A resistor is a passivetwo-terminalelectronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for . Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.
The electrical function of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. The nominal value of the resistance falls within the manufacturing tolerance, indicated on the component.
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There are also E12, E24, E48, E96 and E192 series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade.
There are also E12, E24, E48, E96 and E192 series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade.
There are also E12, E24, E48, E96 and E192 series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade.
There are also E12, E24, E48, E96 and E192 series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade.
There are also E12, E24, E48, E96 and E192 series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade.
Terms the article defines in bold, quoted in context.
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Electric heating
Electrical components
Resistive components
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The notation to state a resistor's value in a circuit diagram varies.
One common scheme is the RKM code following IEC 60062. Rather than using a decimal separator, this notation uses a letter loosely associated with SI prefixes corresponding with the part's resistance. For example, 8K2 as part marking code, in a circuit diagram or in a bill of materials (BOM) indicates a resistor value of 8.2 kΩ. Additional zeros imply a tighter tolerance, for example 15M0 for three significant digits. When the value can be expressed without the need for a prefix (that is, multiplicator 1), an "R" is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω.
Theory of operation
The hydraulic analogy compares electric current flowing through circuits to water flowing through pipes. When a pipe (left) is clogged 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) to drive the same flow (electric current).1
An ideal resistor (i.e. a resistance without reactance) obeys Ohm's law:
Ohm's law states that the voltage () across a resistor is proportional to the current () passing through it, where the constant of proportionality is the resistance (). 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 flows through that resistor.
The (symbol: ) is the unit of , named after . An ohm is equivalent to a per . Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10 Ω), kilohm (1 kΩ = 10 Ω), and megohm (1 MΩ = 10 Ω) are also in common usage.
Nonideal properties
Practical resistors have a series inductance and a small parallel capacitance; these specifications can be important in high-frequency applications. And while even an ideal resistor inherently has Johnson noise, some resistors have worse noise characteristics and so may be an issue for low-noise amplifiers or other sensitive electronics.
In some precision applications, the temperature coefficient of the resistance may also be of concern.
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.9 A family of discrete resistors may also be characterized according to its form factor, that is, the size of the device and the position of its leads (or terminals). This is relevant in the practical manufacturing of circuits that may use them.
Practical resistors are also specified as having a maximum 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. Resistors with higher power ratings are physically larger and may require heat sinks. 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.
A single in line (SIL) resistor package with 8 individual 47 ohm resistors. This package is also known as a SIP-9. 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.
Lead arrangements
Axial resistors with wire leads for through-hole mounting
Through-hole components typically have "leads" (pronounced /liːdz/) leaving the body "axially", that is, on a line parallel with the part's longest axis. Others have leads coming off their body "radially" instead. Other components may be SMT (surface mount technology), while high power resistors may have one of their leads designed into the heat sink.
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.
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.
Potentiometers
Potentiometer with case cut away, showing parts: (A) shaft, (B) stationary carbon composition resistance element, (C) phosphor bronze wiper, (D) shaft attached to wiper, (E, G) terminals connected to ends of resistance element, (F) terminal connected to wiper.
A potentiometer (colloquially, pot) is a three-terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob or by a linear slider.24 The name potentiometer comes from its function as an adjustable voltage divider to provide a variable potential at the terminal connected to the tapping point. Volume control in an audio device is a common application of a potentiometer. A typical low power potentiometer is constructed of a flat resistance element of carbon composition, metal film, or conductive plastic, with a springy wiper contact which moves along the surface. An alternate construction is resistance wire wound on a form, with the wiper sliding axially along the coil. These have lower resolution, since as the wiper moves the resistance changes in steps equal to the resistance of a single turn.
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 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. This allows for the voltages present to be at reasonable levels (generally below 10 volts) but still measurable.
Measuring low-value resistors, such as fractional-ohm resistors, with acceptable accuracy requires 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, 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 called Kelvin clips. Each of the two 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.
Standards
Production resistors
Resistor characteristics are quantified and reported using various national standards. In the US, MIL-STD-20226 contains the relevant test methods to which other standards refer.
There are various standards specifying properties of resistors for use in equipment:
MIL-PRF-39007 (Fixed power, established reliability)
MIL-PRF-55342 (Surface-mount thick and thin film)
MIL-PRF-914
MIL-R-11 Standard Canceled
MIL-R-39017 (Fixed, General Purpose, Established Reliability)
MIL-PRF-32159 (zero ohm jumpers)
UL 1412 (fusing and temperature limited resistors)27
There are other United States military procurement MIL-R- standards.
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 1 square millimeter in cross-section, at 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 . Since 1990 the international resistance standard has been based on the discovered by , for which he won the Nobel Prize in Physics in 1985.
Wheel-based RMA Resistor Color Code guide. Circa 1945–1950.
Axial resistor cases are usually tan, brown, blue, or green (though other colors are occasionally found as well, such as dark red or dark gray), and display three to six colored stripes that indicate resistance (and by extension tolerance), and may include bands to indicate the temperature coefficient and reliability class. In four-striped resistors, the first two stripes represent the first two digits of the resistance in ohms, the third represents a multiplier, and the fourth the tolerance (which if absent, denotes ±20%). For five- and six- striped resistors the third band is the third digit, the fourth is the multiplier and the fifth is the tolerance; a sixth stripe represents the temperature coefficient. The power rating of the resistor is usually not marked and is deduced from its size.
Early 20th century resistors, essentially uninsulated, were dipped in paint to cover their entire body for color-coding. This base color represented the first digit. A second color of paint was applied to one end of the element to represent a second digit, 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.
Preferred values
Common usage patterns
There are several common usage patterns that resistors are commonly configured in.39
Current limiting
Resistors are commonly used to limit the amount of current flowing through a circuit. Many circuit components (such as LEDs) require the current flowing through them to be limited, but do not themselves limit the amount of current. Therefore, oftentimes resistors will be added to prevent overcurrent situations. Additionally, oftentimes circuits do not need the amount of current that would be otherwise flowing through them, so resistors can be added to limit the power consumption of such circuits.
Oftentimes circuits need to provide various reference voltages for other circuits (such as voltage comparators). A fixed voltage can be obtained by taking two resistors in series between two other fixed voltages (such as the source voltage and ground). The terminal between the two resistors will be at a voltage that is between the two voltages, at a linear distance based on the relative resistances of the two resistors. For instance, if a 200 ohm resistor and a 400 ohm resistor are placed in series between 6 V and 0 V, the terminal between them will be at 4 V.
When a circuit is not connected to power, the voltage of that circuit is not zero but undefined (it can be influenced by previous voltages or the environment). A pull-up or pull-down resistor provides a voltage for a circuit when it is otherwise disconnected (such as when a button is not pushed down or a transistor is not active). A pull-up resistor connects the circuit to a high positive voltage (if the circuit requires a high positive default voltage) and a pull-down resistor connects the circuit to a low voltage or ground (if the circuit requires a low default voltage). The resistor value must be high enough so that, when the circuit is active, the voltage source it is attached to does not over influence the function of the circuit, but low enough so that it "pulls" quickly enough when the circuit is deactivated, and does not significantly alter the voltage from the source value.
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 naturally produces 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 value of resistance produces a larger voltage noise, whereas a smaller value of resistance generates more current noise, at a given temperature.
The thermal noise of a practical resistor may also be larger than the theoretical prediction and that increase is typically frequency-dependent. Excess noise of a practical resistor is observed only when current 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 exhibits 1 μV (rms) of excess noise for each volt across the resistor in each frequency decade. Excess noise is thus an example of 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 are often used 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.40 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 tend to average out.
Failure modes
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 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 significant change in its resistance. A safe design generally uses overrated resistors in power applications to avoid this danger.
When overheated, carbon-film resistors may decrease or increase in resistance.42 Carbon film and composition resistors can fail (open circuit) if running close to their maximum dissipation. Fusible or fusing resistors are designed specifically to fail open.43 Decreasing resistance is also possible but less likely with metal film and wirewound resistors.
Resistors can fail due to mechanical stress and adverse environmental factors, including humidity. If not enclosed, wirewound resistors can corrode.
Surface mount resistors can fail due to sulfur ingress into the resistor's internal makeup. This sulfur chemically reacts with the silver layer to produce non-conductive silver sulfide. The resistor's impedance goes to infinity. Sulfur-resistant, anti-corrosive resistors are used in automotive, industrial, and military applications. ASTM B809 is an industry standard that tests a part's susceptibility to sulfur.
Resistors are specified for a maximum voltage drop. Exceeding this voltage causes the resistor to degrade slowly by reducing resistance. The voltage drop across large-value resistors can exceed this limit before power dissipation reaches its limit. Low-power thin-film resistors can be damaged by long-term high-voltage stress, even below maximum specified voltage and below maximum power rating.
Variable resistors can degrade differently, typically through poor contact between the wiper and the resistance body. 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. Potentiometers are somewhat self-cleaning: running the wiper across the resistance may improve contact. Potentiometers which are seldom adjusted, especially in dirty or harsh environments, are most likely to develop this problem. spray may be effective when self-cleaning is insufficient.
2American Radio Relay League (ARRL) (2021). "Fundamental Theory—Circuits and Components". ARRL Handbook for Radio Communications (98 ed.). American Radio Relay League. ISBN 978-1-62595-139-7.
3Doug DeMaw, ed. (1968). "Electrical Laws and Circuits —Resistance". Radio Amateurs Handbook (45 ed.). American Radio Relay League.
4Farago, P.S. (1961) An Introduction to Linear Network Analysis, pp. 18–21, The English Universities Press Ltd.
5Wu, F. Y. (2004). "Theory of resistor networks: The two-point resistance". Journal of Physics A: Mathematical and General. 37 (26): 6653–6673. arXiv:math-ph/0402038. Bibcode:2004JPhA...37.6653W. doi:. .
The total resistance of resistors connected in series is the sum of their individual resistance values.
Equivalently, the total conductance of resistors connected in series is the reciprocal of the sum of the reciprocals of the individual conductances.
The total resistance of resistors connected in parallel is the reciprocal of the sum of the reciprocals of the individual resistances.3
Equivalently, the total conductance of resistors connected in parallel is the sum of the individual conductances.
For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor and a 15 ohm resistor produces 1/1/10 + 1/5 + 1/15 ohms of resistance, or 30/11 = 2.727 ohms.
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. Some complex networks of resistors cannot be resolved in this manner, requiring more sophisticated circuit analysis. Generally, the Y-Δ transform, or matrix methods can be used to solve such problems.456
Power dissipation
At any instant, the power P (watts) consumed by a resistor of resistance R (ohms) is calculated as:
where V (volts) is the voltage across the resistor and I (amps) is the current flowing through it. 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 before its temperature rises excessively.3
Resistors are rated according to their maximum power dissipation. Discrete resistors in solid-state electronic systems are typically rated as 1⁄10, 1⁄8, or 1⁄4 watt. They usually absorb much less than a watt of electrical power and require little attention to their power rating.
An aluminium-encased power resistor rated for dissipation of 50 W when mounted on a heat-sink
Power resistors are required to dissipate substantial amounts of power and are typically used in power supplies, power conversion circuits, and power amplifiers; 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.
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 will not produce flames with any overload of any duration.
Resistors may be specified with higher rated dissipation than is experienced in service to account for poor air circulation, high altitude, or high operating temperature.
All resistors have a maximum voltage rating; this may limit the power dissipation for higher resistance values.7 For instance, among 1⁄4 watt resistors (a very common sort of leaded resistor) one is listed with a resistance of 100 MΩ8 and a maximum rated voltage of 750 V. However even placing 750 V across a 100 MΩ resistor continuously would only result in a power dissipation of less than 6 mW, making the nominal 1⁄4 watt rating meaningless.
VZR power resistor 1.5 kΩ 12 W, manufactured in 1963 in the Soviet Union
Old style "dog bone" resistors with "body, tip, dot" color code marking
Three carbon composition resistors in a 1960s valve (vacuum tube) radio
Carbon composition resistors (CCR) 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.
The resistive element in carbon composition resistors is made from a mixture of finely 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, which is a good conductor, result in lower resistances. Carbon composition resistors were commonly used in the 1960s and earlier, but are not popular for general use now as other types have better specifications, such as tolerance, voltage dependence, and stress. Carbon composition resistors change value when stressed with over-voltages. Moreover, if internal moisture content, such as from exposure for some length of time to a humid environment, is significant, soldering heat creates 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.10 These resistors are non-inductive, which provides benefits when used in voltage pulse reduction and surge protection applications.11 Carbon composition resistors have higher capability to withstand overload relative to the component's size.12
Carbon composition resistors are still available, but relatively expensive. 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.12 They are also in demand for repair of vintage electronic equipment where authenticity is a factor.
Carbon pile
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, such as 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.13 A carbon pile resistor can be incorporated in automatic voltage regulators for generators, where the carbon pile controls the field current to maintain relatively constant voltage.14 This principle is also applied in the carbon microphone.
Carbon film
Carbon film resistor with exposed carbon spiral (Tesla TR-212 1 kΩ)
In manufacturing carbon film resistors, a carbon film is deposited on an insulating substrate, and a helix is cut in the flim to create a long, narrow resistive path. Varying helix shapes, coupled with the resistivity of amorphous carbon (ranging from 500 to 800 μΩ m), can provide a wide range of resistance values with tolerances from ±2% to ±10%, with ±5% being most common. Carbon film resistors feature lower noise compared to carbon composition resistors because of the precise distribution of the pure graphite without binding.15 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 megaohm. 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.12
Printed carbon resistors
Carbon resistors (black rectangles) printed directly onto the SMD pads on the PCB of a Psion Organiser II from 1989
Carbon composition resistors can be printed directly onto printed circuit board (PCB) substrates as part of the PCB manufacturing process. Although 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 resistors.
Thick and thin film
Laser Trimmed Precision Thin Film Resistor Network from Fluke, used in the Keithley DMM7510 multimeter. Ceramic backed with glass hermetic seal cover.
Thick film resistors became popular during the 1970s, and most SMD (surface mount device) resistors today are of this type. The resistive element of thick films is 1000 times thicker than thin films,16 but the principal difference is how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors).
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 photo-sensitive material, 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.
Thick film resistors are manufactured using screen and stencil printing processes.12
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 also varies, consisting of one or more ceramic (cermet) conductors such as tantalum nitride (TaN), ruthenium oxide (RuO 2), lead oxide (PbO), bismuth ruthenate (Bi 2Ru 2O 7), nickel chromium (NiCr), or bismuth iridate (Bi 2Ir 2O 7).
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 1% and 5%, and with temperature coefficients of 5 to 50 ppm/K. They also have much lower noise levels, on the level of 10–100 times less than thick film resistors.17 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-printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C.
When first manufactured, thick film resistors had tolerances of 5%, but standard tolerances have improved to 2% or 1% from the late 1960s, thanks to laser-trimming technology. Temperature coefficients of thick film resistors are typically ±200 or ±250 ppm/K; a 40-kelvin (70 °F) temperature change can change the resistance by 1%.
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.
Metal film
A common type of axial-leaded resistor today is the metal-film resistor. Metal Electrode Leadless Face (MELF) resistors often use the same technology.
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 technique used). The resistance value is determined by cutting a helix through the coating rather than by etching, 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.18 Metal film resistors possess good noise characteristics and low non-linearity due to a low voltage coefficient. They are also beneficial due to long-term stability.12
Metal oxide film
Metal-oxide film resistors are made of metal oxides which results in a higher operating temperature and greater stability and reliability than metal film. They are used in applications with high endurance demands.
Wire wound
High-power wire wound resistors used for dynamic braking on an electric railway car. Such resistors may dissipate many kilowatts for an extended length of time.
Types of windings in wire resistors: common bifilar common on a thin former Ayrton–Perry
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 enamel coating baked at high temperature. These resistors are designed to withstand unusually high temperatures of up to 450 °C.12 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 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 overheats 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.
Because wirewound resistors are coils they have more undesirable inductance than other types of resistor. However, 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.
Applications of wirewound resistors are similar to those of composition resistors with the exception of high frequency applications. The high frequency response of wirewound resistors is substantially worse than that of a composition resistor.12
Metal foil resistor
Metal foil resistor
In 1960, Felix Zandman and Sidney J. Stein19 presented a development of resistor film of very high stability.
The primary resistance element of a foil resistor is a chromium nickel alloy foil several micrometers thick. Chromium nickel alloys are characterized by having a large electrical resistance (about 58 times that of copper), a small temperature coefficient and high resistance to oxidation. Examples are Chromel A and Nichrome V, whose typical composition is 80 Ni and 20 Cr, with a melting point of 1420 °C. When iron is added, the chromium nickel alloy becomes more ductile. The Nichrome and Chromel C are examples of an alloy containing iron. The composition typical of Nichrome is 60 Ni, 12 Cr, 26 Fe, 2 Mn and Chromel C, 64 Ni, 11 Cr, Fe 25. The melting temperature of these alloys are 1350 °C and 1390 °C, respectively.20
Since their introduction in the 1960s, foil resistors have had the best precision and stability of any resistor available. One of the important parameters of stability is the temperature coefficient of resistance (TCR). The TCR of foil resistors is extremely low, and has been further improved over the years. 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 years) 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.21
The thermal stability of this type of resistor also has to do with the opposing effects of the metal's electrical resistance increasing with temperature, and being reduced by thermal expansion leading to an increase in thickness of the foil, whose other dimensions are constrained by a ceramic substrate.22
Ammeter shunts
An 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 across which the voltage drop is measured and interpreted as current. A typical shunt consists of two solid metal blocks, sometimes brass, mounted on an insulating base. Between the blocks, and soldered or brazed to them, are one or more strips of low temperature coefficient of resistance (TCR) manganin alloy. Large bolts threaded into the blocks make the current connections, while much smaller screws provide volt meter 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 to be made to the other parts of the meter.
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 banking for 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.23
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.
High-resolution multiturn potentiometers are used in precision applications. These have wire-wound resistance elements typically wound on a helical mandrel, with the wiper moving on a helical track as the control is turned, making continuous contact with the wire. 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, and can typically achieve three-digit resolution. Electronic analog computers used them in quantity for setting coefficients and delayed-sweep oscilloscopes of recent decades in time included one on their panels.
Typical panel mount potentiometer
An assortment of small through-hole potentiometers designed for mounting on printed circuit boards.
A resistance decade box (A decade here means a range of resistance increase by a factor of 10, like 10 to 100 ohm, or 100 to 1k ohm) 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 100 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.25
Special devices
There are various devices whose resistance changes with various quantities. The resistance of NTC thermistors 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 current surges when equipment is powered on. Similarly, the resistance of a humistor varies with humidity. One sort of photodetector, the photoresistor, has a resistance which varies with illumination.
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 is subjected to mechanical strain. With the strain gauge and a filter, amplifier, and analog/digital converter, the strain on an object can be measured.
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 1012 in response to changes in applied pressure.
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.30
Early resistors were made in more or less arbitrary round numbers; a series might have 100, 125, 150, 200, 300, etc.31 Early power wirewound resistors, such as brown vitreous-enameled types, were made with a system of preferred values like some of those mentioned here. Resistors as manufactured are subject to a certain percentage 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 geometric 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 may make sense to have each resistor about 1.5 times its predecessor, covering a decade in 6 values. (A decade for resistors is a resistance range increasing by a factor of 10.) More precisely, 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 resistance range from 1 to 10 ohm; 0.1–1 and 10–100 are other dacade examples.); these are rounded in practice to 1.5, 2.2, 3.3, 4.7, 6.8, 10; followed by 15, 22, 33, ... in the next decade and preceded by ... 0.47, 0.68, 1 in the previous decade. This scheme has been adopted as the E6 series of the IEC 60063 preferred number values. There are also E12, E24, E48, E96 and E192 series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade. The actual values used are in the IEC 60063 lists of preferred numbers.
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%, might not lie 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 and 10% too high or 5 to 10% too low (but not closer to 100 ohm than that) because any resistors the factory had 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. This process of sorting parts based on post-production measurement is known as "binning", and can be applied to other components than resistors (such as speed grades for CPUs).
SMT resistors
This image shows four surface-mount resistors (the component at the upper left is a capacitor) including two zero-ohm resistors. Zero-ohm links are often used instead of wire links, so that they can be inserted by a resistor-inserting machine. Their resistance is negligible.
Surface mounted resistors of larger sizes (metric 1608 and above) are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance surface-mount technology (SMT) resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten (the number of zeroes). For example:
334 = 33 × 104 Ω = 330 kΩ
222 = 22 × 102 Ω = 2.2 kΩ
473 = 47 × 103 Ω = 47 kΩ
105 = 10 × 105 Ω = 1 MΩ
Resistances less than 100 Ω are written: 100, 220, 470. The final zero represents ten to the power zero, which is 1. For example:
100 = 10 × 100 Ω = 10 Ω
220 = 22 × 100 Ω = 22 Ω
Sometimes these values are marked as 10 or 22 to prevent a mistake.
Resistances less than 10 Ω have 'R' to indicate the position of the decimal point (radix point). For example:
4R7 = 4.7 Ω
R300 = 0.30 Ω
0R22 = 0.22 Ω
0R01 = 0.01 Ω
000 and 0000 sometimes appear as values on surface-mount zero-ohm links, since these have (approximately) zero resistance.
More recent surface-mount resistors are too small, physically, to permit practical markings to be applied.
Precision resistor markings
Many precision resistors, including surface mount and axial-lead types, are marked with a four-digit code. The first three digits are the significant figures and the fourth is the power of ten. For example:
1001 = 100 × 101 Ω = 1.00 kΩ
4992 = 499 × 102 Ω = 49.9 kΩ
1000 = 100 × 100 Ω = 100 Ω
Axial-lead precision resistors often use color code bands to represent this four-digit code.
EIA-96 marking
The former EIA-96 marking system now included in IEC 60062:2016 is a more compact marking system intended for physically small high-precision resistors. It uses a two-digit code plus a letter (a total of three alphanumeric characters) to indicate 1% resistance values to three significant digits.32 The two digits (from "01" to "96") are a code that indicates one of the 96 "positions" in the standard E96 series of 1% resistor values. The uppercase letter is a code that indicates a power of ten multiplier. For example, the marking "01C" represents 10 kOhm; "10C" represents 12.4 kOhm; "96C" represents 97.6 kOhm.3334353637
Code
Series
Letter
Digits
E96
Y / S
X / R
A
B / H
C
D
E
01
1.00
1R00
10R0
100R
1K00
10K0
100K
1M00
02
1.02
1R02
10R2
102R
1K02
10K2
102K
1M02
03
1.05
1R05
10R5
105R
1K05
10K5
105K
1M05
04
1.07
1R07
10R7
107R
1K07
10K7
107K
1M07
05
1.10
1R10
11R0
110R
1K10
11K0
110K
1M10
06
1.13
1R13
11R3
113R
1K13
11K3
113K
1M13
07
1.15
1R15
11R5
115R
1K15
11K5
115K
1M15
08
1.18
1R18
11R8
118R
1K18
11K8
118K
1M18
09
1.21
1R21
12R1
121R
1K21
12K1
121K
1M21
10
1.24
1R24
12R4
124R
1K24
12K4
124K
1M24
11
1.27
1R27
12R7
127R
1K27
12K7
127K
1M27
12
1.30
1R30
13R0
130R
1K30
13K0
130K
1M30
13
1.33
1R33
13R3
133R
1K33
13K3
133K
1M33
14
1.37
1R37
13R7
137R
1K37
13K7
137K
1M37
15
1.40
1R40
14R0
140R
1K40
14K0
140K
1M40
16
1.43
1R43
14R3
143R
1K43
14K3
143K
1M43
17
1.47
1R47
14R7
147R
1K47
14K7
147K
1M47
18
1.50
1R50
15R0
150R
1K50
15K0
150K
1M50
19
1.54
1R54
15R4
154R
1K54
15K4
154K
1M54
20
1.58
1R58
15R8
158R
1K58
15K8
158K
1M58
21
1.62
1R62
16R2
162R
1K62
16K2
162K
1M62
22
1.65
1R65
16R5
165R
1K65
16K5
165K
1M65
23
1.69
1R69
16R9
169R
1K69
16K9
169K
1M69
24
1.74
1R74
17R4
174R
1K74
17K4
174K
1M74
25
1.78
1R78
17R8
178R
1K78
17K8
178K
1M78
26
1.82
1R82
18R2
182R
1K82
18K2
182K
1M82
27
1.87
1R87
18R7
187R
1K87
18K7
187K
1M87
28
1.91
1R91
19R1
191R
1K91
19K1
191K
1M91
29
1.96
1R96
19R6
196R
1K96
19K6
196K
1M96
30
2.00
2R00
20R0
200R
2K00
20K0
200K
2M00
31
2.05
2R05
20R5
205R
2K05
20K5
205K
2M05
32
2.10
2R10
21R0
210R
2K10
21K0
210K
2M10
33
2.15
2R15
21R5
215R
2K15
21K5
215K
2M15
34
2.21
2R21
22R1
221R
2K21
22K1
221K
2M21
35
2.26
2R26
22R6
226R
2K26
22K6
226K
2M26
36
2.32
2R32
23R2
232R
2K32
23K2
232K
2M32
37
2.37
2R37
23R7
237R
2K37
23K7
237K
2M37
38
2.43
2R43
24R3
243R
2K43
24K3
243K
2M43
39
2.49
2R49
24R9
249R
2K49
24K9
249K
2M49
40
2.55
2R55
25R5
255R
2K55
25K5
255K
2M55
41
2.61
2R61
26R1
261R
2K61
26K1
261K
2M61
42
2.67
2R67
26R7
267R
2K67
26K7
267K
2M67
43
2.74
2R74
27R4
274R
2K74
27K4
274K
2M74
44
2.80
2R80
28R0
280R
2K80
28K0
280K
2M80
45
2.87
2R87
28R7
287R
2K87
28K7
287K
2M87
46
2.94
2R94
29R4
294R
2K94
29K4
294K
2M94
47
3.01
3R01
30R1
301R
3K01
30K1
301K
3M01
48
3.09
3R09
30R9
309R
3K09
30K9
309K
3M09
Code
Series
Letter
Digits
E96
Y / S
X / R
A
B / H
C
D
E
49
3.16
3R16
31R6
316R
3K16
31K6
316K
3M16
50
3.24
3R24
32R4
324R
3K24
32K4
324K
3M24
51
3.32
3R32
33R2
332R
3K32
33K2
332K
3M32
52
3.40
3R40
34R0
340R
3K40
34K0
340K
3M40
53
3.48
3R48
34R8
348R
3K48
34K8
348K
3M48
54
3.57
3R57
35R7
357R
3K57
35K7
357K
3M57
55
3.65
3R65
36R5
365R
3K65
36K5
365K
3M65
56
3.74
3R74
37R4
374R
3K74
37K4
374K
3M74
57
3.83
3R83
38R3
383R
3K83
38K3
383K
3M83
58
3.92
3R92
39R2
392R
3K92
39K2
392K
3M92
59
4.02
4R02
40R2
402R
4K02
40K2
402K
4M02
60
4.12
4R12
41R2
412R
4K12
41K2
412K
4M12
61
4.22
4R22
42R2
422R
4K22
42K2
422K
4M22
62
4.32
4R32
43R2
432R
4K32
43K2
432K
4M32
63
4.42
4R42
44R2
442R
4K42
44K2
442K
4M42
64
4.53
4R53
45R3
453R
4K53
45K3
453K
4M53
65
4.64
4R64
46R4
464R
4K64
46K4
464K
4M64
66
4.75
4R75
47R5
475R
4K75
47K5
475K
4M75
67
4.87
4R87
48R7
487R
4K87
48K7
487K
4M87
68
4.99
4R99
49R9
499R
4K99
49K9
499K
4M99
69
5.11
5R11
51R1
511R
5K11
51K1
511K
5M11
70
5.23
5R23
52R3
523R
5K23
52K3
523K
5M23
71
5.36
5R36
53R6
536R
5K36
53K6
536K
5M36
72
5.49
5R49
54R9
549R
5K49
54K9
549K
5M49
73
5.62
5R62
56R2
562R
5K62
56K2
562K
5M62
74
5.76
5R76
57R6
576R
5K76
57K6
576K
5M76
75
5.90
5R90
59R0
590R
5K90
59K0
590K
5M90
76
6.04
6R04
60R4
604R
6K04
60K4
604K
6M04
77
6.19
6R19
61R9
619R
6K19
61K9
619K
6M19
78
6.34
6R34
63R4
634R
6K34
63K4
634K
6M34
79
6.49
6R49
64R9
649R
6K49
64K9
649K
6M49
80
6.65
6R65
66R5
665R
6K65
66K5
665K
6M65
81
6.81
6R81
68R1
681R
6K81
68K1
681K
6M81
82
6.98
6R98
69R8
698R
6K98
69K8
698K
6M98
83
7.15
7R15
71R5
715R
7K15
71K5
715K
7M15
84
7.32
7R32
73R2
732R
7K32
73K2
732K
7M32
85
7.50
7R50
75R0
750R
7K50
75K0
750K
7M50
86
7.68
7R68
76R8
768R
7K68
76K8
768K
7M68
87
7.87
7R87
78R7
787R
7K87
78K7
787K
7M87
88
8.06
8R06
80R6
806R
8K06
80K6
806K
8M06
89
8.25
8R25
82R5
825R
8K25
82K5
825K
8M25
90
8.45
8R45
84R5
845R
8K45
84K5
845K
8M45
91
8.66
8R66
86R6
866R
8K66
86K6
866K
8M66
92
8.87
8R87
88R7
887R
8K87
88K7
887K
8M87
93
9.09
9R09
90R9
909R
9K09
90K9
909K
9M09
94
9.31
9R31
93R1
931R
9K31
93K1
931K
9M31
95
9.53
9R53
95R3
953R
9K53
95K3
953K
9M53
96
9.76
9R76
97R6
976R
9K76
97K6
976K
9M76
Industrial type designation
Power Rating at 70 °C
Type no.
Power rating (watts)
MIL-R-11 style
MIL-R-39008 style
BB
1⁄8
RC05
RCR05
CB
1⁄4
RC07
RCR07
EB
1⁄2
RC20
RCR20
GB
1
RC32
RCR32
HB
2
RC42
RCR42
GM
3
-
-
HM
4
-
-
Tolerance code
Industrial type designation
Tolerance
MIL designation
5
±5%
J
2
±20%
M
1
±10%
K
-
±2%
G
-
±1%
F
-
±0.5%
D
-
±0.25%
C
-
±0.1%
B
Steps to find out the resistance or capacitance values:38
First two letters gives the power dissipation capacity.
Next three digits gives the resistance value.
First two digits are the significant values
Third digit is the multiplier.
Final digit gives the tolerance.
If a resistor is coded:
EB1041: power dissipation capacity = 1/2 watts, resistance value = 10×104±10% = between 9×104 ohms and 11×104 ohms.
CB3932: power dissipation capacity = 1/4 watts, resistance value = 39×103±20% = between 31.2×103 and 46.8×103 ohms.
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 different temperatures. This induced DC voltage can degrade the precision of instrumentation amplifiers 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 to mount the resistors horizontally to avoid temperature gradients and to mind the air flow over the board.41
9A 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 fail first from exceeding the maximum voltage rating. See Middleton, Wendy; Van Valkenburg, Mac E. (2002). Reference data for engineers: radio, electronics, computer, and communications (9 ed.). Newnes. pp. 5–10. ISBN 0-7506-7291-9.
10Harter, James H. and Lin, Paul Y. (1982) Essentials of electric circuits. Reston Publishing Company. pp. 96–97. ISBN 0-8359-1767-3.
