(1) Field of the Invention
This invention relates to resistors and, more particularly, to resistors molded of conductive loaded resin-based materials comprising micron conductive powders, micron conductive fibers, or a combination thereof, homogenized within a base resin when molded. This manufacturing process yields a conductive part or material usable within the EMF or electronic spectrum(s).
(2) Description of the Prior Art
Resistors are a basic building block in electrical and electronic systems. Resistors are based on a fundamental property of a material, namely the resistivity or, inversely, the conductivity of the material. Resistivity, or, conversely conductivity, is based on the relative ability, or inability, of a material to conduct current under a voltage bias. Low resistivity materials permit easy current flow and are typically called conductors. Metals, such as copper and aluminum, are examples of excellent conductors. High resistivity materials permit little or no current flow and are typically called insulators. Metal oxides, ceramics, and air are examples of excellent insulators.
The resistance of an object or device is simply a measure of the ratio of voltage to current in that object or a device. Resistance is defined, electrically, by the equation:Resistance=Voltage/Current,and is expressed in Ohms. Resistance depends, mostly, on two factors: (1) the inherent resistivity of the material that makes up the object or device and (2) the physical shape of the device. Resistivity is a material property that is essentially constant, excluding variation due to temperature, for the material once the composition of the material is established. Resistivity (ρ) is expressed in Ohms-cm. The shape of the object is important because actual resistance varies inversely with the available cross sectional area through which current may flow. Resistance may be easily calculated for a simple object once the physical shape and size are known and once the resistivity is known. Resistance is given by:Resistance=ρ×(Areacross-secton/Length),where ρ is the material resistivity. Another useful metric for resistance calculation is sheet resistance. Sheet resistance is defined as the resistance of a square area of a material and is particularly useful in technologies, such as integrated circuit devices, where resistor structures have one fixed dimension, such as depth, and two variable dimensions, such as length and width. Sheet resistance is given in ohms/square and may be used to calculate a resistance value using the equation:Resistance=Sheet Resistance×(Length/Width).
All objects, even metal conductors, have a measurable resistance value across the object. In a metal conductor, this value is usually considered to be an undesirable feature and is termed “parasitic resistance.” In other cases, the resistance value is not only desired but, further, it is essential to correct operation of the electrical or electronics circuit. In this case the object or device is typically called a resistor. A resistor comprises two terminals, or connection points to the rest of the circuit, that are separated by the resistor bulk region. Resistors are constructed using a variety of techniques and materials.
Resistors may be categorized in a number of ways. For example, resistors may be discrete devices or may be integrated devices. Discrete resistors are manufactured as individual devices and then placed into, or onto, a circuit. Typically, discrete resistors are further electrically and mechanically attached by soldering. Integrated resistors are formed as part of the fabrication process of the overall circuit. For example, in a semiconductor integrated circuit device, a resistor may be formed by as a patterned line in a polysilicon layer where this same polysilicon layer is also fabricated into a transistor gate in another location on the circuit. This type of resistor is completely integrated into the design and manufacture of the article.
Resistor devices are manufactured using any of several approaches in the art. Film resistors are formed by covering a ceramic substrate with a resistive film. Carbon films and metal films, such as nichrome, are frequently used to create high value, low current resistors. Metal oxide resistors are formed by oxidizing a chemical, such as tin-chloride, on a ceramic substrate. Carbon composition resistors comprise a bulk piece of carbon-based material into which terminals are embedded. Wire wound resistors comprise a long metal wire that is wound around a core and encased in an insulator.
Resin-based polymer materials are used in many arts for the manufacture of a wide array of articles. These polymer materials combine many outstanding characteristics, such as excellent strength to weight ratio, corrosion resistance, electrical isolation, and the like, with an ease of manufacture using a variety of well-established molding processes. Many resin-based polymer materials have been introduced into the market to provide useful combinations of characteristics. However, most resin-based polymer materials are electrical insulators. Attempts to increase the conductivity of resin-based materials have been made in the art. However, the manufacture of useful resistors from resin-based materials has not been successful, to date, in the art. An important object of the present invention is to provide a new type of resistor device from a resin-based material.
Several prior art inventions relate to resistors and methods of manufacturing resistors. U.S. Patent Publication US 2003/0197589 A1 to Taguchi et al. teaches a variable resistor and its means of production. The resistor is formed on a substrate by means of screen printing and utilizes a thermosetting binder resin, a solvent for dissolving the binder resin, and carbon black as the conductive filler. U.S. Patent Publication US 2004/0012479 A1 to Yamada et al teaches a method of manufacturing a chip resistor with a superior surge property. U.S. Pat. No. 4,365,230 to Feldman teaches a lead screw type variable resistor wherein the lead screw is a conductive plastic and acts as a resistive element. U.S. Pat. No. 2,901,381 to Teal teaches a method of manufacture for film resistors. U.S. Pat. No. 6,359,545 B1 to Bressers teaches an adjustable resistor with a slider made of conductive rubber or a conductive plastic and formed with dual resistance strips. U.S. Pat. No. 4,036,786 to Tiedemann teaches a resistor comprising a fluorinated carbon composition as the essential conductive component. This invention also teaches 35% by weight of a conductive filler comprising partially fluorinated acetylene black and utilizes a polysulfone resin as the base resin.