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A resistor is an electrical component designed to have an electrical resistance that is independent of the current flowing through it. The common type of resistor is also designed to be independent of
temperature and other factors. Resistors may be fixed or variable. Variable
resistors are also called potentiometers or rheostats (see below).
A few resistor types
Some resistors are long and thin, with the actual resisting material in the centre, and a conducting metal leg on each end.
This is called an axial package. The photo on the right shows a row of commonly used resistors in a bandolier. Resistors
used in computers and other devices are typically much smaller, often in surface-mount (Surface-mount technology) packages without leads. Larger
power resistors come in more sturdy packages designed to dissipate heat efficiently, but they are all basically the same
structure.
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 in ohms, an SI unit. A component has a resistance of 1 ohm if a voltage of 1 volt across the
component results in a current of 1 ampere, or amp, which is equivalent to a flow of
6.25 × 1018 electrons per second in the opposite direction. (see: Current)
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 a vacuum has a resistance that can be measured. Materials that have very high resistance are called insulators.
The relationship between voltage, current, and resistance through an object is given by a simple equation, derived from and
often confused with 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 ohms. 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 over that range. An ideal
resistor has a fixed resistance across all frequencies and amplitudes of voltage or current.
Superconducting materials at very low temperatures have zero
resistance. Insulators (such as air, diamond, or
other non-conducting materials) may have extremely high (but not infinite) resistance, but break down and admit a larger flow of
current under sufficiently high voltage.
The resistance of a component can be calculated from its physical characteristics. Resistance is proportional to the length of the resistor and to the material's resistivity (a physical property of the material) and inversely proportional to cross-sectional area. The
equation to determine resistance of a section of material is:
-
is the resistivity of the material,
is the length, and is the surface area. This can be extended to an integral for
more complex shapes, but this simple formula is applicable to cylindrical wires and most common conductors. This value is subject
to change at high frequencies due to the skin effect, which decreases the
available surface area.
Standard resistors are sold in values from a few milliohms to about a gigohm; only a limited range of values called preferred values are available. In practice, the discrete component sold as
a "resistor" is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (maximum expected
variation from the marked resistance). On color coded
resistors a rightmost silver band denotes 10% tolerance, a gold band 5% tolerance, a red band 2% tolerance, and a brown band 1%
tolerance. Closer tolerance resistors, called precision 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 maximum power which is determined by the
physical size. Common power ratings for carbon composition and metal-film resistors are 1/8 watt, 1/4 watt, and 1/2 watt. Metal-film resistors are more stable than carbon resistors against temperature changes
and age. Larger resistors are able to dissipate more heat because of their larger surface area. Wire-wound and sand-filled
resistors are used when a high power 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.
Resistors in a parallel configuration
each have the same potential difference (voltage). To find their total equivalent resistance (Req):
-
-
The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to simplify equations. For
two resistors,
-
The current through resistors in series
stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to
the total voltage. To find their total resistance:
-
-
A resistor network that is a combination of parallel and series is broken up into smaller parts that are either one or the
other. For instance,
-
-
Variable resistor
The variable resistor is a resistor whose value can be adjusted by a mechanical movement, for example by being turned by
hand.
Variable resistors can be cheap single-turn types or multi-turn types with a helical element. Some even have a mechanical
display to count the turns.
Traditionally, variable resistors have been unreliable, because the wire or metal would corrode or wear. Some modern variable
resistors use plastic materials that do not corrode.
(Another method of control, which is not actually a resistor, but behaves like one, involves a photoelectric sensor system
which measures the optical density of a piece of film. Since the sensor does not touch the film, no wear is possible.)
A rheostat is a variable resistor with two terminals, one fixed and one sliding. It is often used with high
currents.
A potentiometer is a common type of variable
resistor. One common use is as volume controls on audio
amplifiers.
A Metal Oxide Varistor, or M.O.V. is a special type of resistor which has 2 very different resistance values,
a very high resistance at low voltage (below the trigger voltage) and very low resistance at high voltage (above the trigger
voltage). It is usually used for short circuit protection in power strips or lightning bolt "arrestors" on street power poles, or
as a "snubber" in back electromotive force circuits.
See also
External links
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