Abstract:
A low profile mount for a disc varistor. A thermally sensitive switch is provided both for single and multiple electrode embodiments. The switch may be placed in a shorting circuit and include a spring biased conductor prevented from closure by a heat sensitive element which softens in responsive to excessive heat. The varistor may be fused to prevent excessive current from a short circuited but not open circuited varistor. Methods are also provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a surface mount for an electrical component and more particularly to a surface mount for a metal oxide varistor.  
           [0002]    Surface mounting of components on printed circuit boards is common and it is desirable that the mounts be low in profile so that the distance between boards can be minimized and the size of the electronic chassis thus reduced. Where the electrical component is a thin element such as a disc with electrodes on both flat sides, low profile mounts are particularly desirable.  
           [0003]    It is also desirable that the number of components required for mounting be minimized because of the area on the circuit board each component occupies. Thus it is highly desirable that the mount be compatible with components having multiple electrodes.  
           [0004]    The thermal characteristics of the electrical components is generally a concern and it is desirable that the surface mount be compatible with, or integrated with one or more thermally sensitive switches. The thermal characteristics for many electrical circuit components such as varistors are particularly important where the component is coated with epoxy. In the event of a sustained overvoltage condition across the component, the current through the component can cause the temperature to increase sufficiently to cause the epoxy coating to smoke or ignite. Similarly, the epoxy coating can smoke or ignite in the event the component is subjected to a significant current pulse such that the component internally shorts but does not destruct, i.e., does not open-circuit.  
           [0005]    The current solution to this overheating problem is to wire in series with the component a thermal cutout device in sufficiently close physical proximity to respond to the heat in the component to short circuit and thereby disconnect the component. This solution requires the addition of extra components and increases the cost and complexity of the circuit. Since great care must be taken to keep the thermal cutout device in close physical proximity to the component to preserve the responsiveness without overreacting, interference may occur with any surface mount.  
           [0006]    Additionally, the “thermal connection” between the component and the thermal cutout device may be difficult to optimize in a surface mount.  
           [0007]    Finally, low temperature thermal cutout devices may not be able to survive the soldering operation required to attach the component to a printed circuit board or the like, and a surface mount to which the component may be mounted without soldering is highly desirable.  
           [0008]    Many of the above problems are exacerbated where the electrical component is a disc varistor. Disc varistors are well known and generally comprise a thin disc of a metal oxide or other voltage variable resistive material with an electrode on opposite flat sides of the varistor material. Known surface mounts for the smaller sizes, e.g., 7-10 mm, generally include a molded plastic body into which spring electrodes exiting the sides of the body are internally exposed in position to be contacted by the electrodes of the disc varistor when inserted therebetween from the front of the body.  
           [0009]    One example of such prior art mount is illustrated in FIGS. 1 and 2 where the disc varistor  18  comprises a disc of varistor material  20  with surface electrodes  22  and  24  on the flat sides thereof. The varistor  18  may be inserted into the front opening of a plastic body  26  which carries a top contact  28  and a bottom contact  30  conveniently spring biased toward each other and separated by the insertion of the disc varistor. The ends of the contacts  32 ,  34  may be used to electrically connect the circuit to other components on the printed circuit board on which the body  26  may be mounted. Such mounts are expensive to manufacture and are generally higher in profile than desired. Moreover, they do not provide for multiple electrodes, for temperature protection or for a fuse.  
           [0010]    Accordingly, it is an object of the present invention to obviate many of the above problems and to provide a novel surface mount and method for an electrical component on a printed circuit board.  
           [0011]    It is another object of the present invention to provide a novel surface mount and method in which the mount is low profile.  
           [0012]    It is still another object of the present invention to provide a novel surface mount and method in which the component may be connected without soldering.  
           [0013]    It is yet another object of the present invention to provide a novel surface mount and method which is readily adapted for multiple electrical connections.  
           [0014]    It is yet still another object of the present invention to provide a novel surface mount and method which is compatible with a thermal switch.  
           [0015]    It is a further object of the present invention to provide a novel surface mount which is inexpensive and simple in construction.  
           [0016]    It is yet a further object of the present invention to provide a novel surface mount and method which short circuit protection is available.  
           [0017]    It is still a further object of the present invention to provide a novel surface mount and method in which the mount may be coated with a moisture barrier and/or have edge passivation.  
           [0018]    These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a pictorial view of a prior art surface mount package for a varistor disc.  
         [0020]    [0020]FIG. 2 is a section in elevation taken through lines  2 - 2  of FIG. 1.  
         [0021]    [0021]FIG. 3 is a side view in elevation of one embodiment of the surface mount of the present invention.  
         [0022]    [0022]FIG. 3A is an exploded pictorial view of the mount of FIG. 3.  
         [0023]    [0023]FIG. 4 is a side view in elevation of a second embodiment of the surface mount of the present invention.  
         [0024]    [0024]FIG. 5 is a side view in elevation of the embodiment of FIG. 4 with a thermal switch in the open position.  
         [0025]    [0025]FIG. 6 is a schematic circuit diagram of the varistor and switch of FIGS. 3 and 4 with a fuse in series with the varistor.  
         [0026]    [0026]FIG. 7 a side view in elevation of a third embodiment of the surface mount of the present invention adapted for two electrodes on the upper surface of the varistor.  
         [0027]    [0027]FIG. 8 is a top plan view of a two component electrode compatible with the mount of FIG. 7.  
         [0028]    [0028]FIG. 9 is a schematic circuit diagram of the varistor of FIG. 8.  
         [0029]    [0029]FIG. 10 is a side view in elevation of a fourth embodiment of the surface mount of the present invention adapted for two electrodes on the lower surface of the varistor.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]    With reference to FIGS. 3 and 3A, a preferred embodiment of the mount of the present invention includes a molded plastic body  36  with a central circular depression dimensioned to receive a disc varistor  18  (varistor material  20  and electrodes  22  and  24 ) to a depth of about one-half the thickness thereof. Centrally located within the depression is a second depression adapted to receive a circular contact  38  which exits the body  36  to provide a first lead  40 . The contact  38  desirably extends upwardly above the bottom of the first depression in order to insure contact with the bottom electrode  24  of the varistor  18 .  
         [0031]    The body  38  is desirably molded with a support  42  for the second contact  44  which may be made from an electrically conductive spring wire so that it is spring biased downwardly into contact with the upper electrode  22  of the disc varistor  18 . The contact extends through the support  42  to provide a second electrical lead  46  of the mount.  
         [0032]    As shown in FIG. 4, the leads  40  and  46  may extend downwardly so as to penetrate through a printed circuit board for electrical connection to an electrical circuit on the lower side thereof.  
         [0033]    As described more fully in applicant&#39;s copending application Ser. No. filed concurrently herewith for “Thermally Protected Metal Oxide Varistor And Method”, the disclosure of which is hereby incorporated herein by reference, thermal protection may be provided by a third contact  50  which may be constructed in the same manner as the contact  44  to provide a third lead  52 , but which includes a thermal element  54  covering at least a portion of the contact  50  to prevent contact with the upper electrode  22  of the varistor  18 . The thermal element  54  may be any suitable conventional insulator responsive to excessive heat in the varistor to either physically dissipate or to lose its insulative characteristics. An example of such a thermally reactive insulator is benzanilide. Benzanilide is generally rigid up to its melting temperature, which is generally in the range of 150-200° C. and is preferentially 163° C. Benzanilide adheres well and breaks down at its melting temperature to a viscosity that is water-like. It has been found desirable for the body  36  to include a second support  56  at the end of the mount opposite to the support  42  to lend rigidity to the contact  50 .  
         [0034]    The circuit including the third contact  50  is shown in FIG. 6 where a shorting circuit is shown with the leads  52  and  40  electrically connected. Under normal operating conditions, current flows through the varistor  18  via the leads  40  and  46 . An undesirable thermal condition in the varistor  18  caused by a excessive current through the varistor from a sustained overvoltage condition across the varistor, or by a high current pulse through the varistor which causes an internal short in the varistor but does not cause the varistor to destruct, i.e., to open circuit. In the event of such an undesirable thermal, the effective disappearance of the insulating means  54  in response thereto permits contact between the electrode  50  and the upper electrode  22  of the varistor and effects a short circuit across the varistor  18 .  
         [0035]    While the spring wire contact has been found desirable, it is to be noted that other biasing means may be adequate, e.g., a discrete spring for the electrical contact or a magnet. The shape of the varistor is not a part of this invention, and it may be square or other forms as desired so long as the depression in the body of the mount is compatible.  
         [0036]    However, the mount of the present invention finds particular utility where the disc varistor electrodes are segmented as shown, e.g., in FIG. 8. With reference to FIG. 8, one of the electrodes, e.g.,the upper electrode  22 , may be segmented into two, three or four parts, each forming a varistor element with the single lower electrode  24 .  
         [0037]    As shown in FIG. 7, an embodiment with two segments on the upper electrode may use two spring wire contacts  60 ,  62  connected respectively to leads  64 ,  66  to form the circuit shown in FIG. 9. As shown in FIG. 10, the identical circuit may be formed using the two segment varistor of FIG. 8 with the segmented electrode facing downwardly and a single contact  44  in contact with the single, now upper, electrode  24 . This embodiment requires that the body  36  include not one central contact  38  as illustrated in FIG. 3A, but two spaced contacts  38 A and  38 B as shown in FIG. 10.  
         [0038]    While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.