resistor

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resistor,

two-terminal electric circuit component that offers opposition to an electric current. Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistanceresistance,
property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. Optimum resistance is provided by a conductor that is long, small in cross section, and of a material that conducts
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). They are available in several common forms: wirewound resistors are formed from windings of fine wire; film resistors, commonly found in consumer electronic devices, use lengths of carbon or metal film deposited on a resistive base; and carbon-composition resistors use a bonded mass of carbon powder with a phenolic binder. Some resistors are made so that their values can be adjusted (see potentiometerpotentiometer.
1 Manually adjustable, variable, electrical resistor. It has a resistance element that is attached to the circuit by three contacts, or terminals. The ends of the resistance element are attached to two input voltage conductors of the circuit, and the third
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; rheostatrheostat
, device whose resistance to electric current depends on the position of some mechanical element or control in the device. Typically a rheostat consists of a resistance element equipped with two contacts, or terminals, by which it is attached to a circuit: a fixed
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). Resistors absorb power from a circuit and convert it into heat; they are normally rated for the maximum amount of power that they can safely handle. Special resistors are also produced for the integrated circuitintegrated circuit
(IC), electronic circuit built on a semiconductor substrate, usually one of single-crystal silicon. The circuit, often called a chip, is packaged in a hermetically sealed case or a nonhermetic plastic capsule, with leads extending from it for input, output,
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.

Resistor

One of the three basic passive components of an electric circuit that displays a voltage drop across its terminals and produces heat when an electric current passes through it. The electrical resistance, measured in ohms, is equal to the ratio of the voltage drop across the resistor terminals measured in volts divided by the current measured in amperes. See Ohm's law

Resistors are described by stating their total resistance in ohms along with their safe power-dissipating ability in watts. The tolerance and temperature coefficient of the resistance value may also be given. See Electrical resistance, Electrical resistivity

All resistors possess a finite shunt capacitance across their terminals, leading to a reduced impedance at high frequencies. Resistors also possess inductance, the magnitude of which depends greatly on the construction and is largest for wire-wound types. See Capacitance, Electrical impedance, Inductance

Resistors may be classified according to the general field of engineering in which they are used. Power resistors range in size from about 5 W to many kilowatts and may be cooled by air convection, air blast, or water. The smaller sizes, up to several hundred watts, are used in both the power and electronics fields of engineering.

Direct-current (dc) ammeters employ resistors as meter shunts to bypass the major portion of the current around the low-current elements. These high-accuracy, four-terminal resistors are commonly designed to provide a voltage drop of 50–100 mV when a stated current passes through the shunt. See Ammeter

Voltmeters of both the dc and the ac types employ scale-multiplying resistors designed for accuracy and stability. The arc-over voltage rating of these resistors is of importance in the case of high-voltage voltmeters. See Voltmeter

Standard resistors are used for calibration purposes in resistance measurements and are made to be as stable as possible, in value, with time, temperature, and other influences. Resistors with values from 1 ohm to 10 megohms are wound by using wire made from special alloys. The best performance is obtained from quaternary alloys, which contain four metals. The proportions are chosen to give a shallow parabolic variation of resistance with temperature, with a peak, and therefore the slowest rate of change, near room temperature. See Electrical units and standards

By far the greatest number of resistors manufactured are intended for use in the electronics field. The major application of these resistors is in transistor analog and digital circuits which operate at voltage levels between 0.1 and 200 V, currents between 1 μA and 100 mA, and frequencies from dc to 100 MHz. Their power-dissipating ability is small, as is their physical size.

Since their exact value is rarely important, resistors are supplied in decade values (0.1, 1, 10, 100 ohms, and so forth) with the interval between these divided into a geometric series, thus having a constant percentage increase. For noncritical applications, values from a series with intervals of 20% (12 per decade) are appropriate. A series with 10% intervals (24 per decade) is often used for resistors having a tolerance of 1%. Where the precise value of a resistor is important, a series with 2.5% intervals (96 per decade) may be used.

Resistors are also classified according to their construction, which may be composition, film-type, wire-wound, or integrated circuit.

The composition resistor is in wide use because of its low cost, high reliability, and small size. Basically it is a mixture of resistive materials, usually carbon, and a suitable binder molded into a cylinder. Copper wire leads are attached to the ends of the cylinder, and the entire resistor is molded into a plastic or ceramic jacket. Composition resistors are commonly used in the range from several ohms to 10–20 M&OHgr;, and are available with tolerances of 20, 10, or 5%.

The film-type resistor is now the preferred type for most electronic applications because its performance has surpassed that of composition resistors and mass-production techniques have reduced the cost to a comparable level. Basically this resistor consists of a thin conducting film of carbon, metal, or metal oxide deposited on a cylindrical ceramic or glass former. The resistance is controlled by cutting a helical groove through the conducting film. This helical groove increases the length and decreases the width of the conducting path, thereby determining its ohmic value. By controlling the conductivity, thickness of the film, and pitch of the helix, resistors over a wide range of values can be manufactured. Film construction is used for very high value resistors, up to and even beyond 1 T&OHgr; (1012 ohms).

Wire remains the most stable form of resistance material available; therefore, all high-precision instruments rely upon wire-wound resistors. Wire also will tolerate operation at high temperatures, and so compact high-power resistors use this construction. Power resistors are available in resistance values from a fraction of an ohm to several hundred thousand ohms, at power ratings from one to several thousand watts, and at tolerances from 10 to 0.1%. The usual design of a power resistor is a helical winding of wire on a cylindrical ceramic former. After winding, the entire resistor is coated in vitreous enamel. Alternatively, the wound element may be fitted inside a ceramic or metal package, which will assist in heat dissipation. The helical winding results in the resistor having significant inductance, which may become objectionable at the higher audio frequencies and all radio frequencies. Precision wire-wound resistors are usually wound in several sections on ceramic or plastic bobbins and are available in the range from 0.1 &OHgr; to 10 M&OHgr;.

Integrated circuit resistors must be capable of fabrication on a silicon integrated circuit chip along with transistors and capacitors. There are two major types: thin-film resistors and diffused resistors. Thin-film resistors are formed by vacuum deposition or sputtering of nichrome, tantalum, or Cermet (Cr-SiO). Such resistors are stable, and the resistance may be adjusted to close tolerances by trimming the film by using a laser. Typical resistor values lie in the range from 100 &OHgr; to 10 k&OHgr; with a matching tolerance of ±0.2% and a temperature coefficient of resistance of ±10 to ±200 ppm/°C.

Diffused resistors are based upon the same fabrication geometry and techniques used to produce the active transistors on the silicon chip or die. A diffused base, emitter, or epitaxial layer may be formed as a bar with contacts at its extremities. The resistance of such a semiconductor resistor depends upon the impurity doping and the length and cross section of the resistor region. In the case of the base-diffused resistor, the emitter and collector regions may be formed so as to pinch the base region to a very small cross-sectional area, thereby appreciably increasing the resistance. The relatively large impurity carrier concentration in n- and p-type regions limits the resistance value. Resistor values between 100 &OHgr; and 10 k&OHgr; are common.

The deposited-film and wire-wound resistors lend themselves to the design of adjustable resistors or rheostats and potentiometers. Adjustable-slider power resistors are constructed in the same manner as any wire-wound resistor on a cylindrical form except that when the vitreous outer coating is applied an uncovered strip is provided. The resistance wire is exposed along this strip, and a suitable slider contact can be used to adjust the overall resistance, or the slider can be used as the tap on a potentiometer. See Potentiometer

Resistor

 

a structural component of an electric circuit (in the form of a finished part), designed primarily to present a known (nominal) resistance to an electric current in order to control the current and voltage.

Resistors are produced in lots by industry. In electronic equipment, they often constitute more than one-half (up to 80 percent) of all the components. Some resistors are used for measuring temperature, and in such resistors the resistance is strongly dependent on the temperature. Other types measure resistances (as one of the measures of electrical quantities), or they may be used as electric heating elements. Resistors produced by industry range in value from 1 to 1012 ohms; the permissible deviation from nominal values ranges from 0.25 to 20 percent, and power dissipations range from 0.01 to 150 watts. The parameters of a resistor are indicated on the resistor’s body, sometimes in the form of a code, such as colored bands.

A resistor’s resistance is dependent on the resistor’s physical properties and the dimensions of the resistive conductor. Depending on the materials from which the resistive conductor is made, resistors are classified as metallic, carbon, liquid, ceramic, or semiconductor. Depending on the design, resistors may have the resistive conductor in the form of a film deposited on the surface of a dielectric or in the form of a wire, ribbon, or plate. In order to protect the resistive conductor from dust, moisture, and mechanical influences, the resistive conductors of low-power resistors are usually coated with a vitreous enamel, which in the case of wire resistors also serves as the insulation between individual windings. Resistors may have a fixed or variable resistance. The value of a variable resistor can change as a result of a mechanical displacement of a slider, as in a rheostat, or as a result of a nonlinear relationship between the current and the voltage, as in a varistor or thermistor.

REFERENCES

Martiushov, K. I., and Iu. V. Zaitsev. Rezistory. Moscow-Leningrad, 1966.
Malinin, R. M. Rezistory, 2nd ed. Moscow, 1969.
Chunikhin, A. A. Elektricheskieapparaty. Moscow, 1975.

T. N. DIL’DLNA

resistor

[ri′zis·tər]
(electricity)
A device designed to have a definite amount of resistance; used in circuits to limit current flow or to provide a voltage drop. Also known as electrical resistor.

resistor

A device used in an electric circuit to control the flow of current.

resistor

an electrical component designed to introduce a known value of resistance into a circuit

resistor

An electronic component that resists the flow of current in an electronic circuit. Resistors are often made out of chunks of carbon or thin films of carbon or other resistive materials. They can also be made of wires wound around a cylinder. The common resistor is a two-wire package with a fixed resistance measured in ohms; however, different types of resistors are adjustable by the circuit designer or the user (see variable resistor).

Discrete or IC
Discrete resistors are individual packages. On a circuit board, discrete axial resistors are commonly used with their two wires soldered into the holes of the board. Generally smaller than axial resistors, discrete surface-mounted resistors are soldered on top of the board. In addition, resistors are built into microprocessors and other integrated circuits (ICs), but they use semiconductor structures for their fabrication in a manner similar to transistors and PN junctions. See variable resistor and thru-hole.


Resistors
There are a wide variety of resistors in use. The chip in the center is a reminder that resistive elements are also used in integrated circuits.







Axial Resistors
Axial resistors are discrete components with wire leads that are soldered to other components or to the circuit board. This type of resistor is easily identified by its color bands, which indicate its resistance and tolerance (see example below).


Axial Resistors
Axial resistors are discrete components with wire leads that are soldered to other components or to the circuit board. This type of resistor is easily identified by its color bands, which indicate its resistance and tolerance (see example below).







Orange-Orange-Yellow-Gold
The color bands on these resistors are interpreted as 3-3-0000-5%, which means 330,000 ohm resistors, plus or minus 5%.







Surface Mount Resistors
Surface mount resistors are soldered on top of the circuit board and are identified by number rather than color bands.
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