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| A <b>resistor</b> is an electrical component that resists the flow of ]. Variable resistors are called '''poteniometers'' or '''rheostats''' (see below). | A <b>resistor</b> is an electrical component that resists the flow of ]. Variable resistors are called '''poteniometers''' or '''rheostats''' (see below). | ||
| Resistors are used as part of ]s and incorporated into ] ] devices. The critical measurement of a resistor is its <i>resistance</i>, which serves as a ratio of voltage to current and is measured by the ] unit ]. A component has resistance 1 ohm if a voltage of 1 ] across the component results in a current of 1 ], or Amp, which is equivalent to 1.6 · 10<sup>19</sup> ]s per ]. | Resistors are used as part of ]s and incorporated into ] ] devices. The critical measurement of a resistor is its <i>resistance</i>, which serves as a ratio of voltage to current and is measured by the ] unit ]. A component has resistance 1 ohm if a voltage of 1 ] across the component results in a current of 1 ], or Amp, which is equivalent to 1.6 · 10<sup>19</sup> ]s per ]. | ||
Revision as of 01:03, 26 August 2002
A resistor is an electrical component that resists the flow of current. Variable resistors are called poteniometers or rheostats (see below).
Resistors are used as part of electrical networks and incorporated into microelectronic semiconductor devices. The critical measurement of a resistor is its resistance, which serves as a ratio of voltage to current and is measured by the SI unit ohm. A component has resistance 1 ohm if a voltage of 1 volt across the component results in a current of 1 ampere, or Amp, which is equivalent to 1.6 · 10 electrons per second.
Any physical object is a kind of resistor. Most metals are conductors, and have low resistance to the flow of electricity. The human body, a piece of plastic, or even air has a resistance that can be measured. Materials that have very high resistance are called insulators.
The relationship between voltage, current, and resistance though an object is given by a simple equation, often mistakenly referred to as Ohm's Law:
V = IR
where V is the voltage across the object in volts, I is the current through the object in amperes, and R is the resistance in ohmss. If V and I have a linear relationship -- that is, R is constant -- along a range of values, the material of the object is said to be ohmic on that range. An ideal resistor has a fixed resistance across all frequencies and amplitudes of voltage or current.
Resistance in theory can range anywhere from zero to infinity. In practice, achieving zero or inifinity is quite difficult or impossible. Superconducting metals at very low temperatures approach zero resistance. Insulators (such as air, ceramic, or other non-conducting materials) may have extremely high resistance, but often break down under sufficiently high voltage.
The resistance of a component can be calculated from its physical characteristics. Resistance is proportional to cross-sectional area and to the material's resistivity, a physical property of the molecules that comprise the resistor. Resistance is inversely proportional to the length of the resistor.
In practice, the discrete component sold as a "resistor" is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (variation from the marked resistance), on color coded resistors a leftmost silver band denotes 10% tolerance, a gold band 5% tolerance, and a red band 2% tolerance. Lower tolerance resistors are also available.
A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (burn up, for instance). Although some resistors have specified voltage and current ratings, most are rated with a wattage rating which is determined by the physical size. Larger resistors are able to dissipate more heat because of the larger surface area. Common wattage ratings for carbon-based resistors are 1/8 watt, 1/4 watt, and 1/2 watt. Wire-wound and sand-filled resistors are used when a high wattage rating is required, such as 20 watts.
Furthermore, all real resistors also introduce some inductance and capacitance, which change the dynamic behavior of the resistor from the ideal equation.
Variable resistor
The variable resistor is a resistor whose value can be adjusted by a mechanical movement, for example by being turned by hand.
A potentiometer is a common form of variable resistor. In this form, the wiper travels the length of a resistor, and both ends of the resistor, as well as the wiper have electrical connections. This provides a variable ratio of resistances, as well as either an increasing or decreasing resistor.
A rheostat is a variable resistor for use with high currents.
The resistive element may be resistance wire, carbon particles in plastic or a ceramic/metal mixture. A movable contact called the wiper touches the resistive element at a variable point along its length, so that the resistance between the wiper and one end of the element can be varied.
Variable resistors can be cheap single-turn types or multi-turn types with a helical element.
Although rotary potentiometers are most common, linear potentiometers also exist.
One very popular form of potentiometer is called a "string pot." It's a multi-turn potentiometer with an attached reel of wire.