Abstract:
PTC electrical heating devices are disclosed which are suitable to vaporize air treatment chemicals from an impregnated slab, such as insect repellents. In one form an electrical contact to a PTC heater element is provided with a bowl-shaped contact head to reduce the risk of edges of the contact cracking the heater element. In other forms peg and recess structures interact with apertures on a heat transfer plate to facilitate assembly of the parts of the structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to positive temperature coefficient (“PTC”) heaters used to vaporize insect control agents (e.g. repellents, insecticides, growth regulators), fragrances, and other air treatment chemicals (e.g. deodorizers) from an impregnated substrate. More particularly it relates to improved electrical contacts and structural assemblies for such devices. 
   A variety of electrical heaters have been developed on which, or through which, substrates impregnated with air treatment chemicals are placed. Heating of the substrate by such devices causes the air treatment chemical, such as allethrin, or any of various other commercially known vaporizable insecticides, to vaporize. While such devices can be powered by battery, they are typically designed to be plugged into electrical outlets. 
   A particularly preferred form of such electrical heaters uses a PTC heating element. However, such devices can experience some maintenance issues associated with the fragile nature of preferred PTC heating element materials and the structure of associated electrical contacts. In this regard, many PTC elements are somewhat fragile and their abutment with associated electrical contacts (particularly those that are stamped) can on occasion lead to breakage of the PTC element due to interaction of that element with an edge of the contact. 
   WO 2006/046209 (the disclosure of which is hereby incorporated by reference as if fully set forth herein) discloses a particularly desirable form of PTC electric heating device for vaporizing insecticides and fragrances impregnated in a solid mat. While this form of heater has many advantages, it still has some drawbacks. 
   For one thing, the electrical terminal/contact which has a pressure contact with the PTC heating element is a stamped part. As a result, there will occasionally be sharp edges formed on the contact. When these edges come into contact with the fragile PTC heater element, there can be an incidence of cracking of the PTC heater element, either when the electric heater device is being manufactured, shipped or handled, or as it heats and cools through use. 
   A variety of heating elements have been developed for these and analogous applications. See generally U.S. Pat. Nos. 4,037,082, 4,251,714, 4,635,026, 4,728,779, 4,814,584, 5,262,619, 6,192,169, 6,374,045, 7,012,222, and international publications WO 97/02054, WO 97/45008, and WO 98/57674; and European patent application EP 965 267. However, these were each deficient in some attribute. 
   Hence, a need still exists for improved PTC electrical heating devices useful for vaporizing air treatment chemicals. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect the invention provides a heating device suitable to vaporize an air treatment chemical. The air treatment chemical can be, by way of non-limiting example, impregnated into or on a wick, a conventional mosquito mat, or other substrate, with the heating device including an arrangement to mount the impregnated substrate where it can be heated when the heating device is activated. The device has a PTC heating element, and a first electrical contact abutting the PTC heating element. In accordance with the invention, the first electrical contact has a domed contact head, having a convexly curved surface presented toward the PTC heating element. 
   In preferred forms there is also a second electrical contact contacting the PTC heating element on a side thereof opposite a side to which the first electrical contact abuts, the first electrical contact also has a spring bend and a circular periphery, in plan view, with respect to the domed contact head, the domed contact head has a continuously curved outer edge, and the domed contact head has a convex side and a concave side, with the convex side being presented toward the PTC heating element. 
   The first electrical contact head may be formed of metal, a suitably resilient stainless steel being preferred, and may have a snap tab below its head. The snap tab can be so placed and sized as to secure the first electrical contact in a housing. Preferably, the domed contact head is capable of elastic deformation such that the domed contact head elastically deforms when pressed against the PTC heating element, reversibly increasing the area of contact of the domed contact head with the PTC heating element. 
   In another aspect the device may also have a first and second heater housings, and a second electrical contact in the form of a plate with an aperture. At least one of the heater housings has a peg and the other heater housing has a receiving recess. The peg passes through the plate aperture and into the receiving recess. 
   In a preferred embodiment, the device is designed to be used with an impregnated mat, such as a common mosquito mat formed of compressed cellulosic fibers and impregnated with an insect control agent such as allethrin that can volatize from the mat when the mat is heated. The art is well aware of a number of other insect control agents commonly used to dose mosquito mats intended for use with a heater device. 
   In another form the invention provides an electrical contact suitable for use with a PTC heating device. The contact has a domed head in the form of a bowl, and a leg extending below the head for linkage to a terminal. There can also be a snap tab located on the leg and a spring bend formed in the leg. 
   It should be appreciated that the use of such domed contact heads, particularly when the head is bowl form, keeps any edge of the head which might have burrs or other sharp remnants from formation, away from the fragile PTC heater. Hence, the risk of such defects causing PTC heater element cracks over time is eliminated. 
   Further, the combination of such a head with associated spring bends and snap tabs renders the contact suitable to optimize pressure against the PTC heater element while also positioning the contact relative to other assembly parts. Preferably, the domed contact head is capable of elastic deformation such that the domed contact head elastically deforms when pressed against the PTC heating element, reversibly increasing the area of contact of the domed contact head with the PTC heating element. 
   Also, the use of the peg and recess assembly technique reduces stress placed on the PTC heating element in the course of the assembly of the device, reducing the danger of cracking the PTC heating element. 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an embodiment of the invention, shown with an impregnated mat about to be installed therein; 
       FIG. 2  is an exploded perspective view of the  FIG. 1  embodiment, albeit without the impregnated mat shown; 
       FIG. 3  is an exploded perspective view of a PTC heater of the  FIG. 1  device; 
       FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 1 ; 
       FIG. 5  is a detail view of detail  5 - 5  of  FIG. 4 ; 
       FIG. 6  is a partially exploded perspective view of the  FIG. 3  PTC heater, albeit only showing certain parts; 
       FIG. 7  is a side view of an electrical contact of the PTC heater; and 
       FIG. 8  is a top view of the electrical contact of  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring first to  FIGS. 1 and 2 , there is shown a vaporizer  10  which includes an apertured cover  12 , a PTC heater  14 , an impregnated mat  16 , plug terminals  18 , and a bottom housing  20 . For convenience of description, the direction toward the cover  12  will be referred to as “up,” even though in actual use the vaporizer may be used in any orientation. Terminals  18  can plug into a typical electrical outlet. It should be understood that while a two-prong plug configuration is suitable for certain countries, the terminals  18  will be modified as necessary to conform to plug standards used in other countries. 
   As will be apparent from  FIG. 3 , PTC heater  14  includes a PTC heating element  22  having a first side  24  and a second side  26  opposite first side  24 . While PTC heating element  22  is shown in a round, “pill” form, other conventional PTC element shapes could also be used. In any event, as indicated in  FIG. 5 , PTC heating element  22  can have metallization  28  on side  26 , and a similar metallization can be provided along first side  24  (not shown). 
   A heat transfer plate  30  contacts first side  24 , and electrical contact  32  contacts second side  26  (through their respective metallization surfaces, if metallization is present). Contact  32  preferably includes an elongated leg  34 , a spring bend  44 , a connector segment  33  above the spring bend  44 , and an upwardly curved dome  36  having a contact portion  38 . Note that dome  36  exposes no edge surfaces to the PTC heating element  22 . Hence, no remnants from stamping or other forming operations along the lower edge of the dome  36  can contribute to any breakage of the PTC heating element  22 . In use, the contact portion  38  is pressed against side  26  of the PTC heating element  22 , making firm contact. The dome  36  can be made of a resilient material, such as a sufficiently thin metal, such that it elastically deforms when pressed against the PTC heating element  22 , reversibly flattening or even inverting a small area or the contact portion  38  (not shown). This increases the area of contact of the dome  36  with the PTC heating element  22 , allowing for a more effective electrical contact and for a spring-like action that adjusts to changes in the size of the PTC heating element as it expands upon heating or contracts upon cooling. 
   As shown in  FIG. 7 , the dome  36  can have a convex side  40  and a concave side  42 . While dome  36  is shown in one form, it should be appreciated that it could be more spherical, and in any event can have a non-constant plan view radius (e.g. elliptical or other curved contact surfaces). Note that there can also be a snap tab  45  to facilitate positioning as shown in  FIG. 4 . 
   Note that transfer plate  30  has a conductive plate  46  which contacts first side  24 . This conductive plate  46  can comprise planar central portion  48  with sides  50  and  52 . There is also a perimeter  54  and apertures  55 . 
   As shown in  FIGS. 3 and 6  there is also a first heater housing  56 , and a second heater housing  58  forming a clamshell type housing. First heater housing  56  has pegs  60 , and second heater housing  58  has receiving holes  62 , such that the heater housing parts can be assembled by inserting the pegs into the holes, optionally followed by sonic or heat welding. Note that the pegs are designed to also fit through apertures  55  which also results in conductive plate  46  being fixed in place. 
   Bent corners  64  on heat transfer plate  30  help maintain a positive pressure contact between heat transfer plate  30  and first heater housing  56 , which facilitates a relatively efficient heat transfer from PTC heating element  22  to first heater housing  56 , and then to mat  16 . Upon heating of the mat the air treatment chemical is dispensed. In this way heat transfer plate  30  acts not only as an electrical contact, it also spreads out and transfers heat generated by PTC heating element  22 . 
   A fusible resistor  66  preferably is included in the electrical circuit by connecting to heat transfer plate  30  at contact  67 . Should the heater  14  overheat, the fusible resistor  66  opens, halting the flow of electrical current through PTC heating element  22 , thus providing protection for the overall device. The fusible resistor  66  and contact  32  via elongated leg  34  connect into plug terminals  18 . As shown in FIG.  4 , ribs  68  of cover  12  hold mat  16  against PTC heater  14  when the mat  16  is installed in vaporizer  10 . 
   To use the device of the present invention, one plugs terminals  18  into an electrical outlet, thereby causing heat to be generated by PTC heating element  22 , thereby heating the impregnated mat and causing air treatment chemicals to be dispensed thereby. The nature of the substrate used for the mat is not critical, nor is what impregnates it. For purposes of this invention, the key features relate to the domed structure of the contact. 
   While this invention has been described with reference to a particular embodiment, the present invention can be modified within the spirit and scope of this disclosure. For example, the form of the dome  36  of the contact need not be exactly as shown. Rather, the key portion of the concept is that no edge of the contact along the head be positioned where it can come into contact with the PTC resistance heater. 
   This application is therefore intended to cover variations, uses, or adaptations of the invention using its general principles. Thus, the claims should be looked to in order to judge the full scope of the invention. 
   INDUSTRIAL APPLICABILITY 
   The present invention provides a PTC type heater which can be produced inexpensively and has improved maintenance characteristics.