Abstract:
The present invention discloses a varistor which comprises a ceramic case made from insulated refractory and at least one zinc-oxide ceramic housed in the ceramic case. The ceramic case is also filled with an insulated resin having the damp-proof effect and thus serves as a powerful clamp with enough mechanical strength against impact at high temperature. Accordingly, the varistor can be secured from flaming and explosion when the zinc-oxide ceramic is overloaded or ineffective.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a varistor, and more particularly to a varistor having a ceramic case for protecting the varistor from explosion and flaming.  
         [0003]     2. Related Prior Arts  
         [0004]      FIG. 1  shows a traditional varistor which is composed of a ceramic absorber  90 , two electrodes disposed on the opposite sides of the absorber  90 , and two leads  91 ,  92 . In  FIG. 1 ( a ), only the electrode  91  is illustrated, and the leads  91 ,  92 , usually made from tinned copper wires, are welded on the respective electrodes. In  FIG. 1 ( b ), the absorber is embedded with a packaging resin  93  for damp-proof and insulation effects. The packaging resin  93  is usually epoxy resin. For operation, the ceramic absorber  90  may protect the power circuit with grain boundary thereof. However, the ceramic absorber  90  very possibly reaches a high temperature when overloaded or ineffective, and therefore the outside packaging resin  93  would burn as flammability thereof.  
         [0005]     To avoid such dangerous situation, the present invention provides a ceramic case to the traditional varistor. By selecting proper material, the ceramic may have enough mechanical strength and nonflammability so as to protect the power circuit from explosion and flaming.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of the present invention is to provide a varistor having a ceramic case, so that the varistor can be safer from explosion and burning.  
         [0007]     The varistor primarily comprises at least one ceramic absorber, at least two leads, a packaging resin and a ceramic case. The ceramic absorber has a positive electrode and a negative electrode respectively connected to two leads which conducts the surge current to the ceramic absorber from converting electricity into heat. The packaging resin has good damp-proof and insulation properties and can embed the ceramic absorber completely. The ceramic case is preferably made from an insulated ceramic material sintered at high temperature, so as to exhibit superior mechanical strength and nonflammability. The ceramic absorber and the packaging resin are housed in the ceramic case which has at least one opening for the leads extending outside. As a result, the varistor can be secured with the ceramic case even though the ceramic absorber is ineffective.  
         [0008]     Types or positions of the opening of the ceramic case is not restricted, and preferably disposed corresponding to the leads. The ceramic case can be composed of a container and a cover. The opening can be formed on the cover, or beneath the container opposite to the cover. The ceramic case is preferably durable for at least 5 minutes at 700° C., 1 atmosphere. Proper material for the ceramic case includes Al 2 O 3 , SiO 2  or MgO, or a mixture thereof; and preferably contains 25˜99 wt % of Al 2 O 3 . The ceramic case can further house a thermistor.  
         [0009]     The ceramic absorber can be typically disk-shaped, or has a three-layered structure. The packaging resin is normally uses phenolic resin, silicone resin, epoxy resin, or a mixture thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows a traditional varistor and packaged with epoxy resin;  
         [0011]      FIG. 2  shows the varistor of the present invention and a cross view thereof; and  
         [0012]      FIG. 3  shows different types of the cover and the container of the ceramic case. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]      FIG. 2  shows a varistor of the present invention and a cross view thereof. The varistor  10  comprises a ceramic case  20 , a ceramic absorber  30  with electrodes  301 ,  302 , leads  31 ,  32 , and a packaging resin  40 . One end of each of the leads  31 ,  32  is welded to the respective electrodes  301 ,  302  of the ceramic absorber  30 , so that surge current can be conducted to the ceramic absorber  30  via the leads  31 ,  32  and converted into heat. The packaging resin  40  performs both of damp-proof and insulation properties, and can embed the ceramic absorber  30  completely. A major feature of the present invention is the ceramic case  20 , which is made from an insulated ceramic material previously sintered at high temperature, and thus performs superior mechanical strength and nonflammability. The ceramic absorber  30  and the packaging resin  40  are housed in the ceramic case  20 . The ceramic case  20  is composed of a container  2 l and a cover  22 , wherein the cover  22  has two openings  221 ,  222  disposed corresponding to the leads  31 ,  32 .  
         [0014]     A preferred method for producing the varistor is described as follows: 
    1. cleaning the ceramic absorber  30  welded with the leads  31 ,  32  to eliminate residual solder and impurities;     2. adding the packaging resin  40  into the container  21  at a volume of about 5%˜100% of the ceramic case  20 ;     3. placing the ceramic absorber  30  with the leads into the container  21 , then filling the container  21  with the packaging resin  40  to completely cover the ceramic absorber  30 , and engaging the cover  22  into the container  21  with the leads  31 ,  32  extending from the opening  221 ,  222 ;     4. drying and solidifying the packaging resin  40  by baking to achieve the varistor as the above embodiment.    
 
         [0019]     In another producing embodiment, the ceramic absorber  30  is previously dipped in the packaging resin  40  by clamping the leads  31 ,  32 , and then placed into the container  21 .  
         [0020]     Additionally, types and positions of the opening(s) can be arranged optionally for the leads  31 ,  32  extending from the ceramic case  20 .  FIG. 3  illustrates different assemblies of the container and the cover. In  FIG. 3 ( a ), two nicks  231 ,  232  are respectively formed on opposite edges of the cover  23 , and the leads  31 ,  32  can protrude from the nicks  231 ,  232 . In  FIG. 3 ( b ), two nicks  241 ,  242  are formed on the same edge of the cover  24 , and the leads  31 ,  32  can protrude. In  FIG. 3 ( c ), the cover  25  has smooth edges, and two openings  211 ,  212  are formed beneath the container  21 ′. With respective to the other embodiments, the ceramic absorber  30  is placed up side down for the leads  31 ,  32  respectively protruding from the opening  211 ,  212 , and then the container  21 ′ is filled with the packaging resin  40 . At last, the cover  25  is engaged into the container  21 ′.  
         [0021]     In the present invention, the packaging resin is preferably phenolic resin, and other known resins are suitable, for example, silicone resin or epoxy resin. The packaging resin is preferably insulated polymers with high electrical resistance and density, and can protect the ceramic absorber from damp or contaminants. Further, the packaging resin is viscous and mobile, which facilitates to secure the ceramic absorber and the cover in the container and stuff the space between the openings and the leads.  
         [0022]     Though stress from the ceramic absorber will be mostly applied to the two sides with larger surface areas than to the cover, it&#39;s still safer to well design the details. For example, slight space or packaging resin can be provided between the openings (or nicks) and the leads, or between the cover and the container and thus serves as a buffer for heat expansion. Further, the cover is disposed in the container away from the top edges a desired distance, so that the cover can shift upward when heat expansion occurs.  
         [0023]     According to the present invention, the ceramic case can function as a protector of the varistor from explosion or flaming when the ceramic absorber is ineffective due to overloading or aging/worsening of material. That is, the ceramic case is required to be made from a material with good mechanical strength and nonflammability; for example, Al 2 O 3 , SiO 2  or MgO. Particularly, the material containing 25˜99 wt % of Al 2 O 3  is preferred due to advantages as follows: 
    1. Good mechanical strength at high temperature    
 
         [0025]     When surge current is instantly conducted via the leads within micro-seconds, very high temperature is reached on a local area of the ceramic absorber. If the temperature is over 700° C., the ceramic absorber is possibly ineffective and begins burning. However, the ceramic case made from Al 2 O 3  can still remain good mechanical strength even at 1,000° C., and therefore the ceramic absorber can be powerfully clamped within the ceramic case without breakdown, burning and explosion. 
    2. High heat-transfer rate    
 
         [0027]     The traditional varistor is simply packaged with silicone or epoxy resin through which the heat is slowly diffused according to the temperature gradient between the resin and the leads or the air. In the present invention, the ceramic case made from Al 2 O 3  with high coefficient of heat transfer can effectively conduct heat away by contacting the packaging resin. As a result, accumulation of heat is avoided and the varistor can be used for a longer time. 
    3. Flat contact surface to the TCO fuse    
 
         [0029]     In general, a positive temperature coefficient (PTC) thermistor or a thermal cut-off (TCO) fuse will be housed in the ceramic case. The PTC thermistor or TCO fuse can increase resistance to reduce the current or fuse instantly, when the temperature of the varistor is raised due to surge current. The traditional varistor is normally packaged as an inflated tablet with bulgy surfaces. When the TCO fuse is attached to such surface, the contact area is very limited to a point or a line. Therefore, the TCO fuse could not sensitively respond to the real temperature of the varistor and thus fails to protect the power circuit. In the present invention, the flat surfaces of the Al 2 O 3  ceramic case may provide large contact areas to the TCO fuse, and thus improve protection of the TCO fuse. However, it should be noticed that other materials with suitable characteristics can also be applied to the ceramic case in addition to Al 2 O 3 .  
         [0030]     While the present invention has been illustrated with the above preferred embodiments, it should be noticed that any modifications or variations in materials, arrangements and shapes deducing from these embodiments still belong to the scope of the present invention.