Abstract:
An electrical heating device is disclosed that is suitable to vaporize air treatment chemicals from an impregnated substrate. The device is provided with a PCT heater held between electrical contacts and a spring that is compressed between the housing of the heating device and one of the electrical contacts to maintain pressure thereon in spite of thermal expansion of the housing, ensuring good electrical contact between the PCT heater and the electrical contacts.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to heaters, such as positive temperature coefficient (PTC) heaters, used to vaporize air treatment chemicals from a substrate. More particularly it relates to spring assemblies which can adjust for thermal expansion of such heaters. 
         [0004]    Electrical heaters have been developed to heat a substrate (e.g. a mat) impregnated with an air treatment chemical, to thereby dispense the chemical. For example WO 2006/046209 (the disclosure of which is hereby incorporated by reference as if fully set forth herein) discloses one such PTC electrical heater. Also representative of the current state of the art are U.S. Pat. Nos. 4,404,463, 4,431,983, 4,728,779, 4,874,924, 5,256,857 and 5,471,034, and also WO 97/02054. 
         [0005]    However, devices of this type do have some drawbacks. For one thing, the outer housings of such PTC heaters tend to undergo thermal expansion during operation. As most of the electrical components are anchored to the housing, this can cause relative movement of the electrical parts with respect to each other, thereby disrupting electrical contact, or rendering heat transfer less efficient. 
         [0006]    To adjust for this to some extent there have been attempts to incorporate springs into the assembly. One approach is to make a portion of the electrical contact itself into an integral spring-like structure. However, this can increase production costs or complexity, or be difficult to reliably implement if one also wishes to use optimal electrical contact materials. 
         [0007]    There have also been attempts to instead use separate springs which bias an electrical contact against a heater, yet allow some relative movement. However, to date, these assemblies have had their own deficiencies (e.g. complexity; cost; inability to adjust for certain types of expansion). 
         [0008]    Apart from this, PTC heater elements are sometimes so fragile that their abutment with associated electrical contacts (particularly those that are stamped) can lead to breakage of the PTC element. A stamped contact can have a sharp edge which, over time, can lead to such cracking. While there have been attempts to avoid the use of stamped contacts with sharp ends (e.g. our company&#39;s U.S. Ser. No. 11/614,645 filed on Dec. 21, 2006), materials which are optimal for that purpose may not be optimal for certain spring related functions. 
         [0009]    Hence, a need still exists for improved electrical heaters useful for vaporizing air treatment chemicals. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    In one aspect the invention provides a heating device having a housing defining an internal cavity and having spaced supports extending there from into the cavity, a heating element positioned in the cavity, a first electrical contact abutting the heating element along a first side of the heating element, a second electrical contact abutting the heating element on a side thereof opposite the first side, and a spring suspended by the spaced supports and having a portion thereof biasing the second electrical contact towards the heater element. 
         [0011]    The spring has a convex surface pointed away from the second electrical contact and rising sufficiently there from so as to be pressed against the interior surface of the housing. As a result, the convex portion of the spring is compressed and remaining parts of the spring apply pressure against the second electrical contact, securing it against the heating element. The spring is sufficiently compressed that, even as the housing expands when heated, the spring remains in contact with the housing and continues to apply pressure against the second electrical contact. As a result, the second electrical contact remains properly engaged against the heating element, and the heating element, in turn, is also urged against the first electrical contact, ensuring uninterrupted electrical connections. 
         [0012]    In preferred forms the heating element is a pill-shaped positive temperature coefficient heating element, the spring has a convex surface pointed away from the second electrical contact and a concave surface pointed towards the second electrical contact, and the first electrical contact has a contact head that bulges towards the heating element. The first electrical contact may also serve a heat diffusion function. 
         [0013]    Our heaters are intended to be used with a substrate positioned against an outside surface of the housing, the substrate bearing a volatilizable air treatment chemical such as an insect control active ingredient, a scent, a deodorizer, or the like. Heating causes the chemical to volatize from the mat, thereby treating the room air. 
         [0014]    For example, in one preferred embodiment the heater could be used with a conventional mosquito control mat formed of compressed cellulosic fibers that have been impregnated with an insect control agent such as allethrin or metofluthrin. The art is well aware of a number of other substrates, and other air treatment chemicals, that can be used with these types of devices (e.g. repellents, fragrances, deodorizers). 
         [0015]    Our devices therefore accommodate thermal expansion, without imposing undesirable design constraints on the electrical contacts. Further, our devices can be inexpensively produced, which may be particularly important to their commercial potential in some third world markets. 
         [0016]    The foregoing and other advantages of the present invention will become apparent from the following description. In that description reference is made to the accompanying drawings which form a part thereof, and in which there is shown by way of non-limiting illustration a preferred embodiment of the invention. The claims which follow thereafter should be looked to in order to judge the full scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view of an embodiment of the invention, shown with an impregnated mat about to be installed therein; 
           [0018]      FIG. 2  is an exploded perspective view of the  FIG. 1  embodiment, albeit without the impregnated mat shown; 
           [0019]      FIG. 3  is an exploded perspective view of a PTC heater portion of the  FIG. 1  device; 
           [0020]      FIG. 4  is another exploded perspective view of the PTC heater of  FIG. 3 ; 
           [0021]      FIG. 5  is a perspective view of part of the PTC heater of  FIG. 4 , but showing how alternate forms of the preferred spring could instead be used; 
           [0022]      FIG. 6  is a cross-sectional view taken along line  4 - 4  of  FIG. 1 ; and 
           [0023]      FIG. 7  is a cross-sectional view taken along line  7 - 7  of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring first to  FIGS. 1-2  and  6 - 7 , there is shown a vaporizer  10  which includes a cover  12  having a air grill  13 , a PTC heater  14 , an impregnated mat  16 , and a housing bottom  18 . As will be apparent from  FIG. 3 , PTC heater  14  includes a first housing part  20 , a second housing part  22 , a first electrical contact  24 , a PTC pill form heating element  26 , a second electrical contact  28 , and most importantly spring  30 . The first housing part  20  supports the first electrical contact  24 , which abuts a second side  25  of the PTC heating element  26 . The second electrical contact  28  abuts a side  27  of the PTC heating element  26  opposite the side abutting the first electrical contact  24 . 
         [0025]    As shown in  FIGS. 3 and 4 , the first housing part  20  and second housing part  22  form a clamshell type housing to define a cavity  33  there between. First housing part  20  has support pegs  32  integrally formed therewith and extending into the cavity  33 . These pass the first electrical contact  24  by virtue of cut outs  37 . The ends of the spring  30  can be pinned or otherwise fastened to the support pegs  32  spanning the distance between them. 
         [0026]    The second housing part  22  also has pegs  34  protruding inwards towards the first housing part  20 . These pegs  34  help fix the first electrical contact  24  in place when the housing is closed. 
         [0027]    The first electrical contact  24  preferably comprises a flat heat diffuser plate section  36  linked by a right angle bend to an elongated leg  38 . The leg  38  forms one terminal of a linkage to the power supply, and thus after assembly extends outside the housing. There are also apertures  40  for receiving additional pegs  32  from the first housing part  20 . 
         [0028]    The PTC heating element  26  includes a first side  27  and a second side  25 . While heating element  26  is shown in a “pill” form, other conventional PTC element shapes could also be used. Further, heating element  26  can have metallization on either or both sides (not shown). 
         [0029]    The second electrical contact  28  preferably comprises a U-shaped plate  50  including an elongated leg  52  extending away from the plate  50  at a right angle. Elongated leg  52  forms the second terminal of a linkage to the power supply and ultimately remains extending outside the housing. There is also a curved dome having a contact portion  56 . The dome can have a convex side  62 . 
         [0030]    A spring  30  is in the form of a stainless steel strip having apertures  68 ,  70  in opposing ends  72 ,  74 . The apertures  68 ,  70  receive the tapered ends  39  of pegs  32  in wedging fashion, to thereby suspend the spring  30  over and against second electrical contact  28 . The middle suspended portion  76  of the spring  30  is convex in shape (i.e., bulges outwardly, away from the second electrical contact  28 ). 
         [0031]    While stainless steel is an optimal metal for the spring in this environment, it is not optimal for the second electrical contact  28 . For that we prefer a metal such as aluminum. 
         [0032]    Referring most specifically to  FIG. 5 , alternate spring embodiments  30 A and  30 B of the spring  30  are shown. For spring  30 A two of the spring  30  elements are attached to each other face-to-face to create a convex bulge  86  and a concave bulge  88 . In spring  30 B the middle portion of a single spring  30  is split lengthwise, leaving three thinner strips  80 ,  82  and  84 . Strips  80  and  82  are convex (curving away from the second electrical contact  28 ), while the middle strip  84  is concave in shape (curving toward the second electrical contact). 
         [0033]    The convex portion  76  of the spring  30  presses against the interior side of the second housing part  22  when the housing is assembled, somewhat compressing the convex portion of the spring. The convex portion  76  is sufficiently compressed that, when the housing expands in response to the heating of the heating element  26 , the convex portion rebounds to cause the spring  30  to remain in contact with the second housing part  22  and to continue to apply pressure against the second electrical contact  28 . The convex portions  80  and  82  of alternative embodiment spring  30 B and  86  of alternative embodiment spring  30 B function in that same way. Thus, regardless of the spring embodiment  30 , 30 A, 30 B used, the spring helps maintain a positive pressure contact between the second electrical contact  28  and the PTC heating element  26 , and thus indirectly between the first electrical contact  24  and the PTC heating element, even during thermal expansion of the housing. In fact, because the spring  30  receives heat directly from the heating element  26  by conduction through the second electrical contact  28 , the spring tends to expand to an extent proportionally greater than the expansion of the housing, which tends to urge the convex portion of the spring even more securely against the housing. The combination of convex and concave portions of alternative spring embodiments  30 A and  30 B allows those curved parts to be compressed so as to achieve the same overall displacement as is achieved by compression of the convex portion of spring  30  but with less distortion of each individual curved part. 
         [0034]    Note also that because the parts are held close together, heat can readily transfer from the pill form heating element  26  to the heat diffusing plate portion of the first electrical contact  24 . Thus, the process of transferring heat to an outer surface  92  of the PTC heater remains efficient even during thermal expansion. Note that the thermal expansion problem occurs even if preferred plastics are used for the outer housing. 
         [0035]    As seen in  FIGS. 6 and 7 , the heater  14  is, after final assembly enclosed by protective cover  12  and housing bottom  18  of the vaporizer  10 . The mat  16  fits between that cover and outer surface  92 . 
         [0036]    While the present invention has been described with reference to a particular embodiment, various other embodiments are possible as well. For example, it is not essential that the heater be in pill form, or even be a PTC heater. 
         [0037]    Thus, the claims should be looked to in order to judge the full scope of the invention. 
       INDUSTRIAL APPLICABILITY 
       [0038]    The present invention provides a heating device with an improved spring/electrical contact assembly to adjust for heat expansion.