Patent Publication Number: US-6707256-B2

Title: Dimmer pack

Description:
FIELD OF THE INVENTION 
     The present invention relates to dimmers for electric lamps, and more particularly to an improved dimmer pack suitable for rack mounted and stand alone stage, studio and architectural lighting applications and that is small in size, reliable and inexpensive to manufacture. 
     DESCRIPTION OF THE PRIOR ART 
     Luminaires for theatrical and architectural applications are provided with power by phase angle dimmers so that the lamps of the luminaires can be dimmed to operate at selected light levels. Phase angle dimmers for this purpose are well known, and typically include solid state switches such as SCRs for interconnecting an AC power source to a lamp load. AC voltage from the source is sinusoidal. A phase control circuit renders a solid state switch conductive at a point during a half cycle of the sinusoid, the point being selected to supply to the lamp a lamp operating pulse having a desired quantity of power in order to produce a desired level of light. 
     Dimmers are often provided in the form of modules or packs suitable for mounting in a rack along with other dimmers and, in some applications, with control modules. Known dimmer modules can include a plurality of dimmer channels in a single modular pack for controlling a plurality of lighting loads. One example of a rack mounted dimmer module of this type is disclosed in U.S. Pat. No. 4,972,125 of Cunningham and Esakoff. The dimmer module disclosed in that patent requires external control and is not self contained. Self contained dimmer packs including all of the functionality required between the mains power supply and the lighting load may be used as stand alone units as well as in racks. In a stand alone application, it would be desirable to include convenient handles integrated into the design. 
     One of the goals in the design of dimmer packs is to combine high power capability and the ability to provide dimming control of multiple lighting channels, while achieving a small size. A difficulty in achieving this goal is heat dissipation. Components of the dimming circuitry, including high speed solid state switching devices and inductive chokes required for EMI suppression, generate substantial heat at high power levels. Small size results in high component and power density. Resulting high temperatures can interfere with dimmer operation and can decrease component life. Adequate cooling is important, but is difficult to achieve, particularly in a small package size operating at high power levels. 
     Another goal in dimmer pack design is to reduce cost. A dimmer pack typically has high voltage solid state power switching circuits, often provided with heat sinks, and inductive chokes. Also included are input interconnections for power supply mains with associated overload protection and switching and outputs for connection to the controlled lighting loads. An input for control network cabling and a user interface are typically used, together with low voltage control circuitry. A major cost factor of known dimmer packs is the labor required to mount and interconnect the many required components. 
     Dimmer packs must withstand rough physical treatment, for example when used for traveling theatrical productions, concerts and the like. Another goal in dimmer pack design is to provide a robust, sturdy construction able to withstand substantial forces without damage. 
     SUMMARY OF THE INVENTION 
     A primary object of the present invention is to provide an improved dimmer pack. Other objects are to provided a dimmer pack that is small in size and has high power capacity with multiple channels but is efficiently cooled; to provide a dimmer pack that that is easy and quick to assemble with low labor cost; to provide a dimmer pack that is sturdy and rugged; to provide a dimmer pack that includes convenient and strong handles integrated into the pack; and to provide a dimmer pack overcoming disadvantages of known dimmer modules and packs. 
     In brief, in accordance with the invention there is provided a dimmer pack for electrical lighting loads including a housing having front and rear walls spaced apart in a longitudinal direction, a bottom wall, a top wall and opposed side walls spaced apart in a lateral direction. A printed circuit main board within the housing overlies the bottom wall and is spaced from the top wall. A printed circuit second board in the housing is adjacent to the main board. The second board extends laterally between the side walls and extends vertically between the main board and the top wall and baffles air flow over the main board between the front and back walls. Cooling air inlet vents are in the housing adjacent the front wall and cooling air outlet vents are in the housing adjacent the rear wall. A high voltage switching circuit assembly and a choke are mounted on the main board at one side of the second board. An air passage opening in the second board defines a cooling air path through the second board between the inlet and outlet vents, the air passage opening being aligned with the switching circuit assembly and choke. A fan supported in the housing moves air along the cooling air path. 
     In brief, in accordance with another aspect of the invention, there is provided a dimmer pack for electrical lighting loads including a housing and a printed circuit board in the housing. A high voltage switching module includes a heat sink. The heat sink includes a planar base portion having first and second opposed surfaces. A plurality of cooling fins extend from the first surface. A solid state high voltage switching circuit is attached to the second surface. A plurality of first electrical connectors connected to the switching circuit project away from the second surface. A plurality of second connectors are mounted on the circuit board. The second connectors are releasably mated with the first connectors for supporting the switching module on the circuit board and for making electrical connections between the circuit board and the switching circuit. 
     In brief, in accordance with another aspect of the invention, there is provided a circuit breaker mounting assembly for an electrical device including a housing having a panel. The panel has opposed parallel flanges. A circuit breaker has front, top and bottom walls with recesses in the top and bottom walls. The circuit breaker is mounted adjacent the panel with the front wall against the panel and the top and bottom walls interfacing with the flanges. Tangs on the flanges are received in the recesses for locking the circuit breaker onto the panel. 
     In brief, in accordance with another aspect of the invention, there is provided a rack mountable electrical unit including a housing having front and side walls defining front housing corners and an integral bracket and handle member at each of the front housing corners. Each of the members includes a planar mounting portion attached to the housing, a mounting bracket projecting laterally from the housing beyond one of the side walls, and a handle spaced from and located in front of the bracket, the handle projecting laterally from the housing beyond one of the side walls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiment of the invention illustrated in the drawings, wherein: 
     FIG. 1 is an isometric front, top and side view of a dimmer pack constructed in accordance with the present invention; 
     FIG. 2 is a front view of the dimmer pack; 
     FIG. 3 is a rear view of the dimmer pack; 
     FIG. 4 is an isometric view like FIG. 1 with the top and rear walls removed to reveal the interior of the dimmer pack; 
     FIG. 5 is a block diagram illustrating the electrical component packaging of the dimmer pack; 
     FIG. 6 is a rear, top and side isometric view of the main and control circuit boards of the dimmer pack; 
     FIG. 7 is a cross sectional view of the dimmer pack taken along the line  7 — 7  of FIG. 2; 
     FIG. 8 is an enlarged cross sectional view of one of the main circuit board mounts taken along the line  8 — 8  of FIG. 7; 
     FIG. 9 is an end view of one of the high voltage switching modules of the dimmer pack; 
     FIG. 10 is a bottom, end and side isometric view of the high voltage switching module; 
     FIG. 11 is a schematic drawing of the circuits of the high voltage switching module; 
     FIG. 12 is a top, rear and side isometric view of the front panel of the housing of the dimmer pack with three circuit breaker switches mounted in place; 
     FIG. 13 is an enlarged sectional view showing the mounting of one circuit breaker switch in the front panel; 
     FIG. 14 is an isometric view of a corner bracket and handle member of the dimmer pack; and 
     FIG. 15 is an enlarged fragmentary top view of the bottom and side walls at a front corner of dimmer pack showing the attachment of the corner bracket and handle member. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Having reference now to the drawings, there is illustrated a dimmer pack generally designated as  20  and constructed in accordance with the principles of the present invention. The dimmer pack  20  is used to control the energization levels of lighting loads, such as luminaires used for stage, studio and architectural applications. The dimmer pack  20  is a self contained multiple channel dimming controller including high voltage phase control switching channels together with user input and network capabilities and low voltage controls. 
     A housing  22  of the dimmer pack  20  includes a front wall  24 , a rear wall  26  (FIG. 3) spaced longitudinally from the front wall  24 , a bottom wall  28 , a top wall  30  and laterally spaced side walls  32  and  34 . The bottom wall  28  and side walls  32  and  34  are portions of a single formed panel having a U-shaped cross section (FIG.  4 ). The rear and top walls  26  and  30  are stamped and formed metal panels. The front wall  24  includes a metal front panel  36  (FIG. 12) and a molded plastic face panel  38  (FIGS.  4  and  7 ). 
     The dimmer pack  20  of the present invention provides self contained, high power, multiple channel dimming in a small, rugged and easily portable package. The housing  22  is sized for slide in, front to back mounting in a industry standard DIN nineteen inch rack. The dimmer pack  20  can be provided in many configurations and power ratings, and can include up to twelve independently controlled dimming channels of up to ten amps per channel. Yet this high power density is accomplished in a package that is only two U (standard rack height unit equal to 1.75 inches), or three and one-half inches, in height. Even at this high power level and small size, the dimmer pack  20  is efficiently and reliably cooled. 
     When rack mounted, or when used as a stand alone unit, the front wall  24  including face panel  38  is readily accessible to the user. The front wall  24  of the dimmer pack  20  includes a row of switch levers  40 , one for each dimming channel, for turning the dimmer channels off and on. Each switch lever  38  is part of a circuit breaker switch  40  mounted upon the front panel  36  (FIGS. 12 and 13) that alos provides overload protection. A similar control circuit switch lever  44  is associated with a control circuit breaker switch  46 . The front wall  24  also provides a user interface  48  with a display  50  and a plurality of user operated input switches  52 . A pair of network connector receptacles  54  are also located at the front wall  24 , permitting the dimmer pack  20  to be connected into a network, such as a DMX network, and permitting the dimmer pack  20  to be daisy chained in the network with other dimmer packs and other network capable devices. 
     Switch levers  40  are shown in FIGS. 1 and 2 in the lower, off position, and switch lever  44  is shown in the upper or on position. Each switch lever  40  and  44  is nested in a panel configuration including a recess  56  that presents the lever  40  or  44  in its lower, off state in a position that is easy for the user&#39;s finger to engage and move upward to the on position. In the on position, as seen with lever  44  in FIGS. 1 and 2, the lever is protected between a pair of abutments  58  that shield the lever and prevent inadvertent movement of the lever from the on position to the off position. Recesses  60  adjacent to the network connector receptacles  54  provide access for fingers to grip network connectors (not shown) mated in the receptacles  54 . Indicator lamps associated with the circuit breaker switches  42  and  46  may be visible at or below the recesses  60 . 
     The rear wall  26  is accessible to the user when the dimmer pack  20  is installed in either a rack mounted or a stand alone unit. The rear wall  26  (FIG. 3) provides an entry or connection point  62  for a mains power supply. The mains entry  62  can be an opening or a knock out for an opening through which a power supply cable enters the housing  22 . The dimmer pack  20  can be configured for two or three phase power using  230  or  110  volt or other power supplies that are conventional for example in the U.S.A. or Europe of elsewhere in the world. The rear wall  26  also includes a series of lighting load output receptacle connectors  64  for each dimming channel. In the illustrated embodiment the six illustrated output connectors  64  are duplex, each serving two independently controlled lighting loads, for a total of twelve loads. The rear wall panel  26  is preferably a modular part that can be tailored to accommodate many different types of power cables and lighting load connectors. 
     Compact and efficient packaging of the components of the dimmer pack  20  contributes to low assembly cost and to high power density in the small size housing  22 . As seen in FIGS. 4-6, the dimmer pack includes a main power printed circuit board or motherboard  66  and a control printed circuit board or daughter board  68 . The main board  66  is mounted within the housing  22  where it is supported upon and overlies the bottom wall  28 . It extends laterally most of the distance between the side walls  32  and  34  and longitudinally a substantial part of the distance between the front and rear walls  24  and  26 . The main board  66  includes circuit paths (not shown) that connect high power circuit components of the dimmer pack  20 , including power supply transformers  70  and  72  and filter capacitors  74 . 
     Edge connectors  75  on the main board  66  support the control board  68  and extend electrical connections to circuit paths (not shown) on the control board. The control board  68  supports a low voltage control circuit  76  including various low voltage circuit components  77  of the dimmer pack  20  as well as a microprocessor  78 . Microprocessor  78  receives inputs from the user interface  48  and/or from one of the network connection receptacles  54  and supplies at its outputs control signals used in phase control dimming of lighting loads connected to the output connectors  64 . 
     The main board  66  also includes three mounting areas or module nests  80  for releasable, plug in connection of high power switching modules  82  best seen in FIGS. 9 and 10. Toroidal chokes  84  are also mounted on the main board  66 . The main board carries arrays  85  and  86  of female electrical terminals located at the front and rear edges of the board  66 . Terminals  84  are used for making connections (seen only schematically in FIG. 5) between the main board  66  and the circuit breaker switches  42 . Terminals  86  are used to make connections (seen only schematically in FIG. 5) between the main board  66  and the output load connectors  64 . 
     As best illustrated in the block packaging diagram of FIG. 5, a small network connector circuit board  88  carries the network receptacles  54 , and is connected to the control circuit board  68  by a wiring harness  90 . The user interface  48  is connected to the control board  68  by a ribbon cable  92  (also seen in FIGS.  4  and  7 ). The control circuit breaker switch  46  is connected to the control board by a wiring harness  94 . The control board  68  is connected to the main or power board  66  by the edge connectors  75 . 
     A group of power entry screw terminal assemblies  96  (FIG. 6) is supported at a rear corner of the main board  66  near the mains power supply point  62 . Individual power supply conductors  98  (FIG. 5) are terminated in these screw terminals  96 . A wiring harness  100  connects the screw terminal assemblies  96  to the circuit breaker switches  42 . The screw terminal assemblies  96  include male terminal tabs  102  for connection to terminals at one end of the harness  100 . The circuit breaker switches  42  include screw terminals  104  (FIG. 12) for connection to the opposite end of the harness  100 . male terminal tabs may be used instead of the screw terminals  104 . The wiring harness connections between the screw terminal assemblies  96  and the circuit breaker switches are selected to configure the dimmer pack  20  for the type of mains power supply used with the dimmer pack  20  and for the number of dimming channels to be employed. The illustrated dimmer pack  20  has twelve independently controlled dimming channels, but fewer channels may be used for some applications. In this case, less than twelve circuit breaker switches  42  are employed. Male terminal tabs  106  of the circuit breaker switches  42  are connected to terminals  85  of the main board  66  by a wiring harness  108 . 
     The control circuit breaker switch  46  has male terminal tabs  109  (FIG. 12) connected by the wiring harness  94  to the main board  66 . The user operates the control circuit breaker switch  46  to place the low voltage control circuit  76  alternatively in a standby, off condition or in an operating or on position. In the on position, power is supplied to the control board  68  through an edge connector  75 . 
     Phase controlled power from the main board  66  is supplied to the output connectors  64  by a wiring harness  110 . At one end of the harness  110 , conductors are connected to terminals  86  at the rear of the main board  66 . At the other end of harness  110 , the conductors are terminated to suitable terminals associated with the output connectors  64 . 
     The term wiring harness is used to mean any type of cable or group of conductors, whether in a cable or discrete, used to interconnect the described components of the dimmer pack  20 . For the most part these harnesses are shown only in schematic form and only in FIG. 5 in order that they not obscure other views of the drawings. The conductors of each harness preferably include electrical connectors and terminals able to mate with the illustrated terminals such as, for example, terminals  85 ,  86 ,  102 ,  104  and  106 . In this way, a minimum of time and effort is required to configure, assemble and interconnect the components of the dimmer pack  20 . 
     The high power switching modules  82  are received in the modules nests  80  with a simple plug in connection. This has the advantages of facilitating initial assembly as well as field replacement of the modules  82 . One nest  80  is illustrated in detail in FIG.  6 . It includes an array of eight female quick connect terminals  112  on the main board  66 . The nest  80  also includes a female in line pin receptacle connector or header  114 . 
     A switching module  82  is seen in FIGS. 9-11. The module  82  includes an extruded metal heat sink  116  made of a material such as aluminum having good heat transfer characteristics. The heat sink  116  has a planar base portion  118  with a recess  120  in one surface. A number of parallel heat transfer fins  122  project from the side of the base  118  opposite the recess  120 . High voltage switching circuits  124  and a heat sensing unit  126  (FIG. 11) are encapsulated in recess  120  by potting compound  128 . 
     In the illustrated twelve channel embodiment of the invention, each switching module  82  includes four dimmer channels, and thus includes four high voltage switching circuits  124 . Other configurations can have fewer modules or fewer switching circuits per module. 
     As seen in FIG. 11, each switching circuit  124  includes a pair of SCRs  130  with gate biasing resistors  132  for controlling conduction during alternate power supply half cycles between a pair of main power terminals  134 . The SCRs  130  are controlled by control signals received at control input terminals  136  relative to a ground input terminal  138 . Optically isolated switches  140  and resistors  141  couple control signals from the terminals  136  to the SCRs  130 . The low voltage control circuit  76  including microprocessor  78  supplies control signals from the control board  68  and through edge connectors  75  to operate the SCRs  130  to achieve selective lighting load phase angle dimmed levels in accordance with inputs received from the user interface  48  and/or the network connected to a connector receptacle  54 . 
     The switching module  82  also includes the onboard heat sensing unit  126 . Unit  126  is connected to terminals  142  and provides a temperature responsive signal to the low voltage circuit  76  and microprocessor  78  through a circuit including edge connectors  75 . This signal may used for example to discontinue operation of one or more dimming channels in response to an excessive temperature condition. 
     As seen in FIGS. 9 and 10, the main power terminals  134  of the module  82  extend from base  118  of the heat sink  116  in a direction opposite from the cooling fins  122 . The terminals  136 ,  138  and  140  are pin terminals and extend in the same direction, parallel to the power terminals  143 , and are arrayed in a line. 
     The switching module  82  is seated in the module nest  80  and is attached to and supported by the main board  66  with a simple plug in operation. When the module  82  is pressed into the nest  80 , the power terminals  134  are slideably and frictionally received into the female nest terminals  112 . Similarly the pin terminals  136 ,  138  and  140  are slideably and frictionally received into the pin receptacle connector  114 . This simple plug in operation makes all of the electrical connections between the circuit board  66  and the module  82 , and no other connections need to be made in any other way from the module  82  to other components of the dimmer pack  20 . In addition, the electrical connections between the module  82  and the module nest  80  provide the entire mechanical attachment and retention of, and support for, the switching module  82  upon the board  66 . No fasteners or hold downs are needed. Assembly of the switching modules  82  into the dimmer pack  20  is easy and fast. In addition, in the event of failure of an SCR  130  or other failure, it is a simple matter in the field to unplug a faulty switching module  82  and replace it with a new module. 
     The electrical components of the dimmer pack  20  radiate heat in use. The high voltage switching circuits  124 , particularly the SCRs  130 , and the chokes  84  are primary sources of generated heat. The small size and the high power density of the dimmer pack  20  impose a requirement for substantial and efficient cooling. The dimmer pack  20  includes a highly effective cooling system generally designated as  144  permitting continuous operation of the multiple dimmer channels at high output levels, even under adverse ambient conditions. 
     Cooling system  144  includes cooling air intake vents  146  and  148  located at the front wall  24  of the housing  22 . An upper vent  146  (FIG. 2) spans the full width of the housing  22  between the top wall  30  and the face panel  38 . A similar lower vent  148  extends the full width of the housing  22 , except that it is interrupted at the location of the user interface  48 . Vents  146  and  148  provide a large area for entry of cooling air from the front of the housing  22 . Heated cooling air is exhausted from the rear portion of the housing  22 . The side walls  32  and  34  include rearward outlet vents  150 . The rear wall  26  also includes a series of outlet vents  152 . The outlet vents  150  and  152  also provide a large area for the removal of heated air. 
     The main power board  66  is mounted upon the bottom wall  28 , and the flow of air through the housing  22  is above the board  66 . The control circuit board extends laterally across the housing  22  between the side walls  32  and  34 , and extends vertically from the main board  66  to the top wall  30 . The control board  68  therefore acts as a baffle and prevents uncontrolled air flow from the front to the back of the housing  22 . 
     A window or opening  154  in the control board  68  permits flow through the control board  68  from the front to the back of the housing  22 . As best seen in FIG. 6, the opening  154  is aligned with the primary heat generating components of the dimmer pack  20 , namely the switching modules  82  and the chokes  84 . As a result, substantially all of the cooling air flow is directed where it is most needed. Heat transfer into the air flow is maximized. 
     Effective heat transfer is augmented by the configurations, locations and orientations of the switching modules  82  and chokes  84 . The fins  122  of the module heat sinks  116  are aligned longitudinally, front to back, in the housing  22 . Cooling air flowing from the opening  154  flows without change of direction through the spaces between the fins  122  to maximize heat transfer from the heat sinks  116  and minimize air flow obstruction. 
     The chokes  84  include windings  156  wound on toroidal cores  158  of magnetic metal material. Each choke  84  has a central opening  160 . In the illustrated configuration there are twelve chokes  84 , one for each dimming channel. Other configurations may use fewer chokes  84 . The chokes  84  are arrayed in rows on the main board  66  between and adjacent to the module nests  82 . Each choke  84  in each row is oriented so that the axis of its central opening  160  is disposed longitudinally, front to back. The central openings  160  of the chokes  84  in each row are aligned. Cooling air flows around and over the chokes  84  and through the central openings  160  without change of direction in order to maximize heat transfer from the chokes  84  to the air flow and in order to minimize obstruction to the air flow. 
     Cooling air is forced through the housing  22  by a fan  162 . The fan  162  is an elongated cross flow fan oriented laterally in the housing  22  and mounted upon the control board  68  over the opening  154 . The fan may be a Panasonic Model FCB34 fan available from Matsushita Electric Corporation of America, Secaucus, N.J. The fan  162  is connected to receive power from the main board  66  by a wiring harness  164  (FIG.  5 ). The elongated exit  166  (FIG. 7) of the cross flow fan  162  extends laterally across the width of the housing  22  and directly communicates with the opening  154 . As a result the fan  162  draws cooling air from the region in front of the control board  68  and forces the air through the opening  154  into the region behind the control board  68 . 
     The cooling air flow path is best seen in FIG.  7 . Cool air enters at the front of the housing  22  through upper and lower inlets  146  and  148 . This air enters the cross flow fan  162  and is forced through the fan exit  166  and opening  154  in control board  68  to the rear of the board  68 . The baffle effect of the control board  68  prevents air from bypassing this cooling path. As a result, substantially all of the cooling air is directed from the opening  154  across the switching modules  82  and the chokes  84  for efficient, maximized cooling of these heat generating components. Air exits at and near the rear of the housing  22  through vents  150  and  152 . In both rack mounted and stand alone applications, the exhausting of heated air from the rear of the dimmer pack is advantageous and avoids the reentry of heated air into the front of the housing  22 . 
     The construction of the dimmer pack  20  is strong and rugged in order to withstand forces, shocks and stresses encountered in everyday use. In addition to discrete components such as the relatively heavy transformers  70  and  72 , the main power board  66  supports the control board  68 , including the fan  162  and other components on the board  68 , the massive chokes  84  and the switching modules  82 . The total weight carried by the board  66  can exceed ten pounds. To prevent damage to the board resulting from this weight, a sturdy one-eighth inch board is used, and it is mounted to the bottom wall  28  using a number of shock absorbing mounts  168  as seen in FIG.  8 . 
     At its corners the main power board  66  is provided with mounting holes  170  for receiving the mounts  168 . The bottom housing wall  28  is provided with an upwardly offset boss  172  aligned with each mounting position. A resilient grommet  174  is received in the hole  170  and receives an enlarged shank portion  176  of a flange head screw  178  threaded down through a hole  180  in the boss  172 . A sleeve  182  limits compression of the grommet  174 . The resilient grommet  174  mounted in this manner dampens shock transfer between the housing  22  and the main power board  66 . 
     A snap in mounting system  184  (FIGS. 12 and 13) is provided for the circuit breaker switches  42  and  46 . Each switch  42  and  44  includes opposed recesses  186  in its top and bottom walls. The metal front panel  36  has upper and lower flanges  188  and  190  that receive and closely overlie the tops and bottoms of the circuit breaker switches  42  and  46 . The flange portions  188  and  1909  are subdivided by slots  192  into flexible tab portions  194 . The tabs  194  are provided with locking projections or tangs  196  extending down from the upper flange  188  and up from the lower flange  190 . A circuit breaker switch  42  or  46  is mounted to the panel  36  simply by pressing it forward as the tabs  194  flex until the tangs  196  snap into the recesses  186  and lock the circuit breaker switch in place as seen in FIG.  13 . Openings  197  in the panel  36  permit the levers  40  and  44  to extend through the panel where they are accessible to the user. Openings  195  may be provided for viewing indicator lights on the front of the circuit breaker switches  42  and  46 . The snap in mounting is quick and easy and the expense and labor cost of using fasteners is avoided. 
     Each of the front corners of the dimmer pack  20  is provided with an integral handle and mounting flange body  198  (FIGS.  14  and  15 ). The bodies  198  are stamped and formed from sturdy, thick sheet metal. Each has a planar mounting portion including a forward side panel  200  and an inwardly offset rear side panel  202 . The forward panel  200  has a pair of weld access holes  204 , while the rear panel  202  has a boss  206  with a third weld access hole  208 . A mounting bracket  210  extends outwardly from the front panel  200 , and a curved and outwardly extending handle portion  212  is formed in front of the bracket  210 . Handle portion  212  is smoothly curved and is convex toward the front of the dimmer pack  20 . The bodies  198  are symmetrical top to bottom around a central horizontal plane, so that identical bodies  198  can be used at both front corners. 
     The bodies  198  are secured to the housing side panels  32  and  34 . As seen in FIG. 15, a slot in the side panel permits the forward panel  200  to lie against the exterior surface of the side panel while the rear panel  202  lies against the interior surface. A boss  215  (FIG. 1) on the side panel registers with the boss  206  of the rear panel  202  and in cooperation with the slot in the side wall accurately positions the body  198 . A three point weld is made at the weld access openings  204  and  208  for a very strong interconnection of the bodies  198  to the housing  22 . The handles  212  are used for transporting and handling the dimmer pack  20 . For rack mounting, the mounting flanges can be fastened to the rack using openings  214  in the flange. Openings  216  in the handle are aligned with the openings  214  (see FIG. 2) to afford access to fasteners located at the flange openings  214 . 
     While the present invention has been described with reference to the details of the embodiment of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.