Abstract:
A circuit assembly and package incorporates a front cover with power contacting blades extending from a front surface thereof for electrical engagement in a receptacle having a standard peripheral dimension. A housing is attached to the front cover and extends perpendicularly therefrom. The housing contains an electrical circuit connected to the power contacting blades which is contained on a plurality of circuit boards mounted substantially perpendicular to the front cover. The housing and front cover create a footprint less than the peripheral dimension of the receptacle. A connecting cable extends from the housing and is connected to the electrical circuit.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 11/149,118 filed on Jun. 8, 2005 having the same title as the present application. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to the field of compact circuit assemblies and packaging and, more particularly, to a packaged circuit for direct attachment to a wall plate duplex receptacle as a male plug having lateral dimensions within the receptacle periphery.  
       BACKGROUND OF THE INVENTION  
       [0003]     Most electronic circuits which are designed to be directly powered by 110V AC circuit outlets are packaged within a rectangular module connected to the outlet receptacle with either a cord extending from the module or a plug arrangement integral with the module having blades extending therefrom for connection to the 110V AC receptacle with the module extending substantially over the entire wall plate or encroaching on the second receptacle in a duplex receptacle wall plate. Power supplies for portable computers and chargers for cellular phones and battery packs are exemplary of this type of device. While circuit improvements have reduced the size of these modules, the footprint required for direct plug arrangements is still greater than the dimension of standard duplex receptacles. This results in the ability to only use one of the receptacles in a duplex outlet or using only a two blade plug arrangement without ground pin to allow inverting the module when plugged into a top receptacle to allow use of the lower receptacle. This type of arrangement typically still encroaches on the adjacent receptacle in a four receptacle faceplate arrangement.  
         [0004]     It is therefore desirable to have circuit module packaging and associated circuits which provide a footprint within the dimensions of a standard receptacle to allow full use of a duplex outlet while providing the ability to use a ground pin for full circuit ground implementation, where required, and plug stability provided by the additional structure of the ground pin.  
       SUMMARY OF THE INVENTION  
       [0005]     A circuit assembly and package according to the present invention incorporates a front cover with power contacting blades extending from a front surface thereof for electrical engagement in a receptacle having a standard peripheral dimension. A housing is attached to the front cover and extends perpendicularly therefrom. The housing contains an electrical circuit connected to the power contacting blades which is contained on a plurality of circuit boards mounted substantially perpendicular to the front cover. The housing and front cover create a footprint less than the peripheral dimension of the receptacle. A connecting cable extends from the housing distal the front plate and is connected to the electrical circuit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
         [0007]      FIG. 1  is a front view of a National Electrical Manufacturers Association (NEMA) face place for a duplex receptacle;  
         [0008]      FIG. 2  is an isometric view of a circuit assembly and packaging according to the present invention;  
         [0009]      FIG. 3A  is a side view of the circuit assembly and packaging of the embodiment of  FIG. 2  with the tapered housing removed;  
         [0010]      FIG. 3B  is a top view of the circuit assembly and packaging of the embodiment of  FIG. 2  with the tapered housing removed;  
         [0011]      FIG. 4  is an isometric view of the tapered housing;  
         [0012]      FIG. 5A  is a front view of the circuit assembly and packaging of the embodiment of  FIG. 2  with the front cover and associated blades and ground pin removed;  
         [0013]      FIG. 5B  is a front view as in  FIG. 4   a  with the socket and header board interconnection removed to show cable attachment;  
         [0014]      FIG. 6A  is an isometric view of the front cover with the connection blades and ground pin;  
         [0015]      FIG. 6B  is a side view of the front cover with the connection blades and ground pin;  
         [0016]      FIG. 7  is a side view of the connection blade configuration;  
         [0017]      FIG. 8A  is a top view of an exemplary circuit board for use in an embodiment of the invention;  
         [0018]      FIG. 8B  is a side view of the circuit board of  FIG. 9A ;  
         [0019]      FIG. 9A  is a pictorial view of two circuit assembly and packaging units according to the present invention plugged into a standard duplex receptacle;  
         [0020]      FIG. 9B  is a rear view of the two circuit assembly and packaging units of  FIG. 9  plugged into a standard duplex receptacle;  
         [0021]      FIG. 10  is a block diagram of an exemplary 6 volt 500 milliamp charging circuit for use in an embodiment of the present invention;  
         [0022]      FIGS. 11A and 11B  are a circuit schematic of the exemplary 6 volt 500 milliamp charging circuit of  FIG. 10 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Referring to the drawings,  FIG. 1  shows a standard National Electrical Manufacturers Association (NEMA) duplex device front cover with associated dimensions. This front cover is defined by the NEMA 5-15R wallplate receptacle dimensions which accepts male plug features conforming to NEMA 5-15P. This duplex receptacle arrangement is prevalent in the majority of homes and workplaces in the United States. The wallplate  10  incorporates two receptacles  12  each having a general dimension of a 1.343 inch diameter circle truncated on the top and bottom by horizontal chords spaced at 1.125 inches from the center.  
         [0024]      FIG. 2  shows an embodiment of a circuit assembly and packaging unit according to the present invention. The unit includes body  14  having a front cover  16  with power connection blades  18  and a ground pin  20  extending from a front surface  22 . A tapered housing  24  engages and extends from the front cover opposite the blades and houses the circuit elements of the unit. The peripheral dimensions of the front surface and housing are approximately 0.010″ less than the NEMA duplex receptacle periphery as defined by the aperture in the NEMA standard duplex receptacle wallplate drawing in  FIG. 1  for the embodiment shown. The tapered housing terminates in a cylindrical extension  26  which engages a strain relief  28  for connection to cord  30 . A charger plug  32  having a standard male DC connector  34  is attached to the connection cord. The DC connector shown in the current embodiment is compatible with most Nokia® phones, but other DC connectors may be used for compatibility with other manufacturer&#39;s phones.  
         [0025]     Details of the internal arrangement of the unit for the exemplary embodiment are shown in  FIGS. 3A and 3B . For this embodiment, the circuit assembly is contained on two circuit boards, an upper circuit board  36  and lower circuit board  38 . The power connection blades  18  incorporate a vertical arm  40  which engages and supports the circuit boards at a first end. Two posts  42  support the circuit boards at a second end opposite the front cover. For the embodiment shown herein, posts  42  are connected by a web  43  (as also shown in  FIG. 5B ) having an aperture for transition of the conductors of the connection cord. The strain relief for the connection cord has a slightly tapered ferule  44  extending into a tail  46  which is integrally molded into the sheathing of the connection cord for structural integrity. Interconnection between the circuit boards is accomplished by a header  48  depending from the upper board which is received in a socket  49  mounted to the opposing surface of the lower board. The header and socket provide additional structural support and rigidity between the primary structural support attachments at the board ends. By adding additional sockets to the upper circuit board  36  a third circuit board with associated headers may be mounted above upper circuit board  36 . By adding additional sockets to lower circuit board  38 , a fourth circuit board with associated headers may be mounted below lower circuit board  38 .  
         [0026]     The tapered housing containing the electrical circuits, as shown in  FIG. 4 , has a truncated circular cross section footprint to fit within the NEMA wallplate aperture dimensions. Two sets of parallel ribs  50  extend from the inner circumference of the housing on each side to provide channels receiving the lateral edges of the circuit boards as best seen in  FIGS. 5A and 5B . For the embodiment shown, the housing is molded using a two slide mold with a lateral slide extending through corner cutouts  52  to form engaging tangs  54  on attachment ears  56 . The length of the housing accommodates the circuit boards and then tapers to the cylindrical extension  26  which incorporates a slightly tapered bore  58  to frictionally engage the ferule of the strain relief on the connection cord. Conductors  60  for the connection cord extend from the strain relief ferrule and are connected to circuit output terminals  62 . The strain relief incorporates stepped cylindrical extensions from the ferrule for engagement with the web  43  and associated aperture of rear support posts  42   
         [0027]     Front cover  16 , as best seen in  FIGS. 6A and 6B , houses the blades and ground pin for connection to the 110 VAC outlet receptacle. Ears  64  are formed in the front plate for engagement with the corner cutouts in the housing. Notches  66  receive the attachment ears of the housing with the tang of each ear captured by webs  68  extending across bases of the notches. A central aperture  70  and four vent apertures  72  are present in the front cover to allow filling of the completed circuit assembly and packaging unit with an epoxy encapsulant, as will be described in greater detail subsequently. Two tabs  74  extend from a rear surface  76  of the front cover for positioning engagement on the internal circumference  78  in the periphery of the housing. Additionally, tabs  74  provide a protrusion for engagement with encapsulating material filling the housing, as will be described in greater detail subsequently.  
         [0028]     The geometry of power connection blades  18  is shown in detail in  FIG. 7 . Vertical arms  40  on the blades terminate at both ends in rectangular posts  80  which engage the circuit boards. As shown in  FIGS. 8A and 8B , the circuit boards each have forward circular engagement holes  82  which receive the rectangular posts in an interference fit. Similarly, rear engagement holes  84  receive posts  42  to maintain separation at the rear of the boards. While the embodiment shown herein employs two horizontally spaced boards, three or more boards are stacked in alternative embodiments for more complex circuits. For the embodiment shown herein, the boards have chamfered rear corners for clearance from the tapered rear of the housing.  
         [0029]     The efficacy of a circuit assembly and package according to the present invention is demonstrated in  FIGS. 9A and 9B . Two units of the embodiment of the invention disclosed herein are plugged into the two receptacles of a single duplex face plate  10 . The body  14  of each unit extends from the receptacle to which it is plugged into without interference with the second receptacle. It is unnecessary to invert the unit when plugged into a top receptacle for spacing from the bottom receptacle thereby allowing use of a ground pin both for additional structural support of the unit and electrical connection when required by the circuit assembly.  
         [0030]     An exemplary circuit for use with the present invention is shown in block diagram form in  FIG. 10 . The circuit comprises a 6 volt DC 500 mA charger for devices such as a cell phone or Personal Digital Assistant (PDA). 110V AC is connected to a power entry circuit  102  which supplies a start-up regulator  104  and a 5 VDC power supply  106 . Startup regulator  104  provides a limited amount of current at 15VDC to the integrated circuits controlling both the 5VDC power supply 106, and the 5VDC-6VDC DC/DC converter  108 . The output current of startup regulator  104  in the present embodiment is limited to about 10 mA typically. A 5VDC to 6VDC DC/DC converter and isolation circuit  108  is powered by the 5VDC power supply and provides the desired charging current output. The start-up regulator provides DC biasing supply currents for both the 5VDC power supply circuit  106  and the converter and isolation circuit  108  which both operate from DC voltages and require an initial DC voltage supply to initiate operation.  
         [0031]     A schematic of the components contained in the circuits described in  FIG. 10  is shown in  FIG. 11 . While described herein with respect to 110 VAC power, the circuit embodiment disclosed herein provides universal voltage input compliance (110VAC, 60 Hz/220VAC, 50 Hz). Power from the 110 VAC receptacle is received on pins P 1 A and P 1 B of the power entry circuit  102  and is series connected through fuse FS 1  to provide a failsafe mechanism for disconnecting the 110VAC input in the case of either an internal short circuit or an output short circuit. For clarity in the drawings, P 1 A and P 1 B are shown as + and − respectively, however those skilled in the art will recognize in standard AC wiring circuits these comprise power, or hot, and neutral. The power entry circuit also contains a parallel connected transient protection diode TPD 1  which protects the internal electronic devices against line surge voltages and plug/unplug transient voltages. The output of power entry circuit  102  supplies AC power to a start-up regulator  104  and a 5 VDC power supply  106 . Startup regulator  104  provides a limited amount of current at 15VDC to the integrated circuits controlling both the 5VDC power supply  106 , and the 5VDC-6VDC DC/DC converter  108 . In the present embodiment, the startup regulator  104  comprises a first diode bridge rectifier DB 1 , a bank of high voltage capacitors C 1   a -C 1   g , and a regulation circuit, for the embodiment herein an LR 8  integrated circuit from Supertex, Inc., which regulates the 110VAC rectified and filtered raw DC output down to 15VDC linearly. Feedback resistors R 1  and R 2  set the output DC voltage level and output capacitors C 2 , C 2   a  provide additional filtering and leveling of the DC startup supply voltage, Vin. The output current of startup regulator  104  in the present embodiment is limited to about 10 mA typically.  
         [0032]     AC power from the power entry circuit  102  is also provided to a second diode bridge DB 2  in the 5 VDC power supply. Output from the second bridge is filtered with capacitor bank C 3 A-c and provided to a power FET U 3 . FET U 3  is switched by a FET driver output signal, (OUT) from Pulse Width Modulation (PWM) controller circuit U 2  which is powered by “Vin” from the regulator.  
         [0033]     The PWM control circuit governs the amount of power delivered to output inductor L 3  and the load by varying the duty cycle of a constant frequency square wave applied to the gate, or control input of power FET switch U 3 . Resistor R 5  connected to the “RT” input of PWM control circuit U 2  sets the frequency of this internal oscillator, in this case at approximately 1 MHz. When power FET U 3  is switched “ON”, by driver output “OUT” from PWM controller circuit U 2 , inductor L 1  is energized and conducts current which is then accumulated on capacitor bank C 8 A-d and C 20 - 32 . As the voltage on the capacitor bank charges towards 5VDC, resistors R 7  and R 6  provide a feedback signal to PWM circuit U 2 . The voltage divider comprised of R 7  and R 6  reduces the nominal 5VDC to 1.25VDC which is compared against the internal 1.25VDC reference in the PWM controller IC. With the power FET in the “ON” condition the voltage at the 5VDC supply output will begin to go above 5VDC. When this occurs, the feedback resistive divider comprised of R 7  and R 6  will cause the input at the voltage feedback input (Vfb) of PWM circuit U 2  to exceed 1.25VDC , thus causing the internal comparator to switch and drive the gate input of power FET U 3  “LOW” so that it will switch into the “OFF” condition, and thereby foreshortening the pulse width of the positive half of the output square wave (therefore, “Pulse Width Modulation”). During the period the power FET U 3  is “OFF”, the energy stored in inductor L 3  by virtue of its current conduction is discharged and supplied to the load and to charge the output capacitor bank through Schottky rectifier U 4 .  
         [0034]     When the load on the 5VDC output causes the voltage to drop as it discharges the output capacitor bank, the process is reversed, with the voltage feedback input “Vfb” being driven below 1.25VDC, and the internal comparator switching to a “HIGH” state and driver output “OUT” switching to a “HIGH” state, causing power FET U 3  to turn “ON” and repeating the cycle. In this manner the operation continues, adjusting and adapting to the varying load conditions by varying the amount of time FET U 3  is turned “ON” during each cycle of the PWM control circuit U 2 &#39;s oscillator. The duty cycle of the PWM controller can typically vary up to 85% to provide maximum power to the load.  
         [0035]     A soft-start capability is provided by capacitor C 4  connected to the “SS” input of PWM circuit U 2  in conjunction with internal circuitry to reduce the level of inrush current on a plugging event. Resistors R 3  and R 4  divide the “Vin” input to be compared against the under voltage lockout threshold internal to the PWM circuit U 2  at input “UVL”. If the voltage at “Vin” drops too low to provide proper operation of U 2 , this mechanism will trigger the UV Lockout provision and shut down the circuit, providing a failsafe condition. Resistor R 10  is connected in series with the DC return path to the diode bridge, DB 2  to provide an overcurrent sense mechanism. If the voltage across R 10  indicates an overcurrent condition in the load, an internal comparator connected to the “CS”input will trigger and shut down the output drive “OUT” until proper conditions are reestablished. This overcurrent sense voltage is coupled back to the PWM controller “CS” input via resistor R 9  and capacitor C 9 , which provide a time delay and filtering so the “CS” input does not respond to noise or transient voltages.  
         [0036]     Compensation for duty cycles in excess of 50% is achieved by modifying the signal at the voltage feedback input “Vfb” through a network comprised of C 6 , C 7 , and R 8  connected between the “COMP” and “Vfb” inputs of the PWM controller U 2 . The startup regulator circuit  104  supplies DC power to the PWM controller circuit through the “Vcc” input. A DC return path for the PWM IC is established by the connection of the PWM controller “GND” input to the common negative voltage reference point at the terminal of diode bridge DB 2 . The 5VDC supply circuit  106  as described herein is an example of a “Buck” or “stepdown “switching regulator.  
         [0037]     The 6 VDC converter and isolation circuit receives the 5VDC power from the 5VDC power supply at pin  3  of the primary winding of transformer TR 1 . Use of the transformer provides a basic insulation isolation from the 110VAC line voltage to any point accessible to the end user. Basic insulation isolation is necessary to comply with Underwriters Laboratory requirements for consumer safety. PWM controller IC U 5  and power switching FET U 6  act in much the same manner as described above for the 5VDC supply circuit  106 , with noted exceptions. Notably, the use of a 1:1.5 step-up transformer TR 1  allows the output voltage of the secondary winding at pin  7  of TR 1  to be greater than the input voltage, and therefore as high as 7.5VDC given a 5VDC input voltage. Additionally, the positioning of the transformer primary winding between the input DC supply and the drain of power switching FET U 6 , makes the FET a “Low Side” switch, simplifying the gate drive requirements, and requiring the use of a “catch” diode SD 1  connected across the primary winding to reduce the potential for a possibly damaging high voltage transient at the drain of FET U 6  when it is switched from “ON” to “OFF”. Catch diode SD 1  also provides a conduction path for the energy stored in the primary winding inductance to provide power to the load through the magnetically coupled secondary winding when power FET switch U 6  is turned “OFF” by a “LOW” from the PWM circuit “OUT” output.  
         [0038]     Output rectifier diode SD 2  is connected to the secondary winding to rectify the output signal, and capacitor bank C 19 A-j filters and levels the 6VDC output. One other point of note is the method of feedback to PWM controller IC U 6 .  
         [0039]     In order not to lose the approximately 1500V isolation achieved by the use of transformer TR 1 , an optocoupler OP 1  is used to feedback an appropriate control signal to the PWM control IC U 5  voltage feedback input “Vfb”. Resistors R 20  and R 21  divide the nominal 6VDC output voltage to 3VDC at the inverting (−) input to voltage comparator U 7 . The non-inverting (+) input to voltage comparator U 7  is connected to a 3VDC bandgap reference biased from the nominal 6VDC output through resistor R 22 . Thus, if the output rises above 6VDC, the comparator (−) input will be above 3VDC, and the voltage comparator output at pin  7  will be driven to a “LOW” state, removing the drive current from the Light Emitting Diode (LED) between pins  1  and  3  of optocoupler OP 1 . With no optical signal present at the base of the phototransistor between pins  6  and  4  of optocoupler OP 1 , the output at pin  6  will be in a high impedance state, and thus will be driven to 2.5VDC by the resisitive voltage divider (⅙) combination formed by R 16  and R 14  and the 15VDC startup supply output, “Vin”. Since the internal reference is at 1.25VDC, the output drive from PWM control circuit U 6  “OUT” will be driven “LOW” and the power switching FET U 6  turned “OFF”, thus providing negative feedback and maintaining excellent isolation.  
         [0040]     When the nominal 6VDC output sinks below 6VDC, the (−) input to voltage comparator U 7  sinks below 3VDC, and the output of voltage comparator U 7  transitions to a high impedance state, and is pulled “HIGH” towards 6VDC through pullup resistor R 19 . The actual voltage will be determined by the forward current (˜2 mA) through the LED between pins  1  and  3  of optocoupler OP 1 . With the now substantial optical power incident on the phototransistor base, and the high gain of the phototransistor between pins  6  and  4  at the second side of optocoupler OP 1 , the voltage at the optocoupler output pin  6  is quickly driven to the saturation voltage of the phototransistor (&lt;0.4VDC). This will cause the output of PWM control circuit U 5  “OUT” to be driven “HIGH”, thus turning power switch FET U 6  “ON”, reenergizing the primary winding of transformer TR 1 , and repeating the cycle anew as the nominal 6VDC voltage output is driven higher. Capacitor C 14  and resistor combination R 14  and R 16  behave as an integrating circuit, delaying both the rising voltage and falling voltage at the voltage feedback input “Vfb” of PWM control IC U 5 , and therefore consideration must be given to compensate the feedback loop appropriately via the “COMP” input to PWM IC U 5   
         [0041]     Besides the noted exceptions, the remainder of the PWM IC operates as described previously and will not be repeated here. This DC/DC converter topology is commonly referred to as a “Boost” or “Flyback” converter. Values for exemplary components of the circuits and various feedback control components for the circuits described above and shown in  FIGS. 11A and 11B  are provided in table 1. The design has been effected in such a manner as to allow interfacing with both the US standard 110VAC and many of the international 220VAC power mains. Suitable passive plug adaptors may be used to effect the mating to a number of different international plug receptacle standards.  
                                                       TABLE 1                       Component   Value   Part no./Type                                R10, R18   0.33   Ohm   ERJ-3RQFR33V       R9, R14, R17   1   K   ERJ-3EKF1001V       R2   1.82   K   ERJ-3EKF1821V       R3, R11, R19   2   K   ERJ-3EKF2001V       R6   3.01   K   MCR03EZPFX3011       R16, R20, R21, R22   4.99   K   MCR03EZPFX4991       R5   6.19   K   ERJ-3EKF6191V       R7   9.09   K   MCR03EZPFX9091       R4, R8, R12, R15   15   K   ERJ-3EKF1502V       R1, R13   20   K   ERJ-3EKF2002V       C6, C15   220   pF   ECJ-1VC1H221J       C7, C16   3.3   nF   C1608C0G1H332J       C4, C12   0.01   uF   ECJ-1VB1E103K       C2, C5, C9, C11, C13,   0.1   uF   MCH182CN104KK       C14, C17, C18, C33       C1a-C1g, C3A-C3c   0.56   uF   501S49W564KV6E       C2a, C8A-C8d,   22   uF   C3225X5R1E226K       C19A-C19j, C26-C32       C20-C25   220   uF   ECEV1AA221XP       L1   68   uH   MSS1260-683MX            TR1   Transformer   PA1032       DB1, DB2   Diode Bridge   HD04           400 V 0.8 A       U1   450 V Linear Reg.   LR8N8           10 mA       U2, U5   100 V PWM   LM5020MM-1           Controller       U3, U6   N-Ch Pwr MOSFET   STD1NB60           600 V 1 A DPAK       U4   Fast Recovery Rectifier   SMBY01-400           400 V 1 A       U7   Voltage Comparator   LM311M       U8   Voltage Reference   LM4040EIM3X-3.0           3.0 V SOT-23       SD1, SD2   Schottky Diode   ZHCS2000           40 V 2 A SOT23-6       OP1   Optocoupler   TLP181       FS1   FUSE 1025TD   1025TD250mA           250 VAC 250 mA       TPD1   Trans. Voltage Processor   P4SMA350CA           350 V, 400 W       P3   2 mm 5-pin   2063-01-01-P2           Receptacle       P4   2 mm 5-pin   2163-01-01-P2           Header Straight                  
 
         [0042]     For the embodiment described herein, a simplified method of manufacture on the unit is created by the form of the packaging components. Power blades  18  and ground pin  20  are integrally molded into front cover  16 . Assembly of the circuits on circuit boards  36  and  38  is accomplished by conventional pick and place and soldering methods. The connecting cable strain relief is engaged to web  43  interconnecting support posts  42  with the stepped cylindrical extension inserted through the aperture in the web. The conductors of the connecting cable are connected to associated lower board terminals. The two circuit boards are then mounted to pins  80  on the vertical arms of the power blades with front mounting holes  82 , as previously described, and then soldered for electrical connection. Coincident with mounting to the vertical arms, the socket and header on the boards are mated and posts  42  are inserted in the rear mounting holes on the boards and soldered for structural support and rigidity at the rear of the multi-board assembly.  
         [0043]     The connecting cable is threaded through the tapered bore in the cylindrical extension of the housing. The tapered ferule  44  of the strain relief engages the tapered bore to preclude pull through of the cable assembly and to provide a liquid tight seal. The printed circuit boards are inserted into the channels formed by ribs  50  and sliding engage the channels while the cable is drawn through the bore. The housing is snap fit onto the front cover employing attachment ears  56  which are received by the notches  66  in the front cover with the tangs  54  on the ears then constrained by the webs  68  in the notches. Ears  64  on the front cover are closely received in corner cutouts  52  in the housing.  
         [0044]     Upon completion of mechanical assembly, the unit is positioned vertically with the front cover at the top. A high thermal conductivity encapsulating compound is then injected through central aperture  70 , using a syringe or comparable injection tool, with venting through apertures  72  providing encapsulation of the circuit boards and connections for additional structural rigidity of the entire unit as well as shock protection and thermal conduction for the circuit elements on the circuit boards. Tabs  74  on the front cover are engaged by the encapsulating material to provide additional structural connection to the housing.  
         [0045]     Having now described the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.