Abstract:
A temperature sensor includes a plastic bottom shell, a plastic cap mating the plastic shell, and a temperature responsive element mounted to the plastic cap and received in a cavity of the plastic bottom shell. The temperature responsive element is positioned in the plastic bottom shell by the cap. An insulating coating is applied to the temperature responsive element. The insulating coating and the plastic bottom shell provide dual electrical insulation for the temperature responsive element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/274,844 filed on Aug. 21, 2009. The entire disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a temperature sensor, and particularly to a temperature sensor for use in household appliance applications. 
       BACKGROUND 
       [0003]    This section provides background information related to the present disclosure which is not necessarily prior art. 
         [0004]    A temperature sensors for household appliance applications are typically required to meet certain industry safety standards, such as the IEC 60335-1 safety of electrical household appliances standard. 
         [0005]    Generally speaking, such temperature sensors include a dielectric housing and a temperature responsive element that is received in the housing. In order to meet industry-accepted standards for such a construction, the housing must have a minimum wall thickness of at least 2 mm around the temperature responsive element and all components that can or will come in contact with the sensing medium. This construction requires increased material costs and slows down the thermal time constant for the temperature sensor. 
         [0006]    Alternatively, to meet industry-accepted standards, a “dual insulation” construction can be used. In such a configuration, in addition to a dielectric housing, which may have a wall thickness of only 1 mm, a second and separate insulative coating of any thickness is applied to the temperature responsive element and all components that can or will come in contact with the sensing medium before they are assembled into the housing. Such a construction, however, requires time consuming and costly manufacturing operations of pre-coating and curing the temperature responsive element and other components with the secondary insulator. 
       SUMMARY 
       [0007]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0008]    The temperature sensor of the present disclosure includes a two-piece housing structure and a temperature responsive element received inside the two-piece housing structure. The two-piece housing structure includes a bottom shell and a top connector, both made from a dielectric material, such as plastic. The bottom shell has a uniform wall thickness of 1 mm, particularly in areas where there are components that can or will come in contact with the sensing medium. Conducting portions of the temperature responsive element are covered by an epoxy. 
         [0009]    To assemble the temperature sensor, the temperature responsive element is mounted to the connector to form a subassembly. Epoxy is injected into a cavity of the bottom shell. Before the epoxy is cured, the subassembly is inserted into the cavity of the bottom shell. The bottom shell and the connector have mating structures that guide and locate the temperature responsive element within the cavity. The temperature responsive element is centered in the cavity, positioned an equal distance from the walls of the bottom shell so that the epoxy fills the space between the temperature responsive element and the cavity walls. The temperature sensor of the present disclosure achieves the IEC 60335-1 safety of electrical household appliances standard, Class II insulation requirements, maintains a fast thermal time constant, and can be easily manufactured consistently in a high-volume production environment. 
         [0010]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0011]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0012]      FIG. 1  is front isometric view of a temperature sensor according to the present disclosure; 
           [0013]      FIG. 2  is a partially-exploded isometric view of the temperature sensor shown in  FIG. 1 ; 
           [0014]      FIG. 3  is a top view a bottom shell portion for the temperature sensor according to the present disclosure; 
           [0015]      FIG. 4  is a front view of a temperature responsive element for the temperature sensor according to the present disclosure; 
           [0016]      FIG. 5  is a side view of the temperature responsive element of  FIG. 4 ; 
           [0017]      FIG. 6  is an exploded front view, in partial cross-section, of a temperature sensor according to the present disclosure; and 
           [0018]      FIG. 7  is a cross-sectional front view of an assembled temperature sensor according to the present disclosure. 
       
    
    
       [0019]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0020]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0021]    Referring generally to  FIGS. 1 to 7 , a temperature sensor  10  according to the present disclosure is shown. The temperature sensor  10  is an assembly that generally has a two piece housing  14  including a top connector  18  and a bottom shell  16 . A terminal subassembly  12  including a temperature responsive element, such as temperature sensitive thermistor  30  is contained and particularly located within the housing  14 . As best seen in  FIG. 7 , a dielectric material  36 , such as epoxy, fills a portion of the bottom shell  16  and surrounds the thermistor  30 . Design features included in the top connector  18  and bottom shell  16  facilitate the orientation and positioning of the terminal subassembly  12  (and hence, the thermistor  30 ) in the temperature sensor  10  in a manner that is repeatable with the consistency necessary for manufacture of the temperature sensor  10  in a high-volume production environment with acceptable scrap rates. 
         [0022]    As shown in  FIGS. 1 ,  2 ,  6  and  7 , the top connector  18  of the housing  14  includes a terminal connector portion  38  at its upper end, and an extension portion  20 , such as a post, at its lower end. Intermediate the connector portion  38  and the extension portion  20  is a cylindrical portion  23 . The extension portion  20  projects away from the connector portion  38 . A pair of slots  40  are included at the lower end of the cap portion  18  at locations that are adjacent to the extension portion  20 . An outer wall  42  of the terminal connector portion  38  of the top connector  18  forms a shoulder  44  that is adjacent to the slots  40 . 
         [0023]    At  FIGS. 1-3  and  6 - 7 , the bottom shell  16  of the housing  14  includes an upper surface  46  on which are located a plurality projections  48  extending upwardly from the upper surface  46 . A first aperture  50  in the upper surface  46  of the bottom shell  16  opens to a ledge portion  52 . The ledge portion  52  has a second aperture  51  that opens to an inner surface  45 . Further into the interior of the bottom shell  16  is a cavity  24  defined by inner walls  53  in the bottom shell  16  within which the terminal subassembly  12  is contained in bottom shell  16  when the temperature sensor  10  is fully assembled. The wall thickness for the portion of the bottom shell  16  that surrounds the cavity  24  generally exhibits a uniform, thin-walled structure having a thickness of approximately 1 mm. 
         [0024]    The top connector portion  18  and the bottom shell  16  are each made from a dielectric material, which can include a variety of plastic materials. A preferred plastic material from which the bottom shell  16  and the top connector  18  may be made is polypropylene. Both the top connector portion  18  and the bottom shell  16  can be molded components, and manufactured with closely held tolerances. 
         [0025]    Referring now to  FIGS. 2 ,  4  and  5 , the terminal subassembly  12  is shown and includes a temperature responsive element, such as a thermistor component  30 . A pair of current conducting terminals  34  are electrically connected to the temperature responsive element  30  being joined at current conducting leads  32  located on opposite sides of the temperature responsive element  30 . The terminals  34  can be joined to the leads  32 , for example, by soldering or any other suitable means. A suitable thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature and is suitable for use in the temperature sensor  10  may be obtained from Therm-O-Disc, Incorporated of Mansfield, Ohio. Of course, determining a particular thermistor that is best suited for a given application ultimately depends on the temperature sensor&#39;s anticipated use. 
         [0026]    The assembly of the temperature sensor  10  is understood with reference to  FIGS. 2 ,  6  and  7 .  FIG. 2  shows a partially exploded isometric view of the temperature sensor  10 . As illustrated, the terminal subassembly  12  is pre-assembled to the top connector  18  to form a subassembly  22 . Subassembly  22  is assembled by inserting the terminals  34  of the terminal subassembly  12  into the slots  40  of the top connector  18 . The sizing of the terminals  34  and the slots  40  can be such that the terminals  34  are received in the slots  40  with a slight interference fit. The terminals  34  are inserted into the slots  40  until the temperature responsive element (e.g., thermistor  30 ) of the terminal subassembly  12  abuts the distal end  54  of the extension portion  20  of the top connector  18 , as shown in  FIGS. 2 and 7 . More specifically, as best seen in  FIG. 2 , the distal end  54  of the extension portion  20  is formed to match the shape of the temperature responsive element  30  to ensure a close fit between the components and to assist in locating the temperature responsive element  30  relative to the extension portion  20 . When the top connector  18  and the terminal subassembly  12  are fully assembled, the temperature responsive element  30  is specifically located relative to the features of the top connector  18 , including the shoulder  44  and cylinder portion  23 . 
         [0027]    The bottom shell  16  is then prepared for joining to the subassembly  22 . As best seen in  FIG. 6 , a dielectric material  36  is disposed in the cavity  24  of the bottom shell  16 . The dielectric material  36  may take the form of a curable, viscous liquid material. The dielectric material  36  may include, for example, a thermally conductive base plastic material, such as epoxy, that is enhanced with additives. The additives may be electrically isolative and yet have better thermally conductive properties than those of the base plastic material. The dielectric material  36  can be deposited in the cavity  24  at the bottom shell  16 , such as, for example, by injecting a predetermined volume of the dielectric material  36  in the cavity  24 . 
         [0028]    Next, with reference to  FIG. 7 , the subassembly  22  and the bottom shell  16  are assembled. Before the dielectric material  36  has cured, the subassembly  22  is inserted into the cavity  24  of the bottom shell  16 . As the distal end  56  of the subassembly  FIG. 2  nears the bottom of the cavity  24 , it becomes immersed in the dielectric material  36 . The dielectric material  36  flows over, around and between the temperature responsive element  30  and leads  32  to coat those components of the terminal subassembly  12 , as shown in  FIG. 7 . As insertion of the subassembly  22  continues, the cylinder portion  23  passes through the second aperture  51  and is received and guided by the inner surface  45 . Insertion continues until the shoulder  44  of the top connector  18  abuts the ledge portion  52  the bottom shell  16 . In addition, in this condition the outer wall  42  of the top connector  18  fits snugly within the aperture  50  of the bottom shell  16 . As assembled, the features of the top connector  18  and bottom shell  16  position the temperature responsive element  30  in the cavity  24  of the bottom shell  16  at a location that is a substantially equal distance from the inner walls  53  of the cavity  24  (as measured about the perimeter of the cavity  24  in a plane generally perpendicular to the plane shown in  FIG. 7 ). The dielectric material  36  substantially fills the space between the temperature responsive element  30  and the inner walls  53  of the cavity  24 . 
         [0029]    The dielectric material  36  then cures and hardens. After the dielectric material  36  cures, it forms an electrically insulating but thermally conductive coating over a portion of the terminal subassembly  12 , including the thermistor  30  and the leads  32 . In addition, the dielectric material  36  aids in affixing together the separate components of the temperature sensor  10  and provides a barrier to moisture. 
         [0030]    As described, the construction of the temperature sensor  10  provides by two layers of dielectric insulation over the temperature responsive element  30  and the leads  32 . With its two layer insulation configuration, the thermal time constant of the temperature sensor of the present disclosure is enhanced. A primary insulation layer is provided by the dielectric material (e.g., a plastic material) forming the bottom shell  16  of the temperature sensor  10 . The bottom shell  16  preferably has a constant wall thickness of 1.0 mm at and around the location the bottom part of the bottom shell  16  where the temperature responsive element  30  and the leads  32  are disposed. 
         [0031]    A secondary insulation layer is provided by the dielectric material  36  serving as an electrically insulating, thermally conductive coating over at least portions of the terminal subassembly  12 . When assembled to form the temperature sensor  10 , the construction and dimensions of the plastic bottom shell  16 , the top connector  18 , and the properties of the dielectric material  36 , enable consistent positioning of, and protection for, the terminal subassembly  12 , and particularly the temperature responsive element  30 , within the temperature sensor  10 . Thus, the temperature sensor  10  is suitable for manufacture in a high-volume production environment while still obtaining a repeatable consistency in temperature sensing performance. 
         [0032]    A temperature sensor  10  constructed according to the present disclosure has passed a 3,750 VAC dielectric test and met IEC 60335-1 safety of electrical household appliances standard Class II dielectric requirements. 
         [0033]    In another aspect of the temperature sensor  10  of the present disclosure, the temperature sensor  10  includes an overvoltage safety feature that provides a consistent failure mode when an excess voltage is experience by the temperature sensor  10 . In particular, a blow-hole safety feature is provided at the distal end  54  of the extension portion  20  of the top connector  18 . 
         [0034]    In this regard, the distal end  54  of the extension portion  20  has a thin-walled construction as shown in  FIG. 7 . The proximal end  58  of the extension portion  20 , opposite to the temperature responsive element  30 , has an opening  60  to the ambient environment. As such, the extension portion  20  is generally a hollow cylinder, closed at one end and open at the other end. Also, as described above, the temperature responsive element  30  of the terminal subassembly  12  abuts directly against the closed, thin-walled, distal end  54  of the extension portion  20  in the assembled temperature sensor  10  and is otherwise surrounded by the hardened dielectric material  36 . If the temperature sensor  10  is inadvertently subjected to a high voltage, any catastrophic failure of the temperature responsive element  30  will cause the thin wall of the distal end  54  of the extension portion  20  to rupture and fail, since it is the weakest feature around the temperature responsive element  30 . Once ruptured at its distal end  54 , the extension portion  20  provides a venting path to the ambient environment through the opening  60  in the proximal end  58 . 
         [0035]    Consequently, the temperature sensor  10  provides an overvoltage safety feature in a moisture proof package, thereby enabling the temperature sensor to be used in applications such as washing machines. 
         [0036]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.