Abstract:
A capacitor providing a thermal alert includes a wound film capacitor for carrying a large current when coupled to an AC generator. The wound film capacitor includes a hollow core extending from one end to another end of the capacitor. Also included are an in-line thermal switch, which is disposed in the hollow core for sensing a predetermined temperature; and a light indicator, which is coupled to the thermal switch. A single housing is integrally formed from an upper cover and a lower cover for housing the capacitor, the thermal switch and the light indicator. The upper cover of the housing is formed from translucent material. The thermal switch is configured to disconnect the wound film capacitor from the AC generator upon reaching the predetermined temperature, and activate the light indicator to emit a light. The upper cover is effective in transmitting the light from inside the housing to outside of the housing. The light indicator includes an incandescent light bulb, a neon bulb, or a light emitting diode (LED).

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to a wound film capacitor for carrying a large current and having an in-line thermal disconnect device disposed at a hot spot of the capacitor. More specifically, the present invention relates to a wound film capacitor having a thermal in-line switch disposed within a hollow core of the capacitor, in which the thermal switch is coupled to a light indicator to provide a visual alert from the capacitor that a predetermined hot temperature has been reached. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 7,471,498, issued on Dec. 30, 2008 to the present inventors, describes a high current carrying capacitor with a thermal disconnect device. The Patent in its entirety is incorporated herein by reference and FIGS. 1, 2 and 3 of that Patent are also described below. 
     Referring to  FIGS. 1 ,  2  and  3 , there is shown a high current carrying capacitor, generally designated as  10 . Capacitor  10  includes hollow core  18  surrounded by capacitor winding  12 . Hollow core  18  is formed by a non-conducting tubular section  22  which extends slightly beyond the ends of capacitor winding  12 . At opposite ends of capacitor winding  12  are two metallization layers  14  and  16 . 
     On completion of winding  12  of the capacitor roll, the ends of capacitor winding  12  may be sprayed with a high velocity mixture of compressed air and molten fine particles of tin produced from an electric arc gun. This spray forms opposing metallization layers  14  and  16 , which may be considered electrically the same as opposing first and second terminals of the capacitor. Wire leads  23  and  25  may then each be bonded to respective metallization layers  16  and  14  by way of solder terminals  26  and  27 . Metallization layers  14  and  16  completely encircle the outer circumferences of the capacitor roll. 
     Capacitor winding  12  is wound around tubular section  22  in conventional manner. Hollow core  18  may be trimmed to extend approximately 0.2 to 0.3 inches beyond metallization layers  14  and  16 , thereby forming core extensions or collars  11  and  13 . The core extensions, however, are not necessary. 
     As best shown in  FIG. 2 , tubular section  22  includes an inner surface forming the hollow core. This inner surface may be used for anchoring the tubular section to a winding machine. The tubular section is then used as a mandrel for winding the film capacitor into a roll. 
     Fuse  28  together with wire leads  21  and  23  are inserted into hollow core  18 . As shown, fuse  28  is in a closed position which permits electrical current to flow from wire lead  21  to wire lead  23 . In turn, electrical current may flow from wire lead  23  to metallization layer  16  and into a first end metallic winding of capacitor  10  by way of solder terminal  27 . Furthermore, electrical current may flow from a second end metallic winding of capacitor  10  to wire lead  25  by way of metallization layer  14  and solder terminal  26 . In this manner, when fuse  28  is in a closed position, capacitor  10  permits current to flow between terminals  20  and  24 . 
     Tubular section  22  may be formed from a non-conductive material, such as polypropylene. Tubular section  22  forms a continuous passageway through the entire length of hollow core  18 . As an example, the diameter of hollow core  18  may be approximately ½ an inch. 
     When electric current is passed through capacitor winding  12 , thermal energy is generated raising the temperature of capacitor winding  12 . The hottest region of capacitor winding  12  is at its geometric center. The geometric center includes the region containing tubular section  22  and is located at the radial center and the axial center of the hollow core. Thus, hollow core  18  passes directly through the region containing the highest temperature within capacitor winding  12 . This region is also referred to herein as the hot spot of the capacitor winding. 
     Fuse  28  is disposed at the middle of the axial length of hollow core  18 . In this manner, fuse  28  senses the highest temperature, or the hot spot of capacitor winding  12 . As shown in  FIG. 1 , fuse  28  is suspended within hollow core  18 , without need to fasten the fuse to any portion of the tubular section. The fuse may be centrally positioned within hollow core  18  with the aid of wire leads  21  and  23 . 
     The present invention includes a thermal switch (also referred to herein as a fuse, or a thermal cutoff device) for a wound film capacitor, which is different from the prior art, as described below. The present invention also includes a light indicator which is controlled by the thermal switch and provides a visual alert to a user. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a capacitor with an in-line thermal alert including: 
     (a) a wound film capacitor for carrying a large current when coupled to an AC generator, and the wound film capacitor including a hollow core extending from one end to another end of the capacitor; 
     (b) a thermal switch disposed in the hollow core for sensing a predetermined temperature; 
     (c) a light indicator coupled to the thermal switch; and 
     (d) a housing integrally formed from an upper cover and a lower cover for housing the wound film capacitor, the thermal switch and the light indicator. 
     The upper cover of the housing is formed from translucent material. The thermal switch is configured to disconnect the wound film capacitor from the AC generator upon reaching the predetermined temperature, and activate the light indicator to emit a light. The upper cover is effective in transmitting the light from inside the housing to outside of the housing. 
     The light indicator is disposed inside the housing and inside an interior portion of the upper cover. The light indicator may include an incandescent light bulb, a neon bulb, or a light emitting diode (LED). The light indicator may be powered ON by an AC generator, or a DC battery. 
     The thermal switch and the light indicator are fixed into positions within the housing by a translucent resin epoxy inserted into the housing. A reflector is circumferentially positioned at a radial distance from the light indicator, so that the reflector is effective in amplifying the light emitted from the light indicator. 
     The thermal switch includes a first end connected by a first conductor to the AC generator and a second end connected by a second conductor to the wound film capacitor. The first and second conductors are wide strips of flexible metal for conducting a current of at least 50 amperes to the switch. The first and second conductors include first double strips and second double strips, respectively, and the first double strips include first and second strip end portions. The thermal switch includes first and second springing arms positioned to urge the first and second strip end portions, respectively, away from the second double strips. The first and second strip end portions are soldered to the second double strips. When the predetermined temperature is reached, the solder melts and the first and second strip end portions are urged away from the second double strips to disconnect the wound film capacitor from the AC generator. The first and second springing arms are formed from beryllium and have a width substantially equal to a width of the first double strips, and a length substantially in contact with a length of the first double strips. 
     The present invention includes another embodiment having a bank of multiple capacitors providing a thermal alert. Included are a bank of multiple wound film capacitors, each capacitor carrying a large current when coupled to an AC generator, and each capacitor including a hollow core extending from one end to another end of the capacitor. A respective thermal switch is disposed in the hollow core of each of the capacitors for sensing a predetermined temperature. A respective light indicator is coupled to the respective thermal switch. A respective housing is integrally formed from an upper cover and a lower cover for housing each of the capacitors, each of the respective thermal switches and each of the respective light indicators. The upper cover of each of the respective housings is formed from translucent material. Each of the respective thermal switches is configured to disconnect a capacitor from the AC generator upon reaching the predetermined temperature and activate a respective light indicator to emit a respective light. The upper cover of each of the respective housings is effective in transmitting the respective light from inside the respective housing to outside of the respective housing. 
     The multiple capacitors are electrically connected in at least one of series or parallel connections with the AC generator, and are stacked into an array of the respective housings with the upper cover positioned vertically above the lower cover. The upper cover of each of the respective housings is shaped as an inverted-U, or an inverted bucket, in cross-section. A metallic plate is disposed vertically above the array. The shape of the upper cover permits viewing of an emitted respective light from a vertical position below the metallic plate and from a peripheral side of the array. 
     The respective light indicator is disposed inside each of the respective housings and within an interior portion of each of the respective upper covers. The respective light indicator may include an incandescent light bulb, a neon bulb, or a light emitting diode (LED). The respective thermal switch and the respective light indicator inside each of the respective housings are fixed into positions by a translucent resin epoxy inserted into each of the respective housings. 
     The respective thermal switch includes a first end connected by a first conductor to the AC generator and a second end connected by a second conductor to the respective capacitor. The first and second conductors are wide strips of flexible metal for conducting a current of at least 50 amperes to the respective thermal switch. The first and second conductors include first double strips and second double strips, respectively, and the first double strips include first and second strip end portions. The respective thermal switch includes first and second springing arms positioned to urge the first and second strip end portions, respectively, away from the second double strips. The first and second strip end portions are soldered to the second double strips. When the predetermined temperature is reached, the solder melts and the first and second strip end portions are urged away from the second double strips to disconnect the respective capacitor from the AC generator. The first and second strip end portions are soldered to the second double strips. When the predetermined temperature is reached, the solder melts and the first and second strip end portions are urged away from the second double strips and disconnect the wound film capacitor from the AC generator. 
     Yet another embodiment of the invention includes: 
     (a) at least one wound film capacitor for carrying a large current when coupled to the AC source, and the wound film capacitor including a hollow core extending from one end to another end of the capacitor, and the capacitor enclosed within a housing; 
     (b) a double-pole double-throw thermal switch disposed, in-line with the capacitor, and in the hollow core for sensing a predetermined temperature; 
     (c) two first terminals of the thermal switch operating in a normally closed position and having two first conductor leads configured to provide an AC voltage across the wound film capacitor; and 
     (d) two second terminals of the thermal switch operating in a normally open position and having two second conductor leads extending externally from the housing of the wound film capacitor. 
     The two first terminals are configured to disconnect the wound film to capacitor from the AC voltage upon reaching the predetermined temperature. The two second terminals are configured to close for enabling a current to flow through the two second conductor leads and provide an external alert to a user. 
     The wound film capacitor may also be a first capacitor in a plurality of capacitors that are arranged in an array and connected to operate in parallel or in series with the AC source. Each of the plurality of capacitors includes a thermal switch having two first and two second terminals configured in a manner similar to the first capacitor. An external indicator and an external power source are connected to the two second conductor leads for providing the external alert to the user. The thermal switch may include a micro-switch for activating the external indicator. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be understood from the following detailed description when read in connection with the accompanying figures: 
         FIG. 1  is a cross sectional view of a wound film capacitor including a thermal cutoff device. 
         FIG. 2  is a side view of the wound film capacitor shown in  FIG. 1  with the thermal cutoff device having been removed. 
         FIG. 3  is a perspective view of the wound film capacitor shown in  FIG. 1  with the thermal cutoff device having been removed. 
         FIG. 4  is a cross sectional view of a wound film capacitor including a schematic view of an in-line thermal switch and a light indicator, in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross sectional view of the wound film capacitor shown in  FIG. 4  when housed in a unique single housing having upper and lower covers, in accordance with an embodiment of the present invention. 
         FIG. 6  is a schematic view of a thermal switch and a light indicator coupled to an AC source, in accordance with an embodiment of the present invention. 
         FIG. 7   a  is a schematic view of a double-pole double-throw thermal switch including a battery and a light indicator for disconnecting an AC source and operating the light indicator with the battery, in accordance with an embodiment of the present invention. 
         FIG. 7   b  is a cross sectional view of the double-pole double-throw thermal switch including the battery and light indicator, shown schematically in  FIG. 7   a , when housed within upper and lower covers of an integral housing, in accordance with an embodiment of the present invention. 
         FIG. 8  is a cross sectional view of the housing of a wound film capacitor and a light indicator, including a reflector disposed between the wound film capacitor and the light indicator, in accordance with an embodiment of the present invention. 
         FIGS. 9   a  and  9   b  are, respectively, top and side views of an array of capacitors showing a metallic plate disposed above the terminals of the array of capacitors. 
         FIGS. 10   a  and  10   b  are, respectively, top and side views of a wide thermal switch connected to wide strips of a double set of conductors, the thermal switch including double arms forced into an open position using a natural spring force, in accordance with an embodiment of the present invention. 
         FIG. 11   a  is a schematic view of a double-pole double-throw thermal switch for simultaneously disconnecting an AC source and activating an external alert circuit, in accordance with an embodiment of the present invention. 
         FIG. 11   b  is a cross sectional view of the double-pole double-throw thermal switch, shown schematically in  FIG. 11   a , when housed within upper and lower covers of an integral housing, for activating an external alert circuit, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes a thermal cutoff device, or a thermal switch, with a light indicator for a dry film capacitor. The capacitor may be used in systems generating 300-900 volts AC (for example) and produce as much as 150 amperes (for example). One system that carries such high current is a wind generator. Overheating in such a system may cause a catastrophic failure. As will be explained, the present invention provides a new level of control to potential overheating by providing a thermal switch that disconnects the capacitor upon detecting an over-temperature condition and visually alerts the user to that condition. 
     Metallized film capacitors, mainly due to self healing of inherent defects, are reliable and long lasting over the life of the product. However, excessive self healings may create an overheating run-away condition, especially in uncontrolled, unmonitored circuits, and fail catastrophically. The present invention provides added insurance against this type of disastrous failure. 
     In addition, the present invention provides an indication to a user that a specific capacitor has been disconnected, due to detecting a hazardous condition. The indication is provided by a light from an incandescent filament bulb, a neon bulb, or a light emitting diode (LED). As will be explained, upon activation of the thermal switch, the film capacitor is disconnected from the primary power supply, and the light indicator is connected to the primary power supply (or a separate small battery) to provide an alert to the user. In this manner, the user is alerted that the film capacitor has shut-off and the failed capacitor may be rapidly replaced. This is especially advantageous in applications having a ganged array of series and/or parallel connected capacitors, in which it is not readily discernible which specific capacitor has shut-off. As will be explained, the visual light provides a quick means to establish the location of the failed capacitor in the ganged array of multiple capacitors. 
     Referring now to  FIGS. 4 and 5 , there are shown cross-sectional views of capacitor  40  without its housing ( FIG. 4 ) and capacitor  40  disposed within its housing, generally designated as  50  ( FIG. 5 ). Also shown is a functional schematic of an alert circuit  42 , in which some elements of the circuit are disposed within hollow core  48  and the remaining elements are disposed above capacitor  40 .  FIG. 4  depicts terminals  43  and  44  suspended above capacitor  40 , and  FIG. 5  depicts the same terminals pressure-fitted through circular openings in the top surface of housing  50 , marked as terminals T 1  and T 2 . 
     The housing  50  includes upper cover  52  which is pressure-fitted into lower cover  54 . The lower cover  54  may be formed from a cylindrical aluminum can and the upper cover  52  may be formed from translucent material, such as plastic, and may be cylindrically sized for a pressure fit into lower cover  54 . As shown in  FIG. 5 , as an example, upper cover  52  includes a wall with a protruding collar  52   a  having a smaller radius than the radius of the wall of lower cover  54 . In this manner, upper cover  52  may be removably inserted into lower cover  54 , so that the cylindrical wall of upper cover  52  rests upon the cylindrical wall of lower cover  54 . The upper cover forms a generally inverted-U configuration, or an inverted bucket. In addition, the outer surfaces of the upper and lower covers may be flush against each other, so that housing  50  has a generally smooth outer surface, and appears as a single, unitary, integral housing. 
     The present invention provides a translucent upper cover and advantageously permits light emitted from light bulb indicator  45  to be seen externally of housing  50 . Furthermore, the present invention may insert a translucent epoxy into the interior of housing  50  that does not prevent transmission of light from light bulb indicator  45  to the exterior of housing  50 . Moreover, the present invention may mix a red dye with the translucent epoxy to provide red colorization to the light transmitted from light bulb indicator  45 . 
     It will be appreciated that the epoxy includes a resin that is given time to cure, thereby forming a hard, solid volume within housing  50 . In addition, light bulb  45 , conductors  46 , voltage divider  47  (R 1  and R 2 ) and thermal switch  49  of alert circuit  42  may be manually positioned within hollow core  48  and within the interior of upper cover  52 , before the epoxy resin hardens and permanently fixes the locations of the elements of alert circuit  42 . 
     Completing description of  FIGS. 4 and 5 , capacitor  40  includes opposing metallization layers  14  and  16  (similar to metallization layers  14  and  16  shown in  FIGS. 1 ,  2  and  3 ). Also included are solder terminals  26  and  27  bonded to conductor leads  46  and  23 , respectively. Hollow core  48  includes thermal switch  49  and voltage divider  47  comprising resistors R 1  and R 2 .  FIG. 5  shows bolt  56  extending from housing  50  which provides attachment means for mounting the bottom of housing  50  onto a plate (for example; not shown), with radial terminals T 1  and T 2  of capacitor  40  extending vertically above the plate (for example). 
     Various embodiments of circuit  42  including thermal switch  49  will now be described. As shown in  FIG. 6 , circuit  42  includes alternating current (AC) source  60  connected between terminals T 1  and T 2  (designated as  43  and  44 ) of capacitor  40 . Inserted between terminal T 2  and one end of capacitor  40  is thermal switch  49 . Thermal switch  49  is normally closed between switch terminals  1  and  2 , so that capacitor  40  may normally operate with AC source  60 . Upon a thermal overheating condition, thermal switch  49  opens a contact between switch terminals  1  and  2 , and disables capacitor  40 . In addition, thermal switch  49  connects switch terminal  1  to switch terminal  3 , thereby causing bulb  45  to light. As shown, bulb  45 , at one end, is receives a voltage divided between R 1  and R 2  which is provided by AC source  60 . The bulb  45  is connected across R 1 . This circuit is only intended to be exemplary and, of course, other circuits for activating bulb  45  may be utilized. 
     Another embodiment of circuit  42  of the present invention is shown in  FIG. 7   a , depicting an alert circuit generally designated as  65 . As shown, circuit  65  includes a double pole-double throw thermal switch, generally designated as  70 . Thermal switch  70  is normally connected between switch terminals  1  and  2 , while switch terminals  3  and  4  are open. In this manner, capacitor  40  is operating normally, as it is connected across AC source  60 . If an over-temperature condition occurs, thermal switch  70  disconnects capacitor  40  from the AC source, and connects switch terminal  3  to switch terminal  4 . Accordingly, while capacitor  40  is disabled, direct current (DC) battery  72 , resistor R  75  and light emitting diode (LED)  74  complete a circuit path and become operational. The LED  74 , the battery  72 , the resistor  75  and thermal switch  70  are all shown in  FIG. 7   b , as an example, housed within housing  50 . The LED  74  emits a light through the translucent upper cover  52  of housing  50  and provides a visual alert to a user that an over-temperature condition of capacitor  40  has occurred. 
     It will be appreciated that circuit  42  in  FIG. 6  requires a voltage from AC source  60  to activate bulb  45  (or neon bulb  45 ). On the other hand, circuit  65  in  FIG. 7   a  does not require power from AC source  60  and, instead, the battery  72  activates LED  74  to provide the alert to the user of the over-temperature condition. 
     Referring next to  FIG. 8 , there is shown another aspect of the present invention, in which bulb  45  is inserted through an opening (not shown) in reflector  80 , the latter positioned above capacitor  40  disposed in housing  50 . Reflector  80  amplifies or concentrates the light emitted from bulb  45 , so that the light may be easily seen by a user viewing the translucent upper cover of housing  50 . It will be appreciated that bulb  45  may also be a neon bulb, or an LED, such as LED  74  shown in  FIG. 7   b.    
     Referring next to  FIGS. 9   a  and  9   b , there is shown an exemplary advantage in using the present invention with a translucent upper cover  52  disposed above capacitor  40  (as shown in  FIG. 5 ). As shown, an array of capacitors  82  are arranged in a series and/or parallel configuration, in which capacitors  82   a ,  82   b , . . . and  82   n  are shown operating as exemplified by array  82 . The array  82  is disposed on top of surface  84 . Furthermore, metallic plate  86  is disposed above the terminals of the multiple capacitors in array  82 . It will be appreciated that although only one plate  86  is shown disposed above the terminals, there may be two metallic plates disposed one above the other. One metallic plate  86  may be connected to one set of terminals (for example, terminals T 1 ) and another plate (not shown) may be connected to another set of terminals (for example, terminals T 2 ). 
     Because the metallic plates (for example plate  86 ) are typically opaque, it is difficult to determine which capacitor in the array of capacitors  82  has experienced an over-temperature condition. However, the embodiments described herein may include capacitors, each having a housing  50  with a translucent upper cover  52 , as shown in  FIG. 5 . With such a translucent upper cover  52 , or another shaped translucent upper cover in a capacitor housing, the light emitted from bulb  45  may be seen by viewing the array  82  at a peripheral side of the array and, perhaps, below plate  86 . In this manner, a user may be alerted to an over-temperature condition of one of the capacitors in the array. The user may then easily remove plate  86  and replace the capacitor that is emitting the light. 
     Yet another embodiment of the present invention is shown in  FIGS. 10   a  and  10   b  providing wide, double layers of conductor leads connected between terminals T 1  and T 2  of a capacitor and a thermal switch. This embodiment may be used in operational conditions having a large current flow, for example 100 amperes or higher. As shown,  FIG. 10   a  is a top view of thermal switch  90  connected between wide conductor leads  94  and  96 .  FIG. 10   b  is a side view of thermal switch  90 , showing that thermal switch  90  is actually a wide, double layer of conductor leads. As shown, thermal switch  90  is connected between upper conductors  94  and  96  and between lower conductors  102  and  104 . 
     As may be seen, thermal switch  90  is a single pole-double throw switch. When thermal switch  90  is opened ( FIG. 10   b ), upper conductor  94  is disconnected from upper conductor  96 . Similarly, when thermal switch  90  is opened ( FIG. 10   b ), lower conductor  102  is disconnected from lower conductor  104 . Thermal switch  90  includes first and second natural springs  100  connected, respectively, to upper conductor  94  at conductor portion  94   a , and lower conductor  102  at conductor portion  102   a . When thermal switch  90  is closed, first and second natural springs  100  are forced to be substantially parallel to a non-conducting base, designated as  98 . The thermal switch may be forced closed by soldering portion  94   a  and portion  102   a  to upper conductor  96  and lower conductor  104 , respectively. Clamps  92  are effective in fixing respective conductors  94 ,  96 ,  102  and  104  to non-conducting base  98 . 
     Natural spring  100  may be formed from beryllium copper, which is used to provide the spring force. Once solder solidifies to connect conductor portion  94   a  and conductor portion  102   a  to wide conductors  96  and  104 , respectively, there exists a force tending to pull the two arms of the wide conductors apart. This force, or springing action, is due to the natural spring force of spring  100 . When conductor portions  94   a  and  102   a  are soldered to respective conductor leads  96  and  104 , the natural springs are each in a compressed state, or in a non-rest state. In the position shown in  FIG. 10   b , thermal switch  90  is mechanically opened and is in a rest state. In the rest state, conductors  94  and  102  are not connected to respective conductors  96  and  104 . 
     In operation, thermal switch  90  is triggered for action by soldering wide conductor portions  94   a  and  102   a  with wide conductors  96  and  104 , respectively. The thermal switch with its wide soldered conductors may then be placed within hollow core  48  ( FIGS. 4 and 5 ). The thermal switch  90  may be centered at the hot spot of capacitor  40  by using a proper length for each of the conductor leads. The wide conductors  94  and  102  may be fixed, by soldering, to metallization layer  14  ( FIGS. 4 and 5 ) at terminal  27  and another terminal (not shown). The other wide conductors  96  and  104  may be fixed, by soldering, to metallization layer  16  by way of terminal  27  and another terminal (not shown). 
     It will be appreciated that by providing wide conductors  94 ,  96 ,  102  and  104 , the present invention doubles the current-carrying capacity of thermal switch  90 . This is advantageous when the capacitor is expected to carry a very large amount of current, for example 100 amperes, or more. 
     Yet another embodiment of the present invention is shown in  FIGS. 11   a  and  11   b . The circuit  110 , capacitor  40  and housing  50  are similar to circuit  65 , capacitor  40  and housing  50  shown in  FIGS. 7   a  and  7   b . A difference between the two circuits, however, is that only thermal switch  70  is housed within the core of capacitor  40 . Two conductor leads  111   a  and  111   b  are provided externally of housing  50 . In this manner, a user may configure an alert circuit as individually desired. For example, a battery, an LED and a resistor may be connected to leads  111   a  and  111   b  as desired by the user. This provides additional flexibility in the manner that the present invention provides the external alert to the user. 
     It will be appreciated that upper cover  52  in  FIG. 11   b  need not be translucent, since circuit  110  permits the user to configure an external alert indication as individually desired. The thermal switch  70  may include a micro-switch for activating the external alert to the user. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.