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| {{Infobox electronic component
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| | component = Resistor
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| | image = [[File:Resistor.jpg|200px]]
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| | caption = A typical axial-lead resistor
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| | type = [[Passivity (engineering)|Passive]]
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| | working_principle = [[Electric resistance]]
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| | invented =
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| | first_produced =
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| | symbol = [[File:Resistors.svg]]
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| | symbol_caption = ''1. US standard <br>2. IEC standard ''
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| }}
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| [[File:Resistors on tape.jpg|thumb|[[Axis of rotation|Axial]]-lead resistors on tape. The tape is removed during assembly before the leads are formed and the part is inserted into the board. In automated assembly the leads are cut and formed.]]
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| A ''' resistor''' is a [[passivity (engineering)|passive]] [[terminal (electronics)|two-terminal]] [[electronic component|electrical component]] that implements [[electrical resistance]] as a circuit element. Resistors act to reduce current flow, and, at the same time, act to lower voltage levels within circuits. Resistors may have fixed resistances or variable resistances, such as those found in [[thermistor|thermistors]], [[varistor|varistors]], [[trimmer (electronics)|trimmers]], [[photoresistor|photoresistors]] and [[potentiometer|potentiometers]].
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| The [[Electric current|current]] through a resistor is in [[direct proportion]] to the [[voltage]] across the resistor's terminals. This relationship is represented by [[Ohm's law]]:
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| :<math>I = {V \over R}</math>
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| where ''I'' is the current through the [[electrical conductor|conductor]] in units of [[amperes]], ''V'' is the potential difference measured across the conductor in units of [[volts]], and ''R'' is the resistance of the conductor in units of [[ohm]]s (symbol: Ω).
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| The ratio of the voltage applied across a resistor's terminals to the intensity of current in the circuit is called its resistance, and this can be assumed to be a constant (independent of the voltage) for ordinary resistors working within their ratings.
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| Resistors are common elements of [[electrical network]]s and [[electronic circuit]]s and are ubiquitous in [[Electronics|electronic equipment]]. Practical resistors can be composed of various compounds and films, as well as [[resistance wire|resistance wires]] (wire made of a high-resistivity alloy, such as nickel-chrome).
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| Resistors are also implemented within [[integrated circuits]], particularly analog devices, and can also be integrated into [[hybrid circuit|hybrid]] and [[Printed circuit board|printed circuit]]s.
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| The electrical functionality of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine [[orders of magnitude]].
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| When specifying that resistance in an electronic design, the required precision of the resistance may require attention to the [[Engineering tolerance#Electrical component tolerance|manufacturing tolerance]] of the chosen resistor, according to its specific application.
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| The [[temperature coefficient]] of the resistance may also be of concern in some precision applications.
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| Practical resistors are also specified as having a maximum [[Power (physics)|power]] rating which must exceed the anticipated power dissipation of that resistor in a particular circuit: this is mainly of concern in power electronics applications.
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| Resistors with higher power ratings are physically larger and may require [[heat sink]]s. In a high-voltage circuit, attention must sometimes be paid to the rated maximum working voltage of the resistor. While there is no minimum working voltage for a given resistor, failure to account for a resistor's maximum rating may cause the resistor to incinerate when current is run through it.
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| Practical resistors have a series [[inductance]] and a small parallel [[capacitance]]; these specifications can be important in high-frequency applications. In a [[low-noise amplifier]] or [[pre-amp]], the [[Noise (electronics)|noise]] characteristics of a resistor may be an issue. The unwanted inductance, excess noise, and temperature coefficient are mainly dependent on the technology used in manufacturing the resistor. They are not normally specified individually for a particular family of resistors manufactured using a particular technology.<ref>A family of resistors may also be characterized according to its ''critical resistance.'' Applying a constant voltage across resistors in that family below the critical resistance will exceed the maximum power rating first; resistances larger than the critical resistance will fail first from exceeding the maximum voltage rating. See {{cite book |first1=Wendy |last1=Middleton |first2=Mac E. |last2=Van Valkenburg |title=Reference data for engineers: radio, electronics, computer, and communications |edition=9 |publisher=Newnes |year=2002 |isbn=0-7506-7291-9 |pages=5–10}}</ref> A family of discrete resistors is also characterized according to its form factor, that is, the size of the device and the position of its leads (or terminals) which is relevant in the practical manufacturing of circuits using them.
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| ==Units==
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| The [[Ohm (unit)|ohm]] (symbol: [[Ω]]) is the [[SI]] unit of [[electrical resistance]], named after [[Georg Simon Ohm]]. An ohm is equivalent to a [[volt]] per [[ampere]]. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10<sup>−3</sup> Ω), kilohm (1 kΩ = 10<sup>3</sup> Ω), and megohm (1 MΩ = 10<sup>6</sup> Ω) are also in common usage.
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| The reciprocal of resistance R is called [[Electrical conductance|conductance]] G = 1/R and is measured in [[siemens (unit)|siemens]] ([[SI]] unit), sometimes referred to as a [[mho]]. Hence, siemens is the reciprocal of an ohm: <math>S = \Omega^{-1}</math>. Although the concept of conductance is often used in circuit analysis, practical resistors are always specified in terms of their resistance (ohms) rather than conductance.
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| ==Electronic symbols and notation==
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| {{Main|Electronic symbol}}
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| The symbol used for a resistor in a [[circuit diagram]] varies from standard to standard and country to country. Two typical symbols are as follows;
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| <gallery>
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| File:Resistor, Rheostat (variable resistor), and Potentiometer symbols.svg|American-style symbols. (a) resistor, (b) rheostat (variable resistor), and (c) potentiometer
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| File:Resistor_symbol_IEC.svg|[[International Electrotechnical Commission|IEC]]-style resistor symbol
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| </gallery>
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| The notation to state a resistor's value in a circuit diagram varies, too. The European notation avoids using a [[decimal separator]], and replaces the decimal separator with the SI prefix symbol for the particular value. For example, ''8k2'' in a circuit diagram indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example ''15M0''. When the value can be expressed without the need for an SI prefix, an 'R' is used instead of the decimal separator. For example, ''1R2'' indicates 1.2 Ω, and ''18R'' indicates 18 Ω. The use of a SI prefix symbol or the letter 'R' circumvents the problem that decimal separators tend to 'disappear' when [[photocopy]]ing a printed circuit diagram.
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| ==Theory of operation==
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| [[File:ResistanceHydraulicAnalogy2.svg|thumb|The [[hydraulic analogy]] compares electric current flowing through circuits to water flowing through pipes. When a pipe (left) is filled with hair (right), it takes a larger pressure to achieve the same flow of water. Pushing electric current through a large resistance is like pushing water through a pipe clogged with hair: It requires a larger push ([[voltage drop]]) to drive the same flow ([[electric current]]).]]
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| ===Ohm's law===
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| {{Main|Ohm's law}}
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| The behavior of an ideal resistor is dictated by the relationship specified by [[Ohm's law]]:
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| :<math>V=I \cdot R.</math>
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| Ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R).
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| Equivalently, Ohm's law can be stated:
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| :<math>I = \frac{V}{R}.</math>
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| This formulation states that the current (I) is proportional to the voltage (V) and inversely proportional to the resistance (R). This is directly used in practical computations. For example, if a 300 [[ohm]] resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 [[amperes]] (or 40 milliamperes) flows through that resistor.
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| ===Series and parallel resistors===
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| {{Main|Series and parallel circuits}}
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| {{anchor|series|Series}}<!-- do not delete, used by redirects -->
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| In a [[Series and parallel circuits|series]] configuration, the current through all of the resistors is the same, but the voltage across each resistor will be in proportion to its resistance. The potential difference (voltage) seen across the network is the sum of those voltages, thus the total resistance can be found as the sum of those resistances:
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| :[[File:resistors in series.svg|A diagram of several resistors, connected end to end, with the same amount of current going through each]]
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| :<math>
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| R_\mathrm{eq} = R_1 + R_2 + \cdots + R_n.
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| </math>
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| As a special case, the resistance of ''N'' resistors connected in series, each of the same resistance ''R,'' is given by ''NR.'' Thus, if a 100K ohm resistor and a 22K ohm resistor are connected in series, their combined resistance will be 122K ohm— they will function in a circuit as though they were a single resistor with a resistance value of 122K ohm; three 22K ohm resistors (''N''=3, ''R''=22K) will produce a resistance of 3x22K=66K ohms.
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| {{anchor|parallel|Parallel}}<!-- do not delete, used by redirects -->
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| Resistors in a [[Series and parallel circuits|parallel]] configuration are each subject to the same potential difference (voltage), however the currents through them add. The [[Electrical conductance|conductance]]s of the resistors then add to determine the conductance of the network. Thus the equivalent resistance (''R<sub>eq</sub>'') of the network can be computed:
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| :[[File:resistors in parallel.svg|A diagram of several resistors, side by side, both leads of each connected to the same wires]]
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| :<math>
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| \frac{1}{R_\mathrm{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + \cdots + \frac{1}{R_n}.
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| </math>
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| So, for example, a 10 ohm resistor connected in parallel with a 5 ohm resistor and a 15 ohm resistor will produce the inverse of 1/10+1/5+1/15 ohms of resistance, or 1/(.1+.2+.067)=2.725 ohms. The greater the number of resistors in parallel, the less overall resistance they will collectively generate, and the resistance will never be higher than that of the resistor with the lowest resistance in the group (in the case above, the resistor with the least resistance is the 5 ohm resistor, therefore the combined resistance of all resistors attached to it in parallel will never be greater than 5 ohms).
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| The parallel equivalent resistance can be represented in equations by two vertical lines "||" ([[Parallel (geometry)#Symbol|as in geometry]]) as a simplified notation. Occasionally two slashes "//" are used instead of "||", in case the keyboard or font lacks the vertical line symbol. For the case of two resistors in parallel, this can be calculated using:
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| :<math>
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| R_\mathrm{eq} = R_1 \| R_2 = {R_1 R_2 \over R_1 + R_2}.
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| </math>
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| A resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other. For instance,
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| :[[File:Resistors in series and parallel.svg|A diagram of three resistors, two in parallel, which are in series with the other]]
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| :<math>
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| R_\mathrm{eq} = \left( R_1 \| R_2 \right) + R_3 = {R_1 R_2 \over R_1 + R_2} + R_3.
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| </math>
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| However, some complex networks of resistors cannot be resolved in this manner, requiring more sophisticated circuit analysis. For instance, consider a [[cube]], each edge of which has been replaced by a resistor. What then is the resistance that would be measured between two opposite vertices? In the case of 12 equivalent resistors, it can be shown that the corner-to-corner resistance is <sup>5</sup>⁄<sub>6</sub> of the individual resistance.
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| More generally, the [[Y-Δ transform]], or [[Equivalent impedance transforms#2-terminal, n-element, 3-element-kind networks|matrix methods]] can be used to solve such a problem.<ref>Farago, PS, ''An Introduction to Linear Network Analysis'', pp. 18–21, The English Universities Press Ltd, 1961.</ref><ref>{{cite journal|doi=10.1088/0305-4470/37/26/004|title=Theory of resistor networks: The two-point resistance|year=2004|last1=Wu|first1=F Y|journal=Journal of Physics A: Mathematical and General|volume=37|issue=26|page=6653}}</ref><ref>{{cite book|author1=Fa Yueh Wu|author2=Chen Ning Yang|title=Exactly Solved Models: A Journey in Statistical Mechanics : Selected Papers with Commentaries (1963–2008)|url=http://books.google.com/books?id=H-k8dhB7lmwC&pg=PA489|accessdate=14 May 2012|date=15 March 2009|publisher=World Scientific|isbn=978-981-281-388-6|pages=489–}}</ref>
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| One practical application of these relationships is that a non-standard value of resistance can generally be synthesized by connecting a number of standard values in series or parallel. This can also be used to obtain a resistance with a higher power rating than that of the individual resistors used. In the special case of ''N'' identical resistors all connected in series or all connected in parallel, the power rating of the composite resistor is ''N'' times the power rating of the individual resistors.
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| ===Power dissipation===
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| At any instant of time, the power P consumed by a resistor of resistance ''R'' (ohms) is calculated as:
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| <math>
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| P =I^2 R = I V = \frac{V^2}{R}
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| </math>
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| where ''V'' (volts) is the voltage across the resistor and ''I'' (amps) is the [[Ampere|current]] flowing through it. The first form is a restatement of [[Joule's first law]]. Using Ohm's law, the two other forms can be derived. This power is converted into heat which must be dissipated by the resistor's package.
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| The total amount of heat energy released over a period of time can be determined from the integral of the power over that period of time:
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| :<math>
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| W = \int_{t_1}^{t_2} v(t) i(t)\, dt .
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| </math>
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| Therefore one could write the average power dissipated over that particular time period as:
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| :<math> \bar{P} = \frac{1}{t_2-t_1}\int_{t_1}^{t_2} v(t) i(t)\, dt .</math>
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| If the time interval ''t<sub>1</sub> - t<sub>2</sub>'' is chosen to be one complete cycle of a periodic waveform (or an integer number of cycles), then this result is equal to the long-term average power generated as heat which will be dissipated continuously. With a periodic waveform (such as, but not limited to, a [[sine wave]]), then this average over complete cycles (or over the long term) is conveniently given by
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| <math> \bar{P} = I_{rms} V_{rms} = I_{rms}^2 R = \frac{V_{rms}^2}{R} </math> where ''I<sub>rms</sub>'' and ''V<sub>rms</sub>'' are the [[root mean square]] values of the current and voltage. In any case, that heat generated in the resistor must be dissipated before its temperature rises excessively.
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| Resistors are rated according to their maximum power dissipation. Most discrete resistors in solid-state electronic systems absorb much less than a watt of electrical power and require no attention to their power rating. Such resistors in their discrete form, including most of the packages detailed below, are typically rated as 1/10, 1/8, or 1/4 watt.
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| [[File:Danotherm HS50 power resistor.jpg|thumb|An aluminium-housed power resistor rated for 50 W when heat-sinked]]
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| Resistors required to dissipate substantial amounts of power, particularly used in power supplies, power conversion circuits, and power amplifiers, are generally referred to as ''power resistors''; this designation is loosely applied to resistors with power ratings of 1 watt or greater. Power resistors are physically larger and may not use the preferred values, color codes, and external packages described below.
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| If the average power dissipated by a resistor is more than its power rating, damage to the resistor may occur, permanently altering its resistance; this is distinct from the reversible change in resistance due to its [[temperature coefficient]] when it warms. Excessive power dissipation may raise the temperature of the resistor to a point where it can burn the circuit board or adjacent components, or even cause a fire. There are flameproof resistors that fail (open circuit) before they overheat dangerously.
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| Since poor air circulation, high altitude, or high [[operating temperature]]s may occur, resistors may be specified with higher rated dissipation than will be experienced in service.
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| Some types and ratings of resistors may also have a maximum voltage rating; this may limit available power dissipation for higher resistance values.
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| ==Fixed resistor==
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| [[File:Sil resistor.png|thumb|A single in line (SIL) resistor package with 8 individual, 47 ohm resistors. One end of each resistor is connected to a separate pin and the other ends are all connected together to the remaining (common) pin – pin 1, at the end identified by the white dot.]]
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| ===Lead arrangements===
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| [[File:3 Resistors.jpg|thumb|right| Resistors with wire leads for through-hole mounting]]
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| [[Through-hole]] components typically have leads leaving the body axially. Others have leads coming off their body radially instead of parallel to the resistor axis. Other components may be [[Surface-mount technology|SMT]] (surface mount technology) while high power resistors may have one of their leads designed into the [[heat sink]].
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| ===Carbon composition===
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| [[File:Old Radio Resistors.jpg|thumb|Three carbon composition resistors in a 1960s [[Vacuum tube|valve]] (vacuum tube) radio]]
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| Carbon composition resistors consist of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached. The body of the resistor is protected with paint or plastic. Early 20th-century carbon composition resistors had uninsulated bodies; the lead wires were wrapped around the ends of the resistance element rod and soldered. The completed resistor was painted for color-coding of its value.
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| The resistive element is made from a mixture of finely ground (powdered) carbon and an insulating material (usually ceramic). A resin holds the mixture together. The resistance is determined by the ratio of the fill material (the powdered ceramic) to the carbon. Higher concentrations of carbon— a good conductor— result in lower resistance. Carbon composition resistors were commonly used in the 1960s and earlier, but are not so popular for general use now as other types have better specifications, such as tolerance, voltage dependence, and stress (carbon composition resistors will change value when stressed with over-voltages). Moreover, if internal moisture content (from exposure for some length of time to a humid environment) is significant, soldering heat will create a non-reversible change in resistance value. Carbon composition resistors have poor stability with time and were consequently factory sorted to, at best, only 5% tolerance.<ref>James H. Harter, Paul Y. Lin, ''Essentials of electric circuits'', pp. 96–97, Reston Publishing Company, 1982 ISBN 0-8359-1767-3.</ref>
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| These resistors, however, if never subjected to [[overvoltage]] nor overheating were remarkably reliable considering the component's size.<ref name=Vishay08>Vishay Beyschlag [http://www.element-14.com/community/docs/DOC-22086 ''Basics of Linear Fixed Resistors Application Note''], Document Number 28771, 2008.</ref>
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| Carbon composition resistors are still available, but comparatively quite costly. Values ranged from fractions of an ohm to 22 megohms. Due to their high price, these resistors are no longer used in most applications. However, they are used in power supplies and welding controls.<ref name=Vishay08/>
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| ===Carbon pile===
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| A carbon pile resistor is made of a stack of carbon disks compressed between two metal contact plates. Adjusting the clamping pressure changes the resistance between the plates. These resistors are used when an adjustable load is required, for example in testing automotive batteries or radio transmitters. A carbon pile resistor can also be used as a speed control for small motors in household appliances (sewing machines, hand-held mixers) with ratings up to a few hundred watts.<ref>C. G. Morris (ed) ''Academic Press Dictionary of Science and Technology'', Gulf Professional Publishing, 1992 ISBN 0122004000, page 360</ref> A carbon pile resistor can be incorporated in automatic [[voltage regulator]]s for generators, where the carbon pile controls the field current to maintain relatively constant voltage.<ref>''Principles of automotive vehicles'' United States. Dept. of the Army, 1985 page 13-13</ref> The principle is also applied in the [[carbon microphone]].
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| ===Carbon film===
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| [[File:Carbon-resistor-TR212-1.jpg|thumb|Partially exposed Tesla TR-212 1 kΩ carbon film resistor]]
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| A carbon film is deposited on an insulating substrate, and a [[helix]] is cut in it to create a long, narrow resistive path. Varying shapes, coupled with the [[resistivity]] of amorphous carbon (ranging from 500 to 800 μΩ m), can provide a variety of resistances. Compared to carbon composition they feature low noise, because of the precise distribution of the pure graphite without binding.<ref>{{cite web|title=Carbon Film Resistor|url=http://www.resistorguide.com/carbon-film-resistor/|work= The Resistorguide|accessdate=10 March 2013}}</ref> Carbon film resistors feature a power rating range of 0.125 W to 5 W at 70 °C. Resistances available range from 1 ohm to 10 megohm. The carbon film resistor has an [[operating temperature]] range of −55 °C to 155 °C. It has 200 to 600 volts maximum working voltage range. Special carbon film resistors are used in applications requiring high pulse stability.<ref name=Vishay08/>
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| ===Printed carbon resistor===
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| [[File:PCB Carbon Printed Resistor.jpg|thumb|A carbon resistor printed directly onto the SMD pads on a PCB. Inside a 1989 vintage Psion II Organiser]]
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| Carbon composition resistors can be printed directly onto printed circuit board (PCB) substrates as part of the PCB manufacturing process. Whilst this technique is more common on hybrid PCB modules, it can also be used on standard fibreglass PCBs. Tolerances are typically quite large, and can be in the order of 30%. A typical application would be non-critical [[Pull-up resistor|pull-up resistors]].
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| ===Thick and thin film===
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| Thick film resistors became popular during the 1970s, and most [[surface-mount technology|SMD]] (surface mount device) resistors today are of this type. The resistive element of thick films is 1000 times thicker than thin films,<ref name="Film Comparison">{{cite web|title=Thick Film and Thin Film|url=http://www.digikey.com/Web%20Export/Supplier%20Content/Stackpole_738/PDF/Stackpole_ThickFilmXThinFilm.pdf|publisher=Digi-Key (SEI)|accessdate=23 July 2011}}</ref> but the principal difference is how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors).
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| Thin film resistors are made by [[sputtering]] (a method of [[vacuum deposition]]) the resistive material onto an insulating substrate. The film is then etched in a similar manner to the old (subtractive) process for making printed circuit boards; that is, the surface is coated with a [[photoresist|photo-sensitive material]], then covered by a pattern film, irradiated with [[ultraviolet]] light, and then the exposed photo-sensitive coating is developed, and underlying thin film is etched away.
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| Thick film resistors are manufactured using screen and stencil printing processes.<ref name=Vishay08/>
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| Because the time during which the sputtering is performed can be controlled, the thickness of the thin film can be accurately controlled. The type of material is also usually different consisting of one or more ceramic ([[cermet]]) conductors such as [[tantalum nitride]] (TaN), [[Ruthenium(IV) oxide|ruthenium oxide]] ({{chem|RuO|2}}), [[Lead(II) oxide|lead oxide]] (PbO), [[bismuth ruthenate]] ({{chem|Bi|2|Ru|2|O|7}}), [[chromel|nickel chromium]] (NiCr), or [[bismuth iridate]] ({{chem|Bi|2|Ir|2|O|7}}).
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| The resistance of both thin and thick film resistors after manufacture is not highly accurate; they are usually trimmed to an accurate value by abrasive or [[laser trimming]]. Thin film resistors are usually specified with tolerances of 0.1, 0.2, 0.5, or 1%, and with temperature coefficients of 5 to 25 [[Temperature coefficient|ppm/K]]. They also have much lower [[Resistor noise|noise]] levels, on the level of 10–100 times less than thick film resistors.{{Citation needed|date=December 2012}}
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| Thick film resistors may use the same conductive ceramics, but they are mixed with [[sintered]] (powdered) glass and a carrier liquid so that the composite can be [[screen-printing|screen-printed]]. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C.
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| Thick film resistors, when first manufactured, had tolerances of 5%, but standard tolerances have improved to 2% or 1% in the last few decades. Temperature coefficients of thick film resistors are high, typically ±200 or ±250 ppm/K; a 40 [[kelvin]] (70 °F) temperature change can change the resistance by 1%.
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| Thin film resistors are usually far more expensive than thick film resistors. For example, SMD thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when bought in full size reel quantities, are about twice the cost of 1%, 250 ppm/K thick film resistors.
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| ===Metal film===
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| A common type of axial resistor today is referred to as a metal-film resistor. Metal electrode leadless face ([[MELF Electronic Components|MELF]]) resistors often use the same technology, but are a cylindrically shaped resistor designed for surface mounting. Note that other types of resistors (e.g., carbon composition) are also available in MELF packages.
| |
| | |
| Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the material may be applied using different techniques than sputtering (though this is one of the techniques). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix through the coating rather than by etching. (This is similar to the way carbon resistors are made.) The result is a reasonable tolerance (0.5%, 1%, or 2%) and a temperature coefficient that is generally between 50 and 100 ppm/K.<ref>{{cite web
| |
| | url = http://www.kennethkuhn.com/students/ee431/text/ee431lab3.pdf
| |
| | title = Measuring the Temperature Coefficient of a Resistor
| |
| | accessdate = 2010-03-18
| |
| | author = Kenneth A. Kuhn}}</ref> Metal film resistors possess good noise characteristics and low non-linearity due to a low voltage coefficient. Also beneficial are the components efficient tolerance, temperature coefficient and stability.<ref name=Vishay08/>
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| | |
| ===Metal oxide film===
| |
| Metal-oxide film resistors are made of metal oxides such as tin oxide. This results in a higher operating temperature and greater stability/reliability than Metal film. They are used in applications with high endurance demands.
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| | |
| ===Wirewound===
| |
| [[File:KTSU resistors.jpg|thumb|right|High-power wire wound resistors used for dynamic braking on an electric railway car. Such resistors may dissipate many kilowatts for extended times.]]
| |
| [[File:Types of winding by Zureks.png|thumb|Types of windings in wire resistors:<br />1. common<br />2. [[bifilar winding|bifilar]]<br />3. common on a thin former<br />4. [[Ayrton-Perry winding|Ayrton-Perry]]]]
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| | |
| Wirewound resistors are commonly made by winding a metal wire, usually [[nichrome]], around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an [[Vitreous enamel|enamel]] coating baked at high temperature. These resistors are designed to withstand unusually high temperatures of up to +450 °C.<ref name=Vishay08/> Wire leads in low power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used-- if the outer case is ceramic, such resistors are sometimes described as "cement" resistors, though they do not actually contain any traditional [[Portland cement|cement]]. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power dissipation if not used with a heat sink. Large wirewound resistors may be rated for 1,000 watts or more.
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| | |
| Because wirewound resistors are [[Electromagnetic coil|coils]] they have more undesirable [[Electromagnetic induction|inductance]] than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance. Other techniques employ [[bifilar winding]], or a flat thin former (to reduce cross-section area of the coil). For the most demanding circuits, resistors with [[Ayrton-Perry winding]] are used.
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| | |
| Applications of wirewound resistors are similar to those of composition resistors with the exception of the high frequency. The high frequency response of wirewound resistors is substantially worse than that of a composition resistor.<ref name=Vishay08/>
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| | |
| ===Foil resistor===
| |
| The primary resistance element of a foil resistor is a special alloy foil several [[micrometre|micrometers]] thick. Since their introduction in the 1960s, foil resistors have had the best precision and stability of any resistor available. One of the important parameters influencing stability is the temperature coefficient of resistance (TCR). The TCR of foil resistors is extremely low, and has been further improved over the years.
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| One range of ultra-precision foil resistors offers a TCR of 0.14 ppm/°C, tolerance ±0.005%, long-term stability (1 year) 25 ppm, (3 year) 50 ppm (further improved 5-fold by hermetic sealing), stability under load (2000 hours) 0.03%, thermal EMF 0.1 μV/°C, noise −42 dB, voltage coefficient 0.1 ppm/V, inductance 0.08 μH, capacitance 0.5 pF.<ref>{{cite web|url=http://www.alpha-elec.co.jp/e_machine.html |title=Alpha Electronics Corp.【Metal Foil Resistors】 |publisher=Alpha-elec.co.jp |date= |accessdate=2008-09-22}}</ref>
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| | |
| ===Ammeter shunts===
| |
| An [[Shunt (electrical)#Use in current measuring|ammeter shunt]] is a special type of current-sensing resistor, having four terminals and a value in milliohms or even micro-ohms. Current-measuring instruments, by themselves, can usually accept only limited currents. To measure high currents, the current passes through the shunt, where the voltage drop is measured and interpreted as current. A typical shunt consists of two solid metal blocks, sometimes brass, mounted on to an insulating base. Between the blocks, and soldered or brazed to them, are one or more strips of low [[Temperature coefficient of resistivity|temperature coefficient of resistance]] (TCR) [[manganin]] alloy. Large bolts threaded into the blocks make the current connections, while much smaller screws provide voltage connections. Shunts are rated by full-scale current, and often have a voltage drop of 50 mV at rated current. Such meters are adapted to the shunt full current rating by using an appropriately marked dial face; no change need be made to the other parts of the meter.
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| | |
| ===Grid resistor===
| |
| In heavy-duty industrial high-current applications, a grid resistor is a large convection-cooled lattice of stamped metal alloy strips connected in rows between two electrodes. Such industrial grade resistors can be as large as a refrigerator; some designs can handle over 500 amperes of current, with a range of resistances extending lower than 0.04 ohms. They are used in applications such as [[dynamic braking]] and [[Load bank#Resistive load bank|load banking]] for [[Diesel locomotive|locomotives]] and trams, neutral grounding for industrial AC distribution, control loads for cranes and heavy equipment, load testing of generators and harmonic filtering for electric substations.<ref>[http://www.milwaukeeresistor.com/gridresistors.html Milwaukee Resistor Corporation. ''Grid Resistors: High Power/High Current'']. Milwaukeeresistor.com. Retrieved on 2012-05-14.</ref><ref>[http://www.avtron.com/grid_resistors.htm Avtron Loadbank. ''Grid Resistors'']. Avtron.com. Retrieved on 2012-05-14.</ref>
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| | |
| The term ''grid resistor'' is sometimes used to describe a resistor of any type connected to the [[control grid]] of a [[vacuum tube]]. This is not a resistor technology; it is an electronic circuit topology.
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| | |
| ===Special varieties===
| |
| *[[Metal oxide varistor]]
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| *[[Cermet]]
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| *[[Phenolic resin|Phenolic]]
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| *[[Tantalum]]
| |
| *[[Water resistor]]
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| | |
| ==Variable resistors==
| |
| ===Adjustable resistors===
| |
| A resistor may have one or more fixed tapping points so that the resistance can be changed by moving the connecting wires to different terminals. Some wirewound power resistors have a tapping point that can slide along the resistance element, allowing a larger or smaller part of the resistance to be used.
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| | |
| Where continuous adjustment of the resistance value during operation of equipment is required, the sliding resistance tap can be connected to a knob accessible to an operator. Such a device is called a [[rheostat]] and has two terminals.
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| | |
| ===Potentiometers===
| |
| {{Main|Potentiometer}}
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| | |
| A common element in electronic devices is a three-terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob. These variable resistors are known as [[potentiometer]]s when all three terminals are present, since they act as a continuously adjustable [[voltage divider]]. A common example is a volume control for a radio receiver.<ref>Digitally controlled receivers may not have an analog volume control and use other methods to adjust volume.</ref>
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| | |
| Accurate, high-resolution panel-mounted potentiometers (or "pots") have resistance elements typically wirewound on a helical mandrel, although some include a conductive-plastic resistance coating over the wire to improve resolution. These typically offer ten turns of their shafts to cover their full range. They are usually set with dials that include a simple turns counter and a graduated dial. Electronic analog computers used them in quantity for setting coefficients, and delayed-sweep oscilloscopes of recent decades included one on their panels.
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| | |
| ===Resistance decade boxes===
| |
| [[File:Odporová dekáda KURBELWIDERSTAND (A).jpg|thumb|Resistance decade box "KURBELWIDERSTAND", made in former [[East Germany]].]]
| |
| A resistance decade box or resistor substitution box is a unit containing resistors of many values, with one or more mechanical switches which allow any one of various discrete resistances offered by the box to be dialed in. Usually the resistance is accurate to high precision, ranging from laboratory/calibration grade accuracy of 20 parts per million, to field grade at 1%. Inexpensive boxes with lesser accuracy are also available. All types offer a convenient way of selecting and quickly changing a resistance in laboratory, experimental and development work without needing to attach resistors one by one, or even stock each value. The range of resistance provided, the maximum resolution, and the accuracy characterize the box. For example, one box offers resistances from 0 to 24 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.<ref>{{cite web|url=http://www.ietlabs.com/decaderes.html |title=Decade Box – Resistance Decade Boxes |publisher=Ietlabs.com |date= |accessdate=2008-09-22}}</ref>
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| | |
| ===Special devices===
| |
| There are various devices whose resistance changes with various quantities. The resistance of NTC [[thermistor]]s exhibit a strong negative temperature coefficient, making them useful for measuring temperatures. Since their resistance can be large until they are allowed to heat up due to the passage of current, they are also commonly used to prevent excessive [[Inrush current|current surges]] when equipment is powered on. Similarly, the resistance of a [[humistor]] varies with humidity. [[varistor|Metal oxide varistors]] drop to a very low resistance when a high voltage is applied, making them useful for protecting electronic equipment by absorbing dangerous [[voltage surge]]s. One sort of photodetector, the [[photoresistor]], has a resistance which varies with illumination.
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| | |
| The [[strain gauge]], invented by [[Edward E. Simmons]] and [[Arthur C. Ruge]] in 1938, is a type of resistor that changes value with applied strain. A single resistor may be used, or a pair (half bridge), or four resistors connected in a [[Wheatstone bridge]] configuration. The strain resistor is bonded with adhesive to an object that will be subjected to [[Infinitesimal strain theory|mechanical strain]]. With the strain gauge and a filter, amplifier, and analog/digital converter, the strain on an object can be measured.
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| | |
| A related but more recent invention uses a [[Quantum Tunnelling Composite]] to sense mechanical stress. It passes a current whose magnitude can vary by a factor of 10<sup>12</sup> in response to changes in applied pressure.
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| | |
| ==Measurement==
| |
| The value of a resistor can be measured with an [[ohmmeter]], which may be one function of a [[multimeter]]. Usually, probes on the ends of test leads connect to the resistor. A simple ohmmeter may apply a voltage from a battery across the unknown resistor (with an internal resistor of a known value in series) producing a current which drives a [[Galvanometer|meter movement]]. The current, in accordance with [[Ohm's Law]], is inversely proportional to the sum of the internal resistance and the resistor being tested, resulting in an analog meter scale which is very non-linear, calibrated from infinity to 0 ohms. A digital multimeter, using active electronics, may instead pass a specified current through the test resistance. The voltage generated across the test resistance in that case is linearly proportional to its resistance, which is measured and displayed. In either case the low-resistance ranges of the meter pass much more current through the test leads than do high-resistance ranges, in order for the voltages present to be at reasonable levels (generally below 10 volts) but still measurable.
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| | |
| Measuring low-value resistors, such as fractional-ohm resistors, with acceptable accuracy requires [[Four-terminal sensing|four-terminal connections]]. One pair of terminals applies a known, calibrated current to the resistor, while the other pair senses the voltage drop across the resistor. Some laboratory quality ohmmeters, especially milliohmmeters, and even some of the better digital multimeters sense using four input terminals for this purpose, which may be used with special test leads. Each of the two so-called [[Four-terminal sensing|Kelvin clips]] has a pair of jaws insulated from each other. One side of each clip applies the measuring current, while the other connections are only to sense the voltage drop. The resistance is again calculated using Ohm's Law as the measured voltage divided by the applied current.
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| | |
| ==Color code==
| |
| The axial lead carbon resistors measured by the color codes marked on them. Information such as resistance value, tolerance, temperature co-efficient measured by the color codes, and the amount of power (wattage) identified by the size.
| |
| | |
| The color bands of the carbon resistors can be four, five or, six bands, for all the first two bands represent first two digits to measure their value in ohms. The third band of a four-banded resistor represents multiplier and the fourth band as tolerance. Whereas, the five and six color-banded resistors, the third band rather represents as third digit but the fourth and fifth bands represent as multiplier and tolerance respectively. Only the sixth band represents temperature co-efficient in a six-banded resistor.
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| | |
| The measuring digits against color codes given in the following table. The sentence within the quotation marks may be of great help to memorize these color codes in order, "''Big Ben Realized Only Yesterday; Girls, Boys, Very Good Work Saves God''".
| |
| | |
| {|
| |
| |-
| |
| ! 1st. Two Digits- !! Multiplier- !! Tolerance- !! Temp. Co-eff.
| |
| |-
| |
| | Black 0 || Black 1 || Not Used || Not Used
| |
| |-
| |
| | Brown 1 || Brown 10 || Brown 1% || Brown 100
| |
| |-
| |
| | Red 2 || Red 100 || Red +2% || Red 50
| |
| |-
| |
| | Orange 3 || Orange 1K || Not Used || Orange 15
| |
| |-
| |
| | Yellow 4 || Yellow 10K || Not Used || Yellow 25
| |
| |-
| |
| | Green 5 || Green 100K || Not Used || Green 0.5
| |
| |-
| |
| | Blue 6 || Blue 1M || Not Used || Blue 0.25
| |
| |-
| |
| | Violet 7 || Violet 10M || Not Used || Violet 0.1
| |
| |-
| |
| | Grey 8 || Not Used || Not Used || Not Used
| |
| |-
| |
| | White 9 || Not Used || Not Used || Not Used
| |
| |-
| |
| | - || Silver 0.01 || Silver+10% || Not Used
| |
| |-
| |
| | - || Gold 0.1 || Gold +5% || Not Used
| |
| |}
| |
| | |
| E.g. the value of a four band Carbon resistor having color bands Red, Red, Red, Silver will have value 22*100=2200 ohms with 10% tolerance.
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| | |
| ==Standards==
| |
| ===Production resistors===
| |
| Resistor characteristics are quantified and reported using various national standards. In the US, MIL-STD-202<ref>{{cite web|url=http://www.dscc.dla.mil/Downloads/MilSpec/Docs/MIL-STD-202/std202.pdf |title=Test method standard: electronic and electrical component parts |publisher=Department of Defense}}</ref> contains the relevant test methods to which other standards refer.
| |
| | |
| There are various standards specifying properties of resistors for use in equipment:
| |
| *[[BS 1852]]
| |
| *EIA-RS-279
| |
| *MIL-PRF-26
| |
| *MIL-PRF-39007 (Fixed Power, established reliability)
| |
| *MIL-PRF-55342 (Surface-mount thick and thin film)
| |
| *MIL-PRF-914
| |
| *MIL-R-11 [http://www.landandmaritime.dla.mil/Programs/MilSpec/ListDocs.aspx?BasicDoc=MIL-R-11 STANDARD CANCELED]
| |
| *MIL-R-39017 (Fixed, General Purpose, Established Reliability)
| |
| *MIL-PRF-32159 (zero ohm jumpers)
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| | |
| There are other United States military procurement MIL-R- standards.
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| | |
| ===Resistance standards===
| |
| The [[primary standard]] for resistance, the "mercury ohm" was initially defined in 1884 in as a column of mercury 106.3 cm long and {{nowrap|1 square millimeter}} in cross-section, at {{nowrap|0 degrees Celsius}}. Difficulties in precisely measuring the physical constants to replicate this standard result in variations of as much as 30 ppm. From 1900 the mercury ohm was replaced with a precision machined plate of [[manganin]].<ref>[http://nvl.nist.gov/pub/nistpubs/sp958-lide/063-065.pdf Stability of Double-Walled Manganin Resistors]. NIST.gov</ref> Since 1990 the international resistance standard has been based on the [[quantum Hall effect|quantized Hall effect]] discovered by [[Von Klitzing|Klaus von Klitzing]], for which he won the Nobel Prize in Physics in 1985.<ref>Klaus von Klitzing [http://nobelprize.org/nobel_prizes/physics/laureates/1985/klitzing-lecture.pdf The Quantized Hall Effect]. Nobel lecture, December 9, 1985. nobelprize.org</ref>
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| | |
| Resistors of extremely high precision are manufactured for [[calibration]] and [[laboratory]] use. They may have four terminals, using one pair to carry an operating current and the other pair to measure the voltage drop; this eliminates errors caused by voltage drops across the lead resistances, because no charge flows through voltage sensing leads. It is important in small value resistors (100–0.0001 ohm) where lead resistance is significant or even comparable with respect to resistance standard value.<ref>{{cite web|url=http://www.tinsley.co.uk/products/standard-resistors/4737b.htm |title=Standard Resistance Unit Type 4737B |publisher=Tinsley.co.uk |date= |accessdate=2008-09-22}}</ref>
| |
| | |
| ==Resistor marking==
| |
| {{Main|Electronic color code}}
| |
| | |
| Most axial resistors use a pattern of colored stripes to indicate resistance. [[Surface-mount]] resistors are marked numerically, if they are big enough to permit marking; more-recent small sizes are impractical to mark. Cases are usually tan, brown, blue, or green, though other colors are occasionally found such as dark red or dark gray.
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| | |
| Early 20th century resistors, essentially uninsulated, were dipped in paint to cover their entire body for color-coding. A second color of paint was applied to one end of the element, and a color dot (or band) in the middle provided the third digit. The rule was "body, tip, dot", providing two significant digits for value and the decimal multiplier, in that sequence. Default tolerance was ±20%. Closer-tolerance resistors had silver (±10%) or gold-colored (±5%) paint on the other end.
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| | |
| ===Preferred values===
| |
| {{Main|Preferred number}}
| |
| | |
| Early resistors were made in more or less arbitrary round numbers; a series might have 100, 125, 150, 200, 300, etc. Resistors as manufactured are subject to a certain percentage [[Engineering tolerance|tolerance]], and it makes sense to manufacture values that correlate with the tolerance, so that the actual value of a resistor overlaps slightly with its neighbors. Wider spacing leaves gaps; narrower spacing increases manufacturing and inventory costs to provide resistors that are more or less interchangeable.
| |
| | |
| A logical scheme is to produce resistors in a range of values which increase in a [[geometrical progression]], so that each value is greater than its predecessor by a fixed multiplier or percentage, chosen to match the tolerance of the range. For example, for a tolerance of ±20% it makes sense to have each resistor about 1.5 times its predecessor, covering a decade in 6 values. In practice the factor used is 1.4678, giving values of 1.47, 2.15, 3.16, 4.64, 6.81, 10 for the 1–10 decade (a decade is a range increasing by a factor of 10; 0.1–1 and 10–100 are other examples); these are rounded in practice to 1.5, 2.2, 3.3, 4.7, 6.8, 10; followed, by 15, 22, 33, … and preceded by … 0.47, 0.68, 1. This scheme has been adopted as the [[E48 series|'''E6''' series]] of the [[International Electrotechnical Commission|IEC]] 60063 [[preferred number]] values. There are also '''E12''', '''E24''', '''E48''', '''E96''' and '''E192''' series for components of ever tighter tolerance, with 12, 24, 96, and 192 different values within each decade. The actual values used are in the [[International Electrotechnical Commission|IEC]] 60063 lists of preferred numbers.
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| | |
| A resistor of 100 ohms ±20% would be expected to have a value between 80 and 120 ohms; its E6 neighbors are 68 (54–82) and 150 (120–180) ohms. A sensible spacing, E6 is used for ±20% components; E12 for ±10%; E24 for ±5%; E48 for ±2%, E96 for ±1%; E192 for ±0.5% or better. Resistors are manufactured in values from a few milliohms to about a gigaohm in IEC60063 ranges appropriate for their tolerance. Manufacturers may sort resistors into tolerance-classes based on measurement. Accordingly a selection of 100 ohms resistors with a tolerance of ±10%, may not lay just around 100 ohm (but no more than 10% off) as one would expect (a bell-curve), but rather be in two groups – either between 5 to 10% too high or 5 to 10% too low (but non closer to 100 ohm than that). Any resistors the factory measured as being less than 5% off, would have been marked and sold as resistors with only ±5% tolerance or better. When designing a circuit, this may become a consideration.
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| | |
| Earlier power wirewound resistors, such as brown vitreous-enameled types, however, were made with a different system of preferred values, such as some of those mentioned in the first sentence of this section.
| |
| | |
| ===Five-band axial resistors===
| |
| Five-band identification is used for higher [[Accuracy and precision|precision]] (lower tolerance) resistors (1%, 0.5%, 0.25%, 0.1%), to specify a third significant digit. The first three bands represent the significant digits, the fourth is the multiplier, and the fifth is the tolerance. Five-band resistors with a gold or silver 4th band are sometimes encountered, generally on older or specialized resistors. The 4th band is the tolerance and the 5th the temperature coefficient.
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| | |
| ===SMT resistors===
| |
| [[File:Zero ohm resistors cropped.jpg|thumb|This image shows four surface-mount resistors (the component at the upper left is a [[capacitor]]) including two [[zero-ohm resistor]]s. Zero-ohm links are often used instead of wire links, so that they can be inserted by a resistor-inserting machine. Their resistance is non-zero but negligible. ]]
| |
| | |
| [[Surface-mount technology|Surface mounted]] resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance [[surface-mount technology|surface-mount technology (SMT)]] resistors are marked with a three-digit code, in which the first two digits are the first two [[significant digit]]s of the value and the third digit is the power of ten (the number of zeroes). For example:
| |
| | |
| {|
| |
| |-
| |
| |''334''||= 33 × 10<sup>4</sup> ohms = 330 kilohms
| |
| |-
| |
| |''222''||= 22 × 10<sup>2</sup> ohms = 2.2 kilohms
| |
| |-
| |
| |''473''||= 47 × 10<sup>3</sup> ohms = 47 kilohms
| |
| |-
| |
| |''105''||= 10 × 10<sup>5</sup> ohms = 1 megohm
| |
| |}
| |
| | |
| Resistances less than 100 ohms are written: 100, 220, 470. The final zero represents ten to the power zero, which is 1. For example:
| |
| | |
| {|
| |
| |-
| |
| |''100''||= 10 × 10<sup>0</sup> ohm = 10 ohms
| |
| |-
| |
| |''220''||= 22 × 10<sup>0</sup> ohm = 22 ohms
| |
| |}
| |
| | |
| Sometimes these values are marked as ''10'' or ''22'' to prevent a mistake.
| |
| | |
| Resistances less than 10 ohms have 'R' to indicate the position of the decimal point ([[radix point]]). For example:
| |
| | |
| {|
| |
| |-
| |
| |''4R7''||= 4.7 ohms
| |
| |-
| |
| |''R300''||= 0.30 ohms
| |
| |-
| |
| |''0R22''||= 0.22 ohms
| |
| |-
| |
| |''0R01''||= 0.01 ohms
| |
| |}
| |
| | |
| Precision resistors are marked with a four-digit code, in which the first three digits are the significant figures and the fourth is the power of ten. For example:
| |
| | |
| {|
| |
| |-
| |
| |''1001''||= 100 × 10<sup>1</sup> ohms = 1.00 kilohm
| |
| |-
| |
| |''4992''||= 499 × 10<sup>2</sup> ohms = 49.9 kilohm
| |
| |-
| |
| |''1000''||= 100 × 10<sup>0</sup> ohm = 100 ohms
| |
| |}
| |
| | |
| ''000'' and ''0000'' sometimes appear as values on surface-mount [[zero-ohm link]]s, since these have (approximately) zero resistance.
| |
| | |
| More recent surface-mount resistors are too small, physically, to permit practical markings to be applied.
| |
| | |
| ===Industrial type designation===
| |
| '''Format:'''''<nowiki> [two letters]<space>[resistance value (three digit)]<nospace>[tolerance code(numerical – one digit)]
| |
| </nowiki>''<ref>''Electronics and Communications Simplified'' by A. K. Maini, 9thEd., Khanna Publications (India)</ref>
| |
| <br />
| |
| {| class="wikitable" style="float: left; margin-right: 2em;"
| |
| |+Power Rating at 70 °C
| |
| !Type No.
| |
| !Power<br />rating<br />(watts)
| |
| ![[MIL-R-11]]<br />Style
| |
| ![[MIL-R-39008]]<br />Style
| |
| |- style="text-align: center;"
| |
| |BB||{{frac|1|8}}||RC05||RCR05
| |
| |- style="text-align: center;"
| |
| |CB||{{frac|1|4}}||RC07||RCR07
| |
| |- style="text-align: center;"
| |
| |EB||{{frac|1|2}}||RC20||RCR20
| |
| |- style="text-align: center;"
| |
| |GB||1||RC32||RCR32
| |
| |- style="text-align: center;"
| |
| |HB||2||RC42||RCR42
| |
| |- style="text-align: center;"
| |
| |GM||3||-||-
| |
| |- style="text-align: center;"
| |
| |HM||4||-||-
| |
| |}
| |
| {| class="wikitable" style="float: left;"
| |
| |+Tolerance Code
| |
| |- style="text-align: center;"
| |
| !style="width: 75px;"|Industrial type designation
| |
| !style="width: 50px;"|Tolerance
| |
| !style="width: 75px;"|MIL Designation
| |
| |- style="text-align: center;"
| |
| |5||±5%||J
| |
| |- style="text-align: center;"
| |
| |2||±20%||M
| |
| |- style="text-align: center;"
| |
| |1||±10%||K
| |
| |- style="text-align: center;"
| |
| | -||±2%||G
| |
| |- style="text-align: center;"
| |
| | -||±1%||F
| |
| |- style="text-align: center;"
| |
| | -||±0.5%||D
| |
| |- style="text-align: center;"
| |
| | -||±0.25%||C
| |
| |- style="text-align: center;"
| |
| | -||±0.1%||B
| |
| |}
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| {{-}}
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| ==Electrical and thermal noise==
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| In amplifying faint signals, it is often necessary to minimize [[electronic noise]], particularly in the first stage of amplification. As a dissipative element, even an ideal resistor will naturally produce a randomly fluctuating voltage or "noise" across its terminals. This [[Johnson–Nyquist noise]] is a fundamental noise source which depends only upon the temperature and resistance of the resistor, and is predicted by the [[fluctuation–dissipation theorem]]. Using a larger resistor produces a larger voltage noise, whereas with a smaller value of resistance there will be more current noise, assuming a given temperature. The thermal noise of a practical resistor may also be somewhat larger than the theoretical prediction and that increase is typically frequency-dependent.
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| However the "excess noise" of a practical resistor is an additional source of noise observed only when a charge flows through it. This is specified in unit of μV/V/decade – μV of noise per volt applied across the resistor per decade of frequency. The μV/V/decade value is frequently given in dB so that a resistor with a noise index of 0 dB will exhibit 1 μV (rms) of excess noise for each volt across the resistor in each frequency decade. Excess noise is thus an example of [[Flicker noise|1/''f'' noise]]. Thick-film and carbon composition resistors generate more excess noise than other types at low frequencies; wire-wound and thin-film resistors, though much more expensive, are often utilized for their better noise characteristics. Carbon composition resistors can exhibit a noise index of 0 dB while bulk metal foil resistors may have a noise index of −40 dB, usually making the excess noise of metal foil resistors insignificant.<ref>{{cite book
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| | title = Audio Noise Reduction Through the Use of Bulk Metal Foil Resistors – "Hear the Difference"
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| | url = http://www.c-c-i.com/sites/default/files/vse-an00.pdf
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| }}, Application note AN0003, Vishay Intertechnology Inc, 12 July 2005.</ref> Thin film surface mount resistors typically have lower noise and better thermal stability than thick film surface mount resistors. Excess noise is also size-dependent: in general excess noise is reduced as the physical size of a resistor is increased (or multiple resistors are used in parallel), as the independently fluctuating resistances of smaller components will tend to average out.
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| While not an example of "noise" per se, a resistor may act as a [[thermocouple]], producing a small DC voltage differential across it due to the [[thermoelectric effect]] if its ends are at somewhat different temperatures. This induced DC voltage can degrade the precision of [[instrumentation amplifier]]s in particular. Such voltages appear in the junctions of the resistor leads with the circuit board and with the resistor body. Common metal film resistors show such an effect at a magnitude of about 20 µV/°C. Some carbon composition resistors can exhibit thermoelectric offsets as high as 400 µV/°C, whereas specially constructed resistors can reduce this number to 0.05 µV/°C. In applications where the thermoelectric effect may become important, care has to be taken (for example) to mount the resistors horizontally to avoid temperature gradients and to mind the air flow over the board.<ref>{{cite book
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| | title = Op Amp Applications Handbook
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| | author = Walt Jung
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| | url = http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
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| | chapter = Chapter 7 – Hardware and Housekeeping Techniques
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| | chapterurl = http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch7_final_J.pdf
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| | page = 7.11
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| | isbn = 0-7506-7844-5
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| }}</ref>
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| ==Failure modes==
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| The failure rate of resistors in a properly designed circuit is low compared to other electronic components such as semiconductors and electrolytic capacitors. Damage to resistors most often occurs due to overheating when the average power delivered to it (as computed above) greatly exceeds its ability to dissipate heat (specified by the resistor's ''power rating''). This may be due to a fault external to the circuit, but is frequently caused by the failure of another component (such as a transistor that shorts out) in the circuit connected to the resistor. Operating a resistor too close to its power rating can limit the resistor's lifespan or cause a change in its resistance over time which may or may not be noticeable. A safe design generally uses overrated resistors in power applications to avoid this danger.
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| Low-power thin-film resistors can be damaged by long-term high-voltage stress, even below maximum specified voltage and below maximum power rating. This is often the case for the startup resistors feeding the SMPS integrated circuit.{{Citation needed|date=July 2011}}
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| When overheated, carbon-film resistors may decrease or increase in resistance.<ref>{{cite web
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| | title = Electronic components – resistors
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| | work = Inspector's Technical Guide
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| | publisher = US Food and Drug Administration
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| | date = 1978-01-16
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| | url = http://www.fda.gov/ora/Inspect_ref/itg/itg31.html
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| | accessdate = 2008-06-11 |archiveurl = http://web.archive.org/web/20080403111045/http://www.fda.gov/ora/Inspect_ref/itg/itg31.html <!-- Bot retrieved archive --> |archivedate = 2008-04-03}}</ref>
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| Carbon film and composition resistors can fail (open circuit) if running close to their maximum dissipation. This is also possible but less likely with metal film and wirewound resistors.
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| There can also be failure of resistors due to mechanical stress and adverse environmental factors including humidity. If not enclosed, wirewound resistors can corrode.
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| Surface mount resistors have been known to fail due to the ingress of sulfur into the internal makeup of the resistor. This sulfur chemically reacts with the silver layer to produce non-conductive silver sulfide. The resistor's impedance goes to infinity. Sulfur resistant and anti-corrosive resistors are sold into automotive, industrial, and military applications. ASTM B809 is an industry standard that tests a part's susceptibility to sulfur.
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| Variable resistors degrade in a different manner, typically involving poor contact between the wiper and the body of the resistance. This may be due to dirt or corrosion and is typically perceived as "crackling" as the contact resistance fluctuates; this is especially noticed as the device is adjusted. This is similar to crackling caused by poor contact in switches, and like switches, potentiometers are to some extent self-cleaning: running the wiper across the resistance may improve the contact. Potentiometers which are seldom adjusted, especially in dirty or harsh environments, are most likely to develop this problem. When self-cleaning of the contact is insufficient, improvement can usually be obtained through the use of contact cleaner (also known as "tuner cleaner") spray. The crackling noise associated with turning the shaft of a dirty potentiometer in an audio circuit (such as the volume control) is greatly accentuated when an undesired DC voltage is present, often indicating the failure of a DC blocking capacitor in the circuit.
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| ==See also==
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| {{Portal|Electronics}}
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| *[[Circuit design]]
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| *[[Dummy load]]
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| *[[Electrical impedance]]
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| *[[Iron-hydrogen resistor]]
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| *[[Shot noise]]
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| *[[Trimmer (electronics)]]
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| ==References==
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| {{reflist|35em}}
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| ==External links==
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| {{wikibooks
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| | 1 = Electronics
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| | 2 = Resistors
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| | 3 = Resistors
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| }}
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| {{Wiktionary}}
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| {{Commons category|Resistors}}
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| *[http://www.powerstandards.com/4terminal.htm 4-terminal resistors – How ultra-precise resistors work]
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| *[http://sound.westhost.com/pots.htm Beginner's guide to potentiometers, including description of different tapers]
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| *[http://web.archive.org/web/20110401175312/http://www.ese.upenn.edu/rca/calcjs.html Color Coded Resistance Calculator - archived with WayBack Machine]
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| *[http://www.aikenamps.com/ResistorNoise.htm Resistor Types – Does It Matter?]
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| * [http://www.divilabs.com/2013/08/standard-resistors-capicitors-value.html Standard Resistors & Capacitor Values That Industry Manufactures]
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| *[http://www.analog.com/library/analogDialogue/archives/31-1/Ask_Engineer.html Ask The Applications Engineer – Difference between types of resistors]
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| *[http://www.ipass.net/teara/resistor-frm.html Resistors and their uses]
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| *[http://www.thickfilmtech.com Thick film resistors and heaters]
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| {{Electronic component}}
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| [[Category:Electrical components]]
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| [[Category:Resistive components]]
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| {{Link FA|sl}}
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