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Potentiometer

From Wikipedia, the free encyclopedia

This article is about the electrical component.For the measuring instrument,seePotentiometer (measuring instrument).

Potentiometer

A typical single-turn

potentiometer

Type Passive

Electronic symbol

(International)

(US)

A potentiometer ( /ptnimtr/),

informally, a pot, in electronics technology is
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a component, a three-terminalresistor with asliding contact that forms anadjustable voltage divider.

[1]If only two

terminals are used, one end and the wiper, itacts as a variable resistoror rheostat.

In circuit theory and measurement apotentiometer is essentially a voltage dividerused for measuring electric potential (voltage);

the component is an implementation of thesame principle, hence its name.

Potentiometers are commonly used to controlelectrical devices such as volume controls onaudio equipment. Potentiometers operated bya mechanism can be used as

positiontransducers, for example, inajoystick.

Potentiometers are rarely used to directlycontrol significant power (more than a watt),since the power dissipated in the

potentiometer would be comparable to thepower in the controlled load (seeinfiniteswitch). Instead they are used to adjust thelevel of analog signals (e.g. volume controlsonaudio equipment), and as control inputs forelectronic circuits. For example, a

light dimmeruses a potentiometer to control
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the switching of aTRIACand so indirectly tocontrol the brightness of lamps.

User-accessible rotary potentiometers can befitted with a switch which operates usually atthe anti-clockwise extreme of rotation. Beforedigital electronics became the norm such acomponent was used to allow radio and

television receivers and other equipment to beswitched on at minimum volume with anaudible click, then the volume increased, byturning a knob.

Many inexpensive potentiometers areconstructed with a resistive element formedinto an arc of a circle usually a little less thana full turn, and a wiper rotating around the arcand contacting it. The resistive element, with aterminal at each end, is flat or angled. Thewiper is connected to a third terminal, usually

between the other two. On panelpotentiometers, the wiper is usually the centerterminal of three. For single-turnpotentiometers, this wiper typically travels justunder one revolution around the contact. Theonly point of ingress for contamination is the
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narrow space between the shaft and thehousing it rotates in.

Another type is the linear slider potentiometer,which has a wiper which slides along a linearelement instead of rotating. Contaminationcan potentially enter anywhere along the slotthe slider moves in, making effective sealingmore difficult and compromising long-term

reliability. An advantage of the sliderpotentiometer is that the slider position givesa visual indication of its setting. While thesetting of a rotary potentiometer can be seenby the position of a marking on the knob, anarray of sliders can give a visual impression

of, for example, the effect of a multi-channelequaliser.

Multiturn potentiometers are also operated byrotating a shaft, but by several turns ratherthan less than a full turn. Some multiturnpotentiometers have a linear resistive elementwith a slider which moves along it moved bya worm gear; others have a helical resistiveelement and a wiper that turns through 10, 20,or more complete revolutions, moving alongthe helix as it rotates. Multiturnpotentiometers, both user-accessible andpreset, allow finer adjustments; rotation
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through the same angle changes the settingby typically a tenth as much as for a simplerotary potentiometer.

A string potentiometer is a multi-turnpotentiometer operated by an attached reel ofwire turning against a spring, enabling it toconvert linear position to a variableresistance.

PCB mount trimmer potentiometers, or"trimpots", intended for infrequent adjustment.

Potentiometer construction

Potentiometers comprise a resistive element,a sliding contact (wiper) that moves along theelement, making good electrical contact withone part of it, electrical terminals at each endof the element, a mechanism that moves thewiper from one end to the other, and a

housing containing the element and wiper.
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The resistive element of inexpensivepotentiometers is often made of graphite.Other materials used include resistance wire,carbon particles in plastic, and aceramic/metal mixture called cermet.Conductive track potentiometers useconductive polymer resistor pastes thatcontain hard-wearing resins and polymers,

solvents, and lubricant, in addition to thecarbon that provides the conductiveproperties.log audio equipment. Others areenclosed within the equipment and areintended to be adjusted to calibrateequipment during manufacture or repair, and

not otherwise touched. They are usuallyphysically much smaller than user-accessiblepotentiometers, and may need to be operatedby a screwdriver rather than having a knob.They are usually called "presetpotentiometers". Some presets are accessible

by a small screwdriver poked through a holein the case to allow servicing withoutdismantling.

[edit]Resistanceposition relationship:"taper"

The relationship between slider position andresistance, known as the "taper" or "law", is
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controlled by the manufacturer. In principleany relationship is possible, but for mostpurposes linear orlogarithmic (aka "audiotaper") potentiometers are sufficient. A lettercode ("A" taper, "B" taper, etc.) may be usedto identify which taper is used, but the lettercode definitions are not standardised.

[edit]Linear taper potentiometer

A linear taper potentiometer(lineardescribesthe electrical characteristic of the device, notthe geometry of the resistive element) has aresistive element of constant cross-section,resulting in a device where the resistancebetween the contact (wiper) and one end

terminal is proportional to the distancebetween them. Linear taper potentiometersare used when the division ratio of thepotentiometer must be proportional to theangle of shaft rotation (or slider position), forexample, controls used for adjusting the

centering of an analog cathode-ray oscilloscope.

[edit]Logarithmic potentiometer

A logarithmic taper potentiometerhas aresistive element that either 'tapers' in from

one end to the other, or is made from a
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material whose resistivity varies from one endto the other. This results in a device whereoutput voltage is a logarithmic function of theslider position.

Most (cheaper) "log" potentiometers are notaccurately logarithmic, but use two regions ofdifferent resistance (but constant resistivity) toapproximate a logarithmic law. A logarithmic

potentiometer can also be simulated (not veryaccurately) with a linear one and an externalresistor. True logarithmic potentiometers aresignificantly more expensive.

Logarithmic taper potentiometers are oftenused in connection with audio amplifiers as

human perception of audio volume islogarithmic.

A high power wirewound potentiometer. Anypotentiometer may be connected as arheostat.

[edit]Rheostat
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The most common way to vary the resistancein a circuit is to use a rheostat,[2] a two-terminal variable resistor. For low-powerapplications (less than about 1 watt) a three-terminal potentiometer is often used, with oneterminal unconnected or connected to thewiper.

Where the rheostat must be rated for higher

power (more than about 1 watt), they may bebuilt with a resistance wire wound around asemicircular insulator, with the wiper slidingfrom one turn of the wire to the next.Sometimes a rheostat is made fromresistance wire wound on a heat-resisting

cylinder, with the slider made from a numberof metal fingers that grip lightly onto a smallportion of the turns of resistance wire. The"fingers" can be moved along the coil ofresistance wire by a sliding knob thuschanging the "tapping" point. Wire-wound

rheostats made with ratings up to severalthousand watts are used in applications suchas DC motor drives, electric welding controls,or in the controls for generators.

[edit]Digital potentiometer
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A digital potentiometer is an electroniccomponent that mimics the functions ofanalog potentiometers. Through digital inputsignals, the resistance between two terminalscan be adjusted, just as in an analogpotentiometer.

[edit]Membrane potentiometer

A membrane potentiometer uses a conductive

membrane that is deformed by a slidingelement to contact a resistor voltage divider.Linearity can range from 0.5% to 5%depending on the material, design andmanufacturing process. The repeat accuracyis typically between 0.1mm and 1.0mm with atheoretically infinite resolution. The service lifeof these types of potentiometers is typically 1million to 20 million cycles depending on thematerials used during manufacturing and theactuation method; contact and contactless(magnetic) methods are available. Manydifferent material variations are available suchas PET(foil), FR4, and Kapton. Membranepotentiometer manufacturers offer linear,rotary, and application-specific variations. Thelinear versions can range from 9mm to

1000mm in length and the rotary versions
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range from 0 to multiple full turns, with eachhaving a height of 0.5mm. Membranepotentiometers can be used for positionsensing.[3]

[edit]Potentiometer applications

Preset potentiometers are widely usedthroughout electronics wherever adjustmentsmust be made during manufacturing or

servicing.

User-actuated potentiometers are widely usedas user controls, and may control a very widevariety of equipment functions. Thewidespread use of potentiometers in

consumer electronics declined in the 1990s,with digital controls now more common.However they remain in many applications,such as volume controls and as positionsensors.

[edit]Audio control

Linear potentiometers ("faders")
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Low-power potentiometers, both linear androtary, are used to control audio equipment,changing loudness, frequency attenuation andother characteristics of audio signals.

The 'log pot' is used as the volume controlinaudio amplifiers, where it is also called an"audio taper pot", becausethe amplitude response of the humanear is

approximately logarithmic. It ensures that on avolume control marked 0 to 10, for example, asetting of 5 sounds subjectively half as loudas a setting of 10. There is also an anti-logpotor reverse audio taperwhich is simply thereverse of a logarithmic potentiometer. It is

almost always used in a ganged configurationwith a logarithmic potentiometer, for instance,in an audio balance control.

Potentiometers used in combination with filternetworks act as tone controls or equalizers.

[edit]TelevisionPotentiometers were formerly used to controlpicture brightness, contrast, and colorresponse. A potentiometer was often used toadjust "vertical hold", which affected thesynchronization between the receiver's

internal sweep circuit (sometimes
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a multivibrator) and the received picturesignal, along with other things such as audio-video carrier offset, tuning frequency (forpush-button sets) and so on.

[edit]Transducers

Potentiometers are also very widely used as apart of displacementtransducers because ofthe simplicity of construction and because

they can give a large output signal.[edit]Computation

In analog computers, high precisionpotentiometers are used to scale intermediateresults by desired constant factors, or to

set initial conditions for a calculation. A motor-driven potentiometer may be used asa function generator, using a non-linearresistance card to supply approximations totrigonometric functions. For example, theshaft rotation might represent an angle, and

the voltage division ratio can be madeproportional to the cosine of the angle.

[edit]Theory of operation
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A potentiometer with a resistive load, showingequivalent fixed resistors for clarity.

The potentiometer can be used as a voltagedividerto obtain a manually adjustable outputvoltage at the slider (wiper) from a fixed inputvoltage applied across the two ends of thepotentiometer. This is the most common useof them.

The voltage across can be calculated by:

If is large compared to the otherresistances (like the input to an operationalamplifier), the output voltage can be

approximated by the simpler equation:

(dividing throughout by and cancellingterms with as denominator)

As an example, assume

, , , and
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Since the load resistance is largecompared to the other resistances, theoutput voltage will be approximately:

Due to the load resistance, however, itwill actually be slightly lower: 6.623V.

One of the advantages of the potentialdivider compared to a variable resistorin series with the source is that, whilevariable resistors have a maximumresistance where somecurrent willalways flow, dividers are able to vary

the output voltage from maximum ( )to ground(zero volts) as the wipermoves from one end of thepotentiometer to the other. There is,however, always a small amountof contact resistance.

In addition, the load resistance isoften not known and therefore simplyplacing a variable resistor in serieswith the load could have a negligibleeffect or an excessive effect,depending on the load.
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