![Carbon and ceramic resistors of different power ratings](https://upload.wikimedia.org/wikipedia/commons/6/6b/Carbon_and_ceramic_resistors_of_different_power_ratings.jpg)
A resistor is a
passive
Passive may refer to:
* Passive voice, a grammatical voice common in many languages, see also Pseudopassive
* Passive language, a language from which an interpreter works
* Passivity (behavior), the condition of submitting to the influence of o ...
two-terminal electrical component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...
that implements
electrical resistance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to
divide voltages,
bias
Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group ...
active elements, and terminate
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
s, among other uses. High-power resistors that can dissipate many
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for
generators.
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.
Resistors are common elements of
electrical network
An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sources ...
s and
electronic circuits and are ubiquitous in
electronic equipment
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
. Practical resistors as discrete components can be composed of various compounds and forms. Resistors are also implemented within
integrated circuits
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
.
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
An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
. The nominal value of the resistance falls within the
manufacturing tolerance
Engineering tolerance is the permissible limit or limits of variation in:
# a physical dimension;
# a measured value or physical property of a material, manufactured object, system, or service;
# other measured values (such as temperature, hu ...
, indicated on the component.
Electronic symbols and notation
Two typical
schematic diagram
A schematic, or schematic diagram, is a designed representation of the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the key information the sc ...
symbols are as follows:
File:Resistor, Rheostat (variable resistor), and Potentiometer symbols.svg, ANSI
The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
-style: (a) resistor, (b) rheostat (variable resistor), and (c) potentiometer
File:Resistor_symbol_IEC.svg, IEC
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
resistor symbol
The notation to state a resistor's value in a circuit diagram varies.
One common scheme is the
RKM code
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to speci ...
following
IEC 60062
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to specif ...
. Rather than using a
decimal separator
A decimal separator is a symbol used to separate the integer part from the fractional part of a number written in decimal form (e.g., "." in 12.45). Different countries officially designate different symbols for use as the separator. The choi ...
, this notation uses a letter loosely associated with SI prefixes corresponding with the part's resistance. For example, ''8K2'' as part
marking code
Marking may refer to:
Symbols
Marking may refer to human-made symbols and annotations in several contexts:
On vehicles
* Aircraft marking
* Emergency vehicle equipment markings
** Battenburg markings, emergency vehicle patterns
* Vehicle mark ...
, in a
circuit diagram
A circuit diagram (wiring diagram, electrical diagram, elementary diagram, electronic schematic) is a graphical representation of an electrical circuit. A pictorial circuit diagram uses simple images of components, while a schematic diagram s ...
or in a
bill of materials
A bill of materials or product structure (sometimes bill of material, BOM or associated list) is a list of the raw materials, sub-assemblies, intermediate assemblies, sub-components, parts, and the quantities of each needed to manufacture an end ...
(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
Ohm's law
The behaviour of an ideal resistor is described by
Ohm's law:
Ohm's law states that the
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
(
) across a resistor is proportional to the
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
(
) passing through it, where the constant of proportionality is the resistance (
). For example, if a 300-
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
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
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
(symbol:
Ω) is the
SI unit of
electrical resistance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
, named after
Georg Simon Ohm
Georg Simon Ohm (, ; 16 March 1789 – 6 July 1854) was a German physicist and mathematician. As a school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta. Using equipment of his ...
. An ohm is equivalent to a
volt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
per
ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10
−3 Ω), kilohm (1 kΩ = 10
3 Ω), and megohm (1 MΩ = 10
6 Ω) are also in common usage.
Series and parallel resistors
The total resistance of resistors connected in series is the sum of their individual resistance values.
The total resistance of resistors connected in parallel is the reciprocal of the sum of the reciprocals of the individual resistors.
For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor and a 15 ohm resistor produces ohms of resistance, or = 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
The Y-Δ transform, also written wye-delta and also known by many other names, is a mathematical technique to simplify the analysis of an electrical network. The name derives from the shapes of the circuit diagrams, which look respectively like t ...
, or
matrix methods
The matrix method is a structural analysis method used as a fundamental principle in many applications in civil engineering.
The method is carried out, using either a stiffness matrix or a flexibility matrix.
See also
* Direct stiffness method A ...
can be used to solve such problems.
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
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
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.
Resistors are rated according to their maximum power dissipation. Discrete resistors in solid-state electronic systems are typically rated as , , or watt. They usually absorb much less than a watt of electrical power and require little attention to their power rating.
![Danotherm HS50 power resistor](https://upload.wikimedia.org/wikipedia/commons/a/a1/Danotherm_HS50_power_resistor.jpg)
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
A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...
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. For instance, among watt resistors (a very common sort of
leaded
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
resistor) one is listed with a resistance of 100 MΩ 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 watt rating meaningless.
Nonideal properties
Practical resistors have a series
inductance and a small parallel
capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...
; these specifications can be important in high-frequency applications. In a
low-noise amplifier
A low-noise amplifier (LNA) is an electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio. An amplifier will increase the power of both the signal and the noise present at its input, ...
or
pre-amp
A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier ...
, the
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
characteristics of a resistor may be an issue.
In some precision applications, the
temperature coefficient
A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...
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. 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
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
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 sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...
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.
Fixed resistors
Lead arrangements
Through-hole
In electronics, through-hole technology (also spelled "thru-hole") is a manufacturing scheme in which leads on the components are inserted through holes drilled in printed circuit boards (PCB) and soldered to pads on the opposite side, either by ...
components typically have "leads" (pronounced ) 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 heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...
.
Carbon composition
![Old Radio Resistors](https://upload.wikimedia.org/wikipedia/commons/d/d4/Old_Radio_Resistors.jpg)
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. These resistors are non-inductive, which provides benefits when used in voltage pulse reduction and surge protection applications.
[HVR International (ed.)]
"SR Series: Surge Resistors for PCB Mounting"
(PDF; 252 kB), 26 May 2005, retrieved 24 January 2017. Carbon composition resistors have higher capability to withstand overload relative to the component's size.
[Beyschlag, Vishay (2008)]
"Basics of Linear Fixed Resistors Application Note"
Document Number 28771.
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.
[ 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. A carbon pile resistor can be incorporated in automatic voltage regulator
A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...
s for generators, where the carbon pile controls the field current to maintain relatively constant voltage. This principle is also applied in the carbon microphone
The carbon microphone, also known as carbon button microphone, button microphone, or carbon transmitter, is a type of microphone, a transducer that converts sound to an electrical audio signal. It consists of two metal plates separated by granu ...
.
Carbon film
In manufacturing carbon film resistors, a carbon film is deposited on an insulating substrate, and a helix
A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helic ...
is cut in it to create a long, narrow resistive path. Varying shapes, coupled with the resistivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
of amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wi ...
carbon (ranging from 500 to 800 μΩ m), can provide a wide range of resistance values. Carbon film resistors feature lower noise compared to carbon composition resistors because of the precise distribution of the pure graphite without binding. 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.[
]
Printed carbon resistors
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 resistor
In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as switches and transistors, which physically in ...
s.
Thick and thin film
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, 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
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ca ...
(a method of vacuum deposition
Vacuum deposition is a group of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can r ...
) 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
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
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.[
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
A cermet is a composite material composed of ceramic (cer) and metal (met) materials.
A cermet can combine attractive properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to und ...
) conductors such as tantalum nitride
Tantalum nitride (TaN) is a chemical compound, a nitride of tantalum. There are multiple phases of compounds, stoichimetrically from Ta2N to Ta3N5, including TaN.
As a thin film TaN find use as a diffusion barrier and insulating layer between cop ...
(TaN), ruthenium oxide Ruthenium oxide may refer to either of the following:
*Ruthenium(IV) oxide
Ruthenium(IV) oxide is the inorganic compound with the formula Ru O2. This black solid is the most common oxide of ruthenium. It is widely used as an electrocatalyst fo ...
(), lead oxide
Lead oxides are a group of inorganic compounds with formulas including lead (Pb) and oxygen (O).
Common lead oxides include:
* Lead(II) oxide, PbO, litharge (red), massicot (yellow)
* Lead(II,IV) oxide
Lead(II,IV) oxide, also called red lead o ...
(PbO), bismuth ruthenate (), nickel chromium (NiCr), or bismuth iridate ().
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
{{Unreferenced, date=June 2015
Laser trimming is the manufacturing process of using a laser to adjust the operating parameters of an electronic circuit.
One of the most common applications uses a laser to burn away small portions of resistors, ...
. 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
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
levels, on the level of 10–100 times less than thick film resistors. 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% in the last few decades. Temperature coefficients of thick film resistors are typically ±200 or ±250 ppm/K; a 40-kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
(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
Metal electrode leadless face (MELF) is a type of leadless cylindrical electronic surface mount device that is vacuum deposition, metallized at its ends. MELF devices are usually diodes and resistors.
The EN 140401-803 and JEDEC DO-213 standa ...
) 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. 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.[
]
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
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.[ 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
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
. 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
A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. In engineering, the word ''bifilar'' describes wire which is made of two filaments or strands. It is commonly used to denote special types of 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.[
]
Metal foil resistor
In 1960, Felix Zandman
Felix Zandman ( pl, Feliks Zandman; May 7, 1928 – June 4, 2011) was a Polish-born American entrepreneur and founder of Vishay Intertechnology – one of the world's largest manufacturers of electronic components. From 1946 to 1949 he studied in ...
and Sidney J. Stein 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
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
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.
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.
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.
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
A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...
(TCR) manganin
Manganin is a trademarked name for an alloy of typically 84.2% copper, 12.1% manganese, and 3.7% nickel. It was first developed by Edward Weston in 1892, improving upon his Constantan (1887).
Manganin foil and wire is used in the manufacture of ...
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
A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, motor coach, railcar or power car; the ...
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.
The term ''grid resistor'' is sometimes used to describe a resistor of any type connected to the control grid
The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...
of a vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
. This is not a resistor technology; it is an electronic circuit topology.
Special varieties
* Cermet
A cermet is a composite material composed of ceramic (cer) and metal (met) materials.
A cermet can combine attractive properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to und ...
* Phenolic
Phenolic is an adjective and a substantive (noun) that may apply to :
* Phenol (or carbolic acid), a colorless crystalline solid and aromatic compound
* Phenols, a class of chemical compounds that include phenol
* Phenolic content in wine
* Phenol ...
* Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that is ...
* Water resistor
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.
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
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrume ...
and has two terminals.
Potentiometers
A potentiometer
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrum ...
(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. The name ''potentiometer'' comes from its function as an adjustable voltage divider
In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the inp ...
to provide a variable potential
Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...
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 ''(see drawing)'' is constructed of a flat resistance element ''(B)'' of carbon composition, metal film, or conductive plastic, with a springy phosphor bronze
Phosphor bronze is a member of the family of copper alloys. It is composed of copper that is alloyed with 0.5–11% of tin and 0.01–0.35% phosphorus, and may contain other elements to confer specific properties (e.g. lead at 0.5–3.0% to form ...
wiper contact ''(C)'' 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.
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 included one on their panels.
Resistance decade boxes
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 100 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.
Special devices
There are various devices whose resistance changes with various quantities. The resistance of NTC thermistor
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''.
Thermistors are divided based on their conduction ...
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 current surges when equipment is powered on. Similarly, the resistance of a humistor
A humistor is a type of variable resistor whose resistance varies based on humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidit ...
varies with humidity. One sort of photodetector, the photoresistor
A photoresistor (also known as a photocell, or light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The resi ...
, 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
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provid ...
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.
Measurement
The value of a resistor can be measured with an ohmmeter
An analog ohmmeter
An ohmmeter is an electrical instrument that measures electrical resistance (the opposition offered by a circuit or component to the flow of electric current). Multimeters also function as ohmmeters when in resistance-measu ...
, which may be one function of a multimeter
A multimeter is a measuring instrument that can measure multiple electrical properties. A typical multimeter can measure voltage, resistance, and current, in which case it is also known as a volt-ohm-milliammeter (VOM), as the unit is equipped w ...
. 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-202 contains the relevant test methods to which other standards refer.
There are various standards specifying properties of resistors for use in equipment:
* IEC 60062
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to specif ...
(IEC 62) / DIN 40825 / BS 1852 / IS 8186 / JIS C 5062
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to specif ...
etc. (Resistor color code
An electronic color code or electronic colour code (see spelling differences) is used to indicate the values or ratings of electronic components, usually for resistors, but also for capacitors, inductors, diodes and others. A separate code, the ...
, RKM code
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to speci ...
, date code)
* EIA RS-279 / DIN 41429 (Resistor color code)
* IEC 60063
The E series is a system of preferred numbers (also called preferred values) derived for use in electronic components. It consists of the E3, E6, E12, E24, E48, E96 and E192 series, where the number after the 'E' designates the quantity of ...
(IEC 63) / JIS C 5063
JIS may refer to:
Standards
* Japanese Industrial Standards
** JIS screw drive
** JIS semiconductor designation
** JIS encoding
** Shift JIS
Organisations
* Japanese international school
* Jabriya Indian School, Kuwait
* Jakarta International S ...
(Standard E series values)
* MIL-PRF-26
* MIL-PRF-39007 (Fixed power, established reliability)
* MIL-PRF-55342 (Surface-mount thick and thin film)
* MIL-PRF-914
* MIL-R-1
Standard Canceled
* MIL-R-39017 (Fixed, General Purpose, Established Reliability)
* MIL-PRF-32159 (zero ohm jumpers)
* UL 1412 (fusing and temperature limited resistors)
There are other United States military procurement MIL-R- standards.
Resistance standards
The primary standard A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are ...
for resistance, the "mercury ohm" was initially defined in 1884 in as a column of mercury 106.3 cm long and in cross-section, at . 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
Manganin is a trademarked name for an alloy of typically 84.2% copper, 12.1% manganese, and 3.7% nickel. It was first developed by Edward Weston in 1892, improving upon his Constantan (1887).
Manganin foil and wire is used in the manufacture of ...
. Since 1990 the international resistance standard has been based on the quantized Hall effect discovered by Klaus von Klitzing
Klaus von Klitzing (, born 28 June 1943, Schroda) is a German physicist, known for discovery of the integer quantum Hall effect, for which he was awarded the 1985 Nobel Prize in Physics.
Education
In 1962, Klitzing passed the Abitur at the A ...
, for which he won the Nobel Prize in Physics in 1985.
Resistors of extremely high precision are manufactured for calibration
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of know ...
and laboratory
A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physic ...
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.
Resistor marking
Axial resistors' 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 3–6 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 ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
s, 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.
Surface-mount resistors are marked numerically.
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
Early resistors were made in more or less arbitrary round numbers; a series might have 100, 125, 150, 200, 300, etc. 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
In mathematics, a geometric progression, also known as a geometric sequence, is a sequence of non-zero numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the ''common ratio''. For ex ...
, 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. 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 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 E6 series of the IEC
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
60063 preferred number
In industrial design, preferred numbers (also called preferred values or preferred series) are standard guidelines for choosing exact product dimensions within a given set of constraints.
Product developers must choose numerous lengths, distanc ...
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
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
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
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 digit
Significant figures (also known as the significant digits, ''precision'' or ''resolution'') of a number in positional notation are digits in the number that are reliable and necessary to indicate the quantity of something.
If a number expres ...
s 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
A decimal separator is a symbol used to separate the integer part from the fractional part of a number written in decimal form (e.g., "." in 12.45). Different countries officially designate different symbols for use as the separator. The choi ...
). 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 link
Zero-ohm axial lead resistor
Zero-ohm surface-mount resistor
A zero-ohm link or zero-ohm resistor is a wire link packaged in the same physical package format as a resistor. It is used to connect traces on a printed circuit board (PCB). This f ...
s, 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. The two digits (from "01" to "96") are a code that indicates one of the 96 "positions" in the standard E96 series
The E series is a system of preferred numbers (also called preferred values) derived for use in electronic components. It consists of the E3, E6, E12, E24, E48, E96 and E192 series, where the number after the 'E' designates the quantity of ...
of 1% resistor values. The uppercase letter is a code that indicates a power of ten
A power of 10 is any of the integer exponentiation, powers of the number 10 (number), ten; in other words, ten multiplication, multiplied by itself a certain number of times (when the power is a positive integer). By definition, the number one is ...
multiplier. For example, the marking "01C" represents 10 kOhm; "10C" represents 12.4 kOhm; "96C" represents 97.6 kOhm.
Industrial type designation
Steps to find out the resistance or capacitance values:
# 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% = between ohms and ohms.
* CB3932: power dissipation capacity = 1/4 watts, resistance value = ±20% = between and ohms.
Electrical and thermal noise
In amplifying faint signals, it is often necessary to minimize electronic noise
In electronics, noise is an unwanted disturbance in an electrical signal.
Noise generated by electronic devices varies greatly as it is produced by several different effects.
In particular, noise is inherent in physics, and central to the ...
, 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
Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens reg ...
is a fundamental noise source which depends only upon the temperature and resistance of the resistor, and is predicted by the fluctuation–dissipation theorem
The fluctuation–dissipation theorem (FDT) or fluctuation–dissipation relation (FDR) is a powerful tool in statistical physics for predicting the behavior of systems that obey detailed balance. Given that a system obeys detailed balance, the th ...
. 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. 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.
While not an example of "noise" per se, a resistor may act as a thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...
, producing a small DC voltage differential across it due to the thermoelectric effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
if its ends are at different temperatures. This induced DC voltage can degrade the precision of instrumentation amplifier
An instrumentation amplifier (sometimes shorthanded as in-amp or InAmp) is a type of differential amplifier that has been outfitted with input buffer amplifiers, which eliminate the need for input impedance matching and thus make the amplifie ...
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 to mount the resistors horizontally to avoid temperature gradients and to mind the air flow over the board.
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.
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 a switched-mode power supply
A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.
Lik ...
integrated circuit.
When overheated, carbon-film resistors may decrease or increase in resistance.
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.
There can also be failure of resistors due to mechanical stress and adverse environmental factors including humidity. If not enclosed, wirewound resistors can corrode.
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.
An alternative failure mode can be encountered where large value resistors are used (hundreds of kilohms and higher). Resistors are not only specified with a maximum power dissipation, but also for a maximum voltage drop. Exceeding this voltage causes the resistor to degrade slowly reducing in resistance. The voltage dropped across large value resistors can be exceeded before the power dissipation reaches its limiting value. Since the maximum voltage specified for commonly encountered resistors is a few hundred volts, this is a problem only in applications where these voltages are encountered.
Variable resistors can also 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
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
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
Contact cleaner, also known as switch cleaner, is a term for a chemical, or a mixture of chemicals, intended to remove or prevent the build-up of oxides or other unwanted substances on the conductive surfaces of connectors, switches and other el ...
(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.
See also
* Circuit design
The process of circuit design can cover systems ranging from complex electronic systems down to the individual transistors within an integrated circuit. One person can often do the design process without needing a planned or structured design ...
* Dummy load
A dummy load is a device used to simulate an electrical load, usually for testing purposes. In radio a dummy antenna is connected to the output of a radio transmitter and electrically simulates an antenna, to allow the transmitter to be adjust ...
* Electrical impedance
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit.
Quantitatively, the impedance of a two-terminal circuit element is the ratio of the com ...
* High value resistors (electronics)
* Iron-hydrogen resistor
* Piezoresistive effect
The piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. In contrast to the piezoelectric effect, the piezoresistive effect causes a change only in electrical resistance, ...
* Shot noise
* Thermistor
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''.
Thermistors are divided based on their conduction ...
* Trimmer (electronics)
References
External links
4-terminal resistors – How ultra-precise resistors work
* ttps://web.archive.org/web/20110401175312/http://www.ese.upenn.edu/rca/calcjs.html Color Coded Resistance Calculator – archived with WayBack Machine
Resistor Types – Does It Matter?
* ttp://www.analog.com/library/analogDialogue/archives/31-1/Ask_Engineer.html Ask The Applications Engineer – Difference between types of resistors
Resistors and their uses
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Electrical components
Electric heating
Resistive components