Potentiometer
Overview
 
A potentiometer informally, a pot, is a three-terminal
Terminal (electronics)
A terminal is the point at which a conductor from an electrical component, device or network comes to an end and provides a point of connection to external circuits. A terminal may simply be the end of a wire or it may be fitted with a connector or fastener...

 resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

 with a sliding contact that forms an adjustable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

s, for example, in a joystick
Joystick
A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. Joysticks, also known as 'control columns', are the principal control in the cockpit of many civilian and military aircraft, either as a center stick or...

.

Potentiometers are rarely used to directly control significant power (more than a watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

), since the power dissipated in the potentiometer would be comparable to the power in the controlled load (see infinite switch
Infinite switch
An infinite switch is a type of switch that allows variable power output and is so called because its average output is infinitely variable rather than being limited to a few switched levels. They utilize a bi-metallic strip conductive connection across terminals that disconnects with increased...

).
Encyclopedia
A potentiometer informally, a pot, is a three-terminal
Terminal (electronics)
A terminal is the point at which a conductor from an electrical component, device or network comes to an end and provides a point of connection to external circuits. A terminal may simply be the end of a wire or it may be fitted with a connector or fastener...

 resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

 with a sliding contact that forms an adjustable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

s, for example, in a joystick
Joystick
A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. Joysticks, also known as 'control columns', are the principal control in the cockpit of many civilian and military aircraft, either as a center stick or...

.

Potentiometers are rarely used to directly control significant power (more than a watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

), since the power dissipated in the potentiometer would be comparable to the power in the controlled load (see infinite switch
Infinite switch
An infinite switch is a type of switch that allows variable power output and is so called because its average output is infinitely variable rather than being limited to a few switched levels. They utilize a bi-metallic strip conductive connection across terminals that disconnects with increased...

). Instead they are used to adjust the level of analog signals (e.g. volume
Loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...

 controls on audio equipment
Audio equipment
A piece of audio equipment is any device designed principally to reproduce, record or process sound. This includes microphones, radio receivers, AV receivers, CD players, tape recorders, amplifiers, mixing consoles, effects units and loudspeakers....

), and as control inputs for electronic circuits. For example, a light dimmer
Dimmer
Dimmers are devices used to vary the brightness of a light. By decreasing or increasing the RMS voltage and, hence, the mean power to the lamp, it is possible to vary the intensity of the light output...

 uses a potentiometer to control the switching of a TRIAC
TRIAC
TRIAC, from Triode for Alternating Current, is a genericized tradename for an electronic component that can conduct current in either direction when it is triggered , and is formally called a bidirectional triode thyristor or bilateral triode thyristor.TRIACs belong to the thyristor family and are...

 and so indirectly control the brightness of lamps.

Potentiometer construction

A potentiometer is constructed with a resistive element formed into an arc of a circle, and a sliding contact (wiper) travelling over that arc. The resistive element, with a terminal at one or both ends, is flat or angled, and is commonly made of graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

, although other materials may be used. The wiper is connected through another sliding contact to another terminal. On panel potentiometers, the wiper is usually the center terminal of three. For single-turn potentiometers, this wiper typically travels just under one revolution around the contact. "Multiturn" potentiometers are usually constructed of a conventional resistive element wiped through a worm gear
Worm drive
A worm drive is a gear arrangement in which a worm meshes with a worm gear...

, although other types exist with a helical resistive element and a wiper that turns through 10, 20, or more complete revolutions. Besides graphite, materials used to make the resistive element include resistance wire
Resistance wire
Resistance wire is electrical wire used for its property of electrical resistance. It is routinely used at high temperatures, so normally also has high melting point....

, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 particles in plastic, and a ceramic/metal mixture called cermet
Cermet
A cermet is a composite material composed of ceramic and metallic materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used...

.

A string potentiometer
String potentiometer
A string potentiometer is a transducer used to detect and measure linear position and velocity using a flexible cable and spring-loaded spool. Other common names include "string pot", "cable-extension transducer", "draw wire sensor", and "yo-yo sensor"....

 is a multi-turn potentiometer operated by an attached reel of wire turning against a spring, enabling it to convert linear position to a variable resistance.

In a linear slider potentiometer, a sliding control is provided instead of a dial control. The resistive element is a rectangular strip, not semi-circular as in a rotary potentiometer. Due to the large opening slot for the wiper, this type of potentiometer has a greater potential for getting contaminated.

Conductive track potentiometers use conductive polymer resistor pastes that contain hard-wearing resins and polymers, solvents, lubricant and carbon – the constituent that provides the conductive properties. The tracks are made by screen printing the paste onto a paper-based phenolic substrate and then curing it in an oven. The curing process removes all solvents and allows the conductive polymer to polymerize and cross-link. This produces a durable track with stable electrical resistance throughout its working life.

Laws or tapers

Potentiometers can be obtained with either linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

 or logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

ic relations between the slider position and the resistance (potentiometer laws or "taper
Taper
In cymbal making, taper refers to the gradual change in thickness from the bell to the rim of the cymbal. It is one of the key features that determines the tone of the cymbal....

s"). A letter code ("A" taper, "B" taper, etc.) may be used to identify which taper is used, but the letter code definitions are variable over time and between manufacturers.

Linear taper potentiometer

A linear taper potentiometer has a resistive element of constant cross-section, resulting in a device where the resistance between the contact (wiper) and one end terminal is proportional
Proportionality (mathematics)
In mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...

 to the distance between them. Linear describes the electrical characteristic of the device, not the geometry of the resistive element. Linear taper potentiometers are used when an approximately proportional relation is desired between shaft rotation (or slider position) and the division ratio of the potentiometer; for example, controls used for adjusting the centering of (an analog) cathode-ray oscilloscope.

Logarithmic potentiometer

A logarithmic taper potentiometer has a resistive element that either 'tapers' in from one end to the other, or is made from a material whose resistivity varies from one end to the other. This results in a device where output voltage is a logarithmic function of the mechanical angle of the potentiometer.

Most (cheaper) "log" potentiometers are actually not logarithmic, but use two regions of different resistance (but constant resistivity) to approximate a logarithmic law. A logarithmic potentiometer can also be simulated (not very accurately) with a linear one and an external resistor. True logarithmic potentiometers are significantly more expensive.

Logarithmic taper potentiometers are often used in connection with audio amplifiers as human perception of audio volume is logarithmic.

Rheostat

The most common way to vary the resistance in a circuit is to use a rheostat, a two-terminal variable resistor. Rheostats are usually designed to handle much higher voltage, current and power than potentiometers. Typically they are constructed as a resistive wire wrapped to form a toroid
Toroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....

al coil with the wiper moving over the upper surface of the toroid, sliding from one turn of the wire to the next. Sometimes a rheostat is made from resistance wire wound on a heat-resisting cylinder, with the slider made from a number of metal fingers that grip lightly onto a small portion of the turns of resistance wire. The "fingers" can be moved along the coil of resistance wire by a sliding knob thus changing the "tapping" point. They are usually used as variable resistors rather than variable potential dividers.

Any three-terminal potentiometer can be used as a two-terminal variable resistor by leaving one of the end terminals disconnected. However, it is common practice to connect the wiper terminal to the unused end of the resistance track to reduce the amount of resistance variation caused by dirt on the track.

Digital potentiometer

A digital potentiometer is an electronic component that mimics the functions of analog potentiometers. Through digital input signals, the resistance between two terminals can be adjusted, just as in an analog potentiometer.

Membrane Potentiometer

A membrane potentiometer uses a conductive membrane that is deformed by a sliding element to contact a resistor voltage divider. Linearity can range from 0.5% to 5% depending on the material, design and manufacturing process. The repeat accuracy is typically between 0.1mm and 1.0mm with a theoretically infinite resolution. The service life of these types of potentiometers is typically 1 million to 20 million cycles depending on the materials used during manufacturing and the actuation method; contact and contactless (magnetic) methods are available. Many different material variations are available such as PET(foil)
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...

, FR4, and Kapton. Membrane potentiometer manuafacturers offer linear, rotary, and application-specific variations. The linear versions can range from 9mm to 1000mm in length and the rotary versions range from 0° to 360°(multi-turn), with each having a height of 0.5mm. Membrane potentiometers can be used for position sensing.

Potentiometer applications

Potentiometers are widely used as user controls, and may control a very wide variety of equipment functions. The widespread 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 position sensors.

Audio control

One of the most common uses for modern low-power potentiometers is as audio control devices. Both linear potentiometers and rotary potentiometers are regularly used to adjust loudness, frequency attenuation and other characteristics of audio signals.

The 'log pot' is used as the volume control in audio amplifier
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...

s, where it is also called an "audio taper pot", because the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

 response of the human ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

 is also logarithmic. It ensures that, on a volume control marked 0 to 10, for example, a setting of 5 sounds half as loud as a setting of 10. There is also an anti-log pot or reverse audio taper which is simply the reverse of a logarithmic potentiometer. It is almost always used in a ganged configuration with a logarithmic potentiometer, for instance, in an audio balance control.

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

Television

Potentiometers were formerly used to control picture brightness, contrast, and color response. A potentiometer was often used to adjust "vertical hold", which affected the synchronization between the receiver's internal sweep circuit (sometimes a multivibrator
Multivibrator
A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices cross-coupled by resistors or capacitors...

) and the received picture signal, along with other things such as audio-video carrier offset, tuning frequency (for push-button sets) and so on.

Transducers

Potentiometers are also very widely used as a part of displacement
Displacement (vector)
A displacement is the shortest distance from the initial to the final position of a point P. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by P...

 transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

s because of the simplicity of construction and because they can give a large output signal.

Computation

In analog computer
Analog computer
An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...

s, high precision potentiometers are used to scale intermediate results by desired constant factors, or to set initial conditions for a calculation. A motor-driven potentiometer may be used as a function generator
Function generator
A function generator is a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. These waveforms can be either repetitive or single-shot, in which case some kind of triggering source is required...

, using a non-linear resistance card to supply approximations to trigonometric functions. For example, the shaft rotation might represent an angle, and the voltage division ratio can be made proportional to the cosine of the angle.

Theory of operation

The potentiometer can be used as a voltage divider to obtain a manually adjustable output voltage at the slider (wiper) from a fixed input voltage applied across the two ends of the potentiometer. This is the most common use of them.

The voltage across can be calculated by:

If is large compared to the other resistances (like the input to an operational amplifier
Operational amplifier
An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output...

), the output voltage can be approximated by the simpler equation:
(dividing throughout by and cancelling terms with as denominator)

As an example, assume, , , and

Since the load resistance is large compared to the other resistances, the output voltage will be approximately:

Due to the load resistance, however, it will actually be slightly lower: ≈ 6.623 V.

One of the advantages of the potential divider compared to a variable resistor in series with the source is that, while variable resistors have a maximum resistance where some current will always flow, dividers are able to vary the output voltage from maximum () to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 (zero volts) as the wiper moves from one end of the potentiometer to the other. There is, however, always a small amount of contact resistance
Contact resistance
The term contact resistance refers to the contribution to the total resistance of a material which comes from the electrical leads and connections as opposed to the intrinsic resistance, which is an inherent property, independent of the measurement method...

.

In addition, the load resistance is often not known and therefore simply placing a variable resistor in series with the load could have a negligible effect or an excessive effect, depending on the load.

Early patents

  • Mary Hallock-Greenewalt
    Mary Hallock-Greenewalt
    Mary Elizabeth Hallock-Greenewalt was an inventor and pianist who performed with the Philadelphia and Pittsburgh symphonies as a soloist...

     invented a type of nonlinear rheostat for use in her visual-music instrument, the Sarabet

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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