Abstract:
A compact molded case containing an epoxied surge suppressor circuit with extending male prongs and female sockets for interconnection between a wall outlet and a power cord for electronic equipment. The suppressor circuit has a varistor network and associated pico fuses for transient surge suppression. An indicator connected to the suppressor circuit provides an indication of circuit status.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to voltage sensing devices and, more particularly, is directed towards plug-type transient surge suppressors. 
     2. Description of the Prior Art 
     It is desirable to protect electronic equipment against premature failure caused by transient surges. A variety of devices have been designed to sense voltage mangitude and to suppress transients above a predetermined level. Generally, such devices are complex in construction and costly to manufacture. A need has arisen for a transient surge suppressor that is compact, relatively low in cost and simple in construction. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a relatively inexpensive and compact transient surge suppressor device having a high degree of reliable operational characteristics. 
     It is another object of the invention to provide a compact plug-type transient surge suppressor that is configured to be inserted between a wall outlet and a male plug on a power cord connected to electronic equipment. The transient surge suppressor device of the present invention is characterized by a compact molded case that engulfs an epoxied surge suppression circuit. Male prongs and female sockets that are interconnected with the suppressor circuitry and are fixed within the epoxy compound extend from opposite faces of the molded case. The suppressor circuitry includes a varistor network which constitutes a non-linear resistance that decreases as voltage increases for shunting a load component in the presence of voltage surges above a predetermined magnitude. Pico fuses are provided for isolating the varistor network in the event of massive transient surges that exceed the operational limits of the varistor network. An indicator connected to the suppressor circuitry provides an indication of circuit status. 
     It is another object of the present invention to provide a method of fabricating a transient surge suppressor having a molded casing by potting suppressor circuitry in a room temperature cured epoxy and then molding a plastic casing about the potted circuitry. 
     Other objects of the present invention will in part be obvious and will in part appear hereinafter. 
     The invention accordingly comprises the apparatuses and methods, together with their parts, steps, elements and interrelationships that are exemplified in the following disclosure, the scope of which will be indicated in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A fuller understanding of the nature and objects of the present invention will become apparent upon consideration of the following detailed description taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of the male end of a plug-type transient surge suppressor device embodying the present invention; 
     FIG. 2 is a perspective view of the plug-type transient surge suppressor device of FIG. 1 showing the female end; 
     FIG. 3 is a perspective view of the potted suppressor circuitry of the device of FIGS. 1 and 2; and 
     FIG. 4 is a schematic diagram of the suppressor circuitry. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, particularly FIG. 1, there is shown a plug-type transient surge suppressor device 10 embodying the invention. Transient surge suppressor device 10 is configured to be interconnected between a wall outlet connected to a power source and a male plug on a line cord connected to electronic equipment. Surge suppressor 10 comprises a molded casing 12 having a substantially triangular profile in right cross section. A pair of elongated contact members 14 and 16, which are fixed within an epoxied body 18, are exposed at opposite faces of casing 12. Elongated contact member 14 includes a male prong portion 20 and a female socket portion 22. Elongated contact member 16 includes a male prong portion 24 and a female socket portion 26. Male prongs 20, 24 extend from a forward face 28 of casing 12 and female sockets 22, 26 are exposed at a rearward face 30 of the casing. A ground wire 32 with a terminal 34 extends from a side face of casing 12. A ground socket 36, which is connected to ground wire 32, is exposed at rearward face 30. An indicator 38, for example a light emitting diode, extends from an upper face 40 of casing 12. In the illustrated embodiment, casing 12 is composed of a plastic such as polyvinyl chloride. 
     As shown in FIG. 3, elongated contact members 14 and 16 are held in fixed spaced relationship to one another within body 18 which is composed of a room temperature cured epoxy having low thermal conductivity such as that sold by Hardman Incorporated of Belleville, N.J., under the trade designation EP-2305 LK. Contact members 14 and 16 are positioned relative to one another so that male prongs 20 and 24 can be plugged into a wall outlet and a male plug on the end of a line cord can be inserted into female sockets 22 and 26. Transient surge suppressor circuit elements 42, now to be described, are contained within epoxy body 18 and are interconnected with contact members 14, 16 which constitute a pair of electrical conductors. 
     Referring now to FIG. 4, it will be seen that the transient surge suppressor circuitry 42 includes a pair of voltage dependent, non-linear resistive elements 44, 46 having symmetrical conductive properties, a zener diode 48, a linear resistive element 50, a pair of fuse elements 52, 54 and indicator 38. In the illustrated embodiment, voltage dependent, non-linear resistive elements 44 and 46 are metal oxide varistors which conduct above a set recurrent peak applied voltage such as 184 volts AC with RMS applied voltage set at 130 volts AC. Varistor 44 is connected in parallel across contact members 14 and 16, and varistor 46 is connected in parallel across contact member 16 and a ground lead 56 which connects ground socket 36 and ground wire 32. When transient surge suppressor device 10 is inserted into a wall outlet, 115V-130V volts AC is present across contact members 14 and 16. Fuse element 52, a pico fuse, is connected in series between one side of varistor 44 and contact member 14, the junction of varistor 44 and pico fuse 52 being denoted by reference character 58. Fuse element 54, a pico fuse, is connected in series between the other side of varistor 44 and contact member 16, the junction of varistor 44 and pico fuse 54 being denoted by reference character 60. Varistor 46 is connected between junction 60 and ground lead 56. Linear resistive element 50, a resistor, is connected between junction 58 and the anode of zener diode 48. The cathode of zener diode 48 is connected to junction 60. Indicator 38, a light emitting diode, is connected across zener diode 48. 
     Pico fuses 52 and 54, which are rated at 1.5 amp fuses with applied voltage of 125V, are operative to isolate varistors 44 and 46 from the line voltage in the event of a massive transient surge that would render the varistors into the breakdown zone and unreliable. As previously indicated, varistors 44 and 46 are rated at 130 volts having a peak recurrent voltage of 184 volts, transient dissipation 2 Joules, .24 watts and peak current of 200 amperes t p  20 microseconds. Pico fuses 52 and 54 are rated for 4 hours at 100%, 5 seconds at 200% and 200 amperes at 25 microseconds. Light emitting diode 38 is provided to monitor the status of pico fuses 52, 54. When either or both of pico fuses 52 and 54 are blown, light emitting diode 38 is off. Resistor 50, for example a 75 K ohm resistor, is provided for limiting the emitting diode 38 current to the range of 16-20 milliamperes. Zener diode 48 is a 6 volt zener diode. 
     Transient surge suppressor device 12 is used by inserting male prongs 20 and 24 into a wall outlet. A line plug connected to electronic equipment to be protected is inserted into female sockets 22 and 26. Varistors 44 and 46 conduct above a set recurrent peak applied voltage such as 184 volts AC with RMS applied voltage set at 130 volts AC. The operational characteristics of varistors 44 and 46 is that of spaced charged limited currents. When the initial injected carrier density is less than the thermally generated free carrier density, the current-voltage characteristic is ohmic. When the injected carrier density exceeds the thermally generated free carrier density, the current becomes space charge limited and takes the form of the Mott-Gurney square law. The current rises rapidly until it reaches the trap free charge limited value. In other words, the resistance of the varistor drops as the applied voltage is increased. In the presence of transient surges, the resistances of varistors 44 and 46 decrease and are operative to shunt the load in a protective manner to prevent premature failure of the electronic equipment. 
     As previously indicated, plug-type transient surge suppressor device 10 is in the form of molded casing 12 having a substantially triangular profile in right cross section. In the illustrated embodiment, the edges of forward face 28 and rearward face 30 of casing 12 are each approximately 3.5 cm and the depth of casing 12 is approximately 3.0 cm. 
     The steps of fabricating transient surge suppressor device 10 are now presented. First, contact members 14, 16 and suppressor circuitry 42 are set and potted in a polyethylene potting mold using a room temperature cured epoxy such as that sold by Hardman Incorporated of Belleville, N.J. under the trade designation EP-2305 LK. Upon completion of curing, the potted assembly is removed from the potting mold and placed into an encasing mold. Next, polyvinyl chloride at a temperature of 350° is injected into the encasing mold and encapsulates the epoxied circuitry 42 and contact members 14, 16. Finally, the molded casing 12 encapsulating suppressor circuitry 42, indicator 38 extending through an opening 62 formed in face 40, is removed from the encasing mold. The room temperature cured epoxy is operative to protect suppressor circuitry 42 from the thermal shock of 350° F during the molding of polyvinyl chloride casing 12. This method of manufacture is designed to prevent such a thermal shock from rendering the components of suppressor circuitry 42 unreliable and/or defective. 
     Since certain changes may be made in the foregoing disclosure without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description and depicted in the accompanying drawings be construed in an illustrative and not in a limiting sense.