrical enclosure architecture. As consumer products used in classrooms and meeting rooms, overhead projectors must meet a variety of safety standards and regulations. Exemplary standards include the Underwriter's Laboratories UL122 standard in the US and the NE KO-CE EN60335-56 standard in Europe, both of which are hereby incorporated by reference. Innovation in design must include creativity in meeting regulatory and safety agency requirements and keep the cost of the product accessible to consumers . For overhead projectors, two important safety standards focus on the electrical and fire-enclosures. A fire enclosure would contain or retard a fire if for some reason the internal components should catch on fire. An electrical enclosure would prevent dangerous electrical exposure under normal as well as certain defined abnormal conditions. An example of a defined abnormal condition is the condition of an overhead projector after dropping a steel ball weighing more than 1 lb. (o.45 Kg.) from a height of more than 4 ft. (Y.22 m) on to the stage glass. In the condition created by this process, the person should not be able to contact any dangerous electrical contacts with the unit still plugged in. Traditional overhead projector designs include a fire- resistant outer shell having an extremely strong shatter- resistant hardened glass stage. All electrical and fire- risk elements are contained inside of this shell. The shell is often manufactured of flame-rated plastic materials. The basic present design has remained unchanged for many years, as it is viewed as the open design allows freedom in the internal component arrangement, and offers economical and heat management advantages. The enclosure in conventional projectors is designed expressly to impede user access to the enclosure. The only time the user is allowed to penetrate the enclosure is during maintenance operations such as lamp replacement or cleaning of optical components. As electrical components are exposed during this operation, it is recommended that this maintenance be performed when the projector is disconnected from power. It is also often recommended that a qualified technician perform any such maintenance. SUMMARY OF THE INVENTION The present invention proposes a new architecture for overhead projectors. Rather than sealing all of the components inside an outer enclosure, the present invention discloses an overhead projector in which the electrical and fire-risk elements are contained modularly inside a separate, self-contained inner electrical and fire enclosure. In an exemplary embodiment, this inner enclosure includes optical components and optical orientation and alignment features. The use of the self-contained modular enclosure allows for safe access by the user to the inside of the outer enclosure. Ease of access by the user allows the use of modular designs that are easy to service and to assemble. Furthermore, significant cost savings in the manufacture of the outer enclosure, which is no longer required to perform fire and electrical enclosure functions. An embodiment of an overhead projector according to the present invention includes an outer enclosure including a projection stage; a plurality of hazardous components; and an inner enclosure located inside of the outer enclosure, wherein the inner enclosure meets both fire and electrical enclosure requirements and the inner enclosure encloses all hazardous components. The inner enclosure is a modular electrical and optical assembly located inside of the outer enclosure including a main shell compartment, the hazardous components --including a transformer having a core and a primary winding placed inside the main shell compartment— a projection lamp electrically coupled to the first winding. The projection lamp has a light source portion and electrical contacts. The projection lamp is optically aligned with respect to the stage and the Fresnel lens on the stage .. An embodiment of the overhead projector includes a low- voltage fan motor having a different voltage rating than the projection lamp. A second core winding, electrically isolated from the core and the first core winding, is placed around the core. The second core winding is electrically coupled to the fan motor, the second core winding having a number of turns corresponding to the voltage rating of the fan motor. When the transformer is an AC transformer and the fan motor is a DC motor, a rectifier element may be electrically coupled to the second core winding to rectify the output current of the second core winding, In a particular embodiment, the low-voltage motor is an SELV motor, coupled to an SELV circuit, and the motor is located outside the inner enclosure. The light source or lamp includes a light-emitting element and electrical terminals. In an exemplary embodiment, the inner enclosure comprises an upper shell and a fire-resistant pan, wherein the fan motor and the light emitting element are on the fire-resistant pan and outside of the upper shell. The inner enclosure may include alignment features that receive and align the light source and accompanying optical elements within the inner enclosure, wherein the inner enclosure is optically aligned with respect to the stage. The inner enclosure also may have registration features that optically align the inner enclosure within the outer enclosure. In another exemplary embodiment, the inner enclosure has a plurality of compartments, wherein the compartments are interconnected by conduits that meet the requirements of electrical and fire enclosures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an isometric cut-away side view of an overhead projector . Figure 2 is an isometric front view of an embodiment of a modular inner enclosure in accordance with the present invention . Figure 3 is an isometric bottom view of the enclosure illustrated in Fig. 2. Figure 4 is an isometric back view of a second embodiment of a modular inner enclosure in accordance with the present invention . Figure 5 is a schematic circuit diagram of the SELV DC winding and connection of the fan motor of the enclosure illustrated in Fig. 2. DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a cut-away view of an overhead projector 10 in accordance with the present invention. The overhead projector 10 includes an outer shell or enclosure 12. The body of the outer enclosure 12 is formed of a material such as metal or plastic. As the outer shell 12 does not need to act as a fire or electrical enclosure, it may be manufactured of standard materials such as UL HB (Horizontal Burn) rated plastic A stage 14 covers the top portion of the outer shell 12. The stage 14 includes a Fresnel projection lens. The stage 14 is made of a translucent or transparent material, such as glass or translucent plastics (e.g., acrylic or polycarbonate materials). The size of the outer enclosure 12 is determined by the desired size of the stage 14 (currently 10.5 x 10.5 inches - approximately 27 x 27 cms.), and the physical optical limits set by light beam expansion. Again, since the outer enclosure 12 does not need to act as a fire or electrical enclosure, the stage 14 does not need to be manufactured using specially designed hardened glass. The outer shell 12 further defines a main ventilation channel 16 and secondary ventilation channels 18. The overhead projector 10 includes a latch 20 and a hinge 22, which allow the user to easily access the interior of the outer enclosure 12. A fan motor 24 coupled to a fan 26 is located within the interior of the outer enclosure 12 to direct a ventilation flow out the main ventilation channel 16 while drawing an input cool air flow through the secondary ventilation channels 18. The fan 26 exhausts heated air from the outer enclosure 12. A power socket 28 is placed on a side wall of the outer enclosure 12. An inner enclosure 30, shown in Figs. 2-4, is placed within the outer enclosure 12. The inner enclosure 30 physically and electrically couples to the fan motor 24 and the power socket 28. Figures 2 and 3 illustrate a first embodiment of an inner enclosure module 30 in accordance with the present invention. The inner enclosure includes compartments that surround and prevent user access to all hazardous components contained inside of the outer enclosure. Hazardous components are defined as any electrical or fire hazardous component. Electrically hazardous components are defined as component carrying a hazardous voltage. Hazardous voltage is defined as voltage in a circuit that exceeds 42.4 VAC peak (rms) or 60 VDC. Fire hazardous components are defined as components that may cause a fire. The inner enclosure 30 has a first compartment 32 and a second compartment 34. The compartments 32 and 34 of the inner enclosure 30 are constructed to meet the requirement of electrical enclosures, that is, the prevention of user contact with parts at hazardous voltage. The inner enclosure 30 also meets the requirements of fire enclosures. Accordingly, the inner enclosure 30 is constructed to minimize the spread of fire or flames from within the enclosure. It also is designed to contain any hot particles that could potentially drop off electrical components upon failure. Figure 4 illustrates a second embodiment of an inner enclosure 130 of the present invention having a single compartment. Similar elements of the first and second embodiments are referred to using the same reference numerals . In the embodiment illustrated in Figures 2-3, the inner enclosure 30 comprises a first transformer compartment 32 and a second interlock switch compartment 34. The transformer compartment 32 includes an upper shell 36 and a lower pan 42. Power cord connection 38, formed onto a side wall of the upper shell 36, mates to the power socket 28. The upper shell 36 and/or the pan 42 further include registration and retaining features 40, which allow the inner enclosure 30 to be accurately mated at a precise predetermined location to the interior bottom surface of the outer enclosure 12. The fireproof pan 42 is placed beneath the upper shell 36 completing the bottom portion of the transformer compartment 32. The inner enclosure module 30 further includes a lamp 44, and a lamp connection socket 46. The term lamp is meant to include suitable projection light sources, including incandescent, arc-discharge, laser diodes or other types of light sources known in the art. As better appreciated in Fig. 3, in the present embodiment, the first compartment 32 of the inner enclosure 30 surrounds a transformer 50, electrical connections to and from the transformer, the power cord connection 38, a thermostat (safety device) located inside the inner enclosure, adjacent to the transformer 50, and the lamp connection socket 46. The second compartment, illustrated in Fig. 2, encloses an on/off switch 33. Electrically insulated and fire-resistant cabling 48 electrically couples the switch 33 with the transformer 50. To achieve the electrical and fire enclosure requirements, the inner enclosure 30 is formed of metal, ceramic or flame-rated materials, such as preplated steel. Flame-rated materials are defined as materials, other than metal or ceramic, which have been tested and classified by Underwriters Laboratories, to meet fire resistance characteristics of ignition and burning. Furthermore, the inner enclosure 12 accommodates and surrounds all components and or components that pose the risk of ignition due to high temperatures so as to contain potential flames and hot particles. To complete the fire enclosure, the pan 42 of the inner enclosure 30 is placed below all components that are considered a hazard in the transformer compartment 32. This prevents hot or molten particles from falling on to the plastic bottom of the outer enclosure 12. The pan 42 also completes the electrical enclosure for the main module 32. The second compartment 34 includes a metal bracket 35 that contains the on/off switch 33, which doubles as an interlock switch. Although some safety agencies may not require an interlock switch, the present embodiment includes the switch 33 as an extra safety margin for the user. The on/off switch compartment 34 is a fire enclosure in a different way. The components -the switch 33, cabling, and shrink tubing-- used in the enclosure are agency recognized as self-contained fire enclosures for flame. Therefore, the bracket 35 is configured to surround the components and to protect from hot particles. The bracket 35 includes an extended lower shelf 37, which prevents particles from falling on to the plastic outer enclosure 12, thereby completing the fire enclosure requirement. It should be noted that the upper compartment could use standard components if the bracket was closed off with an outer wall of metal or other fire resistant material. Because the upper compartment is considered a fire enclosure, minimum flame rated electrical components and interconnects may be used, thereby reducing costs. Figure 4 illustrates an inner enclosure module 130 having a single compartment. The on/off switch and the connective cabling are all placed inside of this main compartment . An engineering challenge present with the use of an inner enclosure is the positioning of the cooling fan 26, fan motor 24, and of the projection lamp 44. During use, the lamp 44 becomes a primary heat source. To maintain lamp life and prevent heat damage, it is desirable to cool the interior of outer enclosure 12. In traditional, outer enclosure only designs, the lamp and the fan are placed within the plenum formed by the single outer enclosure. This positioning allows for the cooling of the entire plenum. Furthermore, such design has optical advantages in that it allows the lamp 44 to shine unimpeded onto the bottom of the transparent or translucent stage. In the design of the present invention, however, the outer enclosure does not necessarily act as an electrical or fire enclosure. Safety regulations required that all hazardous voltage elements of an overhead projector be enclosed, that is, not be readily accessible to the user during normal and the predetermined abnormal operating conditions. While, in alternative embodiments, it is contemplated to have an inner enclosure that surrounds both the fan 26 and the lamp 44, such arrangement creates optical and cooling challenges. An additional transparent or translucent cover would be required to allow transmission of the light from the lamp 44. A reduced enclosure would reduce the volume of air available for cooling of the lamp 44 and would impose restrictions on the flow of convective cooling caused by fan 26. The embodiments illustrated in Figures 2-4 solves these design challenges by placing the lamp 44, the fan 26, and fan motor 24 outside of the inner enclosure 30. The lamp 44 is a high-intensity lamp, such as a 100 to 350 W FCS, FNT or EVD from Osram in Germany. The lamp 44 is a single end bulb having an encased light-emitting bulb portion 45 and electrical contacts or pins 43 located on a single end of the lamp 44. A low cost embodiment of the overhead projector of the present invention does not have a condenser lens acting as an enclosure over the lamp 44. Absent such an enclosure, part of the electrical contacts, the lampY pins, may be exposed where the pins go into a connection socket. While such an arrangement would not be an issue in an outer enclosure design, the overhead projector of the present invention is designed to allow user access to the inside of the outer enclosure. With the electrical enclosure now on the interior, the exposure of the lamp pins 43 is addressed by enclosing the area of the connections with a folded metal ledge 47 to prevent user contact. In addition, the lamp connection socket 38 is coupled to the interlock switch 33 to offer added safety to users . There remains the challenge of the placement of the fan 26 and fan motor 24, both of which are electrical components. Traditionally, all electrical components are considered both an electrical and fire hazards. However, there exists one condition under which electrical components are not considered a hazard, that is, when the component is connected to a circuit that has limited power. A low-power SELV (safety extra low voltage) electrical device is not required to be placed within an electrical enclosure. An SELV device is a secondary circuit that is designed and protected so that, under normal and single fault conditions, the voltage between the SELV circuit and the ground terminal does not exceed a safe level. A safe-level is defined as voltage that is less than 42.4 VAC peak or 60 VDC. (The lamp voltage, even though it is 24Vac, does not meet this criterion because the lamp circuit is not separated from the primary circuit. That is the reason for designing the metal shield over the lamp pin insertion area.) The present invention addresses the electrical and fire enclosure problem of the fan motor 24 by using a low voltage SELV DC motor and motor circuit, rather than traditional AC higher voltage motors. The transformer 50, illustrated schematically in Fig. 5, comprises a core 52 and a primary winding 54 for supplying the necessary electrical power to the lamp 44. Rather than requiring a second power transformer, the overhead projector of the present embodiment includes a second low voltage isolated transformer winding 56 placed around the core 52 of the main transformer 50. A diode or another rectifier element 58 is used to transform the current from AC to DC. The low voltage winding 56 includes only a few turns of insulated wire on the outside of the transformer core. As may be readily calculated by those skilled in the art, the number of turns of the second winding is selected as to provide the desired voltage. The half wave rectified voltage applied to the DC fan motor 24 is selected to be 3.7 V rms well below the SELV defined by regulatory and safety agencies. The fire hazard of the fan motor is brought into safety agency compliance by the extension of the pan under the fan motor. Additional fire safety is provided by placing the motor is in a short circuit protected circuit. Accordingly, the fan motor 24 and fan 26 may be placed outside the inner enclosure 30, while providing user safety and complying with safety agency requirements. Another aspect of the present invention is the inclusion of optical components onto or into the electrical and fire enclosure. Optical components include the lamp 44, reflectors and lenses, such as a condenser lens. The enclosure 30 includes lamp and reflector alignment and receiving features, such as reflector receiving cavity 45. The inner enclosure modules 30 and 130 include a prepositioned back reflector 60 placed within a receiving aperture of the inner enclosure 30. The back reflector 60 a spherical mirror optically aligned with respect to the lamp socket 46 and the expected position of the filament of the lamp 44. The position of the lamp 44 and the back reflector 60 within the outer enclosure 12 is in optical alignment with respect to the Fresnel lens stage 14. That is, the optical components of the inner enclosure module are physically located such that the projection beam emitted by the lamp 44, and reflected by reflector 60, is directed towards the stage 14. The overhead projector will further include a projection head having projection lenses and a flat mirror arranged to project the light beam from the stage onto a screen. The registration features 40 help ensure correct repeatable positioning and correct alignment. With the inclusion of the optical elements, the inner enclosure 30 becomes part of the optical system, as the inner enclosure 30 both orients and aligns the optical components contained with the outer enclosure 12. Alternative embodiments may further included pre-aligned lenses or other optical components . The enclosure 30 is then a self-contained module. The enclosure 30 may be pre-assembled, which would allow testing of a subassembly during the assembly process. Taking advantage of the safety and modular design of the inner enclosure, the overhead projector sold as a kit for assembly by the user. Alternatively, the modularity of the inner enclosure 30 allows for efficient assembly, replacement, and upgradability . The present design provides both a fire and an electrical enclosure for all electrical components and wires inside of a "double shell" design. The inner enclosure is interior to the outer skin and thereby allows the use of lower cost outer skin materials. Since the size of the inner enclosure is not determined by optical beam expansion and stage size considerations, the inner enclosure may be smaller in size, offering additional material cost reductions. Combined with a more efficient and modular assembly and manufacturing process, the design of the present invention offers significant cost and safety advantages . Those skilled in the art will appreciate that the present invention may be used when coupling a variety of optical devices and even non-optical devices that require precise alignment. While the present invention has been described with a reference to exemplary preferred embodiments, the invention may be embodied in other specific forms. Accordingly, it should be understood that the embodiments described and illustrated herein are only exemplary and should not be considered as limiting the scope of the present invention. Other variations and modifications may be made in accordance with the scope of the present invention.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. An overhead projector (10) comprising: a) an outer enclosure (12) including a projection stage (14) ; b) a plurality of hazardous components; and c) an inner enclosure (30) located inside of the outer enclosure, wherein the inner enclosure meets both fire and electrical enclosure requirements and the inner enclosure encloses all hazardous components. 2. The overhead projector of claim 1, further comprising a) a lamp transformer (50) located inside of the inner electrical enclosure that transforms supplied electrical power for use by a projection lamp (44), the lamp transformer having a core (52) and a first core winding (54) around the core, the first core winding electrically coupled to the projection lamp; b) an low-voltage fan motor (24) having a different voltage rating than the projection lamp; c) a second core winding (56) electrically isolated from the core and the first core winding placed around the core electrically coupled to the fan motor, the second core winding having a number of turns corresponding to the voltage rating of the fan motor. 3. The overhead projector of claim 2, further comprising a rectifier element (58) electrically coupled to rectify the output current of the second core winding, wherein the transformer is an AC transformer and the fan motor is a DC motor. 4. The overhead projector of claim 2, wherein the low-voltage motor is an SELV motor and the motor is located outside the inner enclosure. 5. The overhead projector of claim 3, the overhead projector including a light source (44) including a light-emitting element (45) and electrical terminals (43), the inner enclosure comprising an upper shell (36) and a fire- resistant pan (42), wherein the fan motor and the light emitting element are on the fire-resistant pan and outside of the upper shell. 6. The overhead projector of claim 1, further comprising a light source (44) and at least one optical element (60), wherein the inner enclosure includes alignment features that receive and align the light source and the optical element within the inner enclosure and wherein the inner enclosure is optically aligned with respect to the stage. 7. The overhead projector of claim 1, further comprising a projection lamp (44) having a bulb portion (45) and electrical contacts (43), wherein the bulb portion is outside of the inner enclosure and the electrical contacts are inside of the inner enclosure. 8. The overhead projector of claim 1, the inner enclosure further comprising optical elements (60) and registration features (40) that optically align the optical elements with respect to the projection stage. 9. The overhead projector of claim 1, wherein the hazardous components includes fire hazard components and the inner enclosure includes a bottom pan (42) placed underneath the fire hazard components. 10. The overhead projector of claim 1, wherein the inner enclosure has a plurality of compartments, wherein the compartments are interconnected by conduits (48) that meet the requirements of electrical and fire enclosures. 11. An overhead projector comprising: a) a lamp transformer that transforms supplied electrical power for use by a projection lamp, the lamp transformer having a core and a first core winding around the core, the first core winding electrically coupled to the projection lamp; b) a fan motor having a different voltage rating than the projection lamp; c) a second core winding around the core electrically coupled to the fan motor, the second core winding being electrically isolated from the core and the first core winding, the second winding having a number of turns determined by the voltage rating of the fan motor. 12. The overhead projector of claim 11, further comprising a rectifier element electrically coupled to rectify the output current of the second core winding, wherein the transformer is an AC transformer and the fan motor is a DC motor. 13. An overhead projector comprising: a) an outer enclosure including a projection stage having a Fresnel lens (14); b) a modular electrical and optical assembly located inside of the outer enclosure including i) an inner enclosure comprising a main shell compartment , ii) hazardous components, including a transformer having a core and a primary winding placed inside the main shell compartment, iii) a projection lamp electrically coupled to the first winding, the projection lamp having a light source portion and electrical contacts, iv) wherein the projection lamp is optically aligned with respect to the Fresnel lens and the inner enclosure provides electrical and fire enclosure to all hazardous components. 14. The overhead projector of claim 13 the inner enclosure having registration features that mate with corresponding features on the inside of the outer enclosure, wherein the registration features optically align the inner enclosure with the outer enclosure.

Inventors: BALDWIN DWIGHT G, FESLER ROBERT M, NELSON JEFFREY M, BALDWIN, DWIGHT, G., FESLER, ROBERT, M., NELSON, JEFFREY, M.
Applicants: 3M INNOVATIVE PROPERTIES CO, 3M INNOVATIVE PROPERTIES COMPANY
Application year: 2000
Main classification: G03B21
IPC codes: G03B21