Potentiometer

A potentiometer is a variable tapped resistor that can be used as a voltage divider. In Electronics, a voltage divider (also known as a potential divider) is a simple Linear circuit that produces an output Voltage ( The same term is applied both to an electrical component and to a measuring instrument.

As an electrical component, a potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider ^[1]. An electronic component is a basic electronic element usually packaged in a discrete form with two or more connecting leads or metallic pads If all three terminals are used, it can act as a variable voltage divider. In Electronics, a voltage divider (also known as a potential divider) is a simple Linear circuit that produces an output Voltage ( If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used as controls for electrical devices such as a volume control of a radio. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick. A transducer is a device usually electrical, electronic, Electro-mechanical, Electromagnetic, Photonic, or Photovoltaic 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

A potentiometer instrument for measuring the potential (or voltage) in a circuit taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer. The Mathematical study of potentials is known as Potential theory; it is the study of Harmonic functions on Manifolds This mathematical A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. The sliding tap of the potentiometer is adjusted and the galvanometer briefly connected to both the sliding tap and the unknown potential; the deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact. This null balance method is a fundamental technique of electrical metrology. Metrology (from Ancient Greek metron (measure and logos (study of is the Science of Measurement.

Potentiometer as measuring instrument

Schematic symbol for a potentiometer. The arrow represents the moving terminal, called the wiper.

Before the introduction of the moving coil galvanometer, potentiometers were used in measuring voltage, hence the '-meter' part of their name. A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. Today this method is still important in standards work. Metrology (from Ancient Greek metron (measure and logos (study of is the Science of Measurement. The null-balance principle of measurement is also used in other areas of electronics.

The potentiometer used for measurement is a type of bridge circuit for measuring voltages by comparison between a small fraction of the voltage which could be precisely measured, then balancing the two circuits to get null current flow which could be precisely measured. A bridge circuit is a type of Electrical circuit in which the current in a conductor splits into two parallel paths and then recombines into a single conductor Measurement potentiometers are divided into four main classes listed below.

Constant current potentiometer

This is used for measuring voltages below 1. 5 volts. In this circuit, the unknown voltage is connected across a section of resistance wire the ends of which are connected to a standard electrochemical cell that provides a constant current through the wire, The unknown emf, in series with a galvanometer, is then connected across a variable-length section of the resistance wire using a sliding contact(s). An electrochemical cell is a device used for generating an Electromotive force ( Voltage) and current from chemical reactions. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. The sliding contact is moved until no current flows into or out of the standard cell, as indicated by a galvanometer in series with the unknown emf. A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. The voltage across the selected section of wire is then equal to the unknown voltage. All that remains is to calculate the unknown voltage from the current and the fraction of the length of the resistance wire that was connected to the unknown emf. The galvanometer does not need to be calibrated, as its only function is to read zero. When the galvanometer reads zero, no current is drawn from the unknown electromotive force and so the reading is independent of the source's internal resistance.

Because the resistance wire can be made very uniform in cross-section and resistivity, and the position of the wiper can be measured easily, this method can be analyzed to accurately determine the uncertainties in the measurement. When measuring potentials larger than that produced by a standard cell, an external voltage divider is used to scale the measured voltage down to approximately 1 volt for measurement by the potentiometer; the uncertainties due to the voltage divider construction and the load placed on the source by the voltage divider then become part of the uncertainty of the overall measurement.

Constant resistance potentiometer

The constant resistance potentiometer is a variation of the basic idea in which a variable current is fed through a fixed resistor. These are used primarily for measurements in the millivolt and microvolt range.

Microvolt potentiometer

This is a form of the constant resistance potentiometer described above but designed to minimize the effects of contact resistance and thermal emf. This equipment is satisfactorily used down to readings of 1000 nV or so.

Thermocouple potentiometer

Another development of the standard types was the 'thermocouple potentiometer' especially adapted for temperature measurement with thermocouples. In Electrical engineering and industry thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal Potential [1] Potentiometers for use with thermocouples also measure the temperature at which the thermocouple wires are connected, so that cold-junction compensation may be applied to correct the apparent measured EMF to the standard cold-junction temperature of 0 degrees C.

Potentiometer as electronic component

Construction of a wire-wound circular potentiometer. The resistive element (1) of the shown device is trapezoidal, giving a non-linear relationship between resistance and turn angle. The wiper (3) rotates with the axis (4), providing the changeable resistance between the wiper contact (6) and the fixed contacts (5) and (9). The vertical position of the axis is fixed in the body (2) with the ring (7) (below) and the bolt (8) (above).

A potentiometer is a three terminal resistor where the position of the sliding connection is continuously adjustable. |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | Potentiometers are rarely used to directly control significant power (more than a watt). The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. Instead they are used to adjust the level of analog signals (e. g. volume controls on audio equipment), and as control inputs for electronic circuits. Loudness is the quality of a Sound that is the primary psychological correlate of physical strength (amplitude A piece of audio equipment is any device designed principally to reproduce record or process Sound. For example, a light dimmer uses a potentiometer to control the switching of a triac and so indirectly control the brightness of lamps. A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two Silicon-controlled rectifiers ( SCRs

Potentiometers are sometimes provided with one or more switches mounted on the same shaft. For instance, when attached to a volume control, the knob can also function as an on/off switch at the lowest volume.

Construction of potentiometers

A typical single turn potentiometer

A potentiometer is constructed using a flat semi-circular graphite resistive element, with a sliding contact (wiper). The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. The wiper is connected through another sliding contact to the third terminal. On panel pots, the wiper is usually the centre terminal. For single turn pots, this wiper typically travels just under one revolution around the contact. 'Multiturn' potentiometers also exist, where the resistor element may be helical and the wiper may move 10, 20, or more complete revolutions. A helix (pl helixes or helices) from the Greek word έλιξ, is a special kind of Space curve, i Besides graphite, other materials may be used to make the resistive element. These may be resistance wire, or carbon particles in plastic, or a ceramic/metal mixture called cermet. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 A cermet is a Composite material composed of Ceramic (cer and Metallic (met materials

One form of rotary potentiometer is called a 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 It is a multi-turn potentiometer operated by an attached reel of wire turning against a spring. It is used as a position transducer.

In a linear slider pot, 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. Because of the large opening for the wiper and knob, this type of pot has a greater potential for getting contaminated.

Potentiometers can be obtained with either linear or logarithmic relations between the slider position and the resistance (potentiometer laws or "tapers"). The word linear comes from the Latin word linearis, which means created by lines. In Mathematics, the logarithm of a number to a given base is the power or Exponent to which the base must be raised in order to produce

PCB mount trimmer potentiometers, or "trimpots", intended for infrequent adjustment. A trimmer is a miniature adjustable electrical component It is meant to be set correctly when installed in some device and never seen or adjusted by the device's user

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 to the distance between them. This article is about proportionality the mathematical relation Linear taper 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 and the division ratio of the potentiometer; for example, controls used for adjusting the centering of (an analog) cathode-ray oscilliscope.

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 (or inverse logarithmic depending on type) function of the mechanical angle of the pot.

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

Logarithmic taper potentiometers are often used in connection with audio amplifiers.

A high power toroidal wirewound rheostat.

Rheostats

A rheostat is a two-terminal variable resistor. Often these are designed to handle much higher voltage and current. Typically these are constructed as a resistive wire wrapped to form a toroid 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 not connecting to the 3rd terminal. 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 control

Digitally Controlled Potentiometers (DCP's), or simply Digital Potentiometers can be used in analog signal processing circuits to replace potentiometers. A digital potentiometer is a digitally-controlled electronic component that mimics the analog functions of a Potentiometer. Signal processing is the analysis interpretation and manipulation of signals Signals of interest include sound, images, biological signals such as They allow small adjustments to be made to the circuit by software, instead of a mechanical adjustment. It is usually controlled via a serial interface, like I²C or SPI. In Telecommunication and Computer science, serial communication is the process of sending data one Bit at one time sequentially over a Communication I²C ( Inter-Integrated Circuit) is a multi-master serial Computer bus invented by Philips that is used to attach low-speed peripherals Interface The SPI bus specifies four logic signals SCLK &mdash Serial Clock (output from master MOSI/SIMO &mdash Master Output Slave Input (output Some types have non-volatile memory to enable them to retain their last settings before power loss when the power is restored.

The same idea can be used to create Digital Volume Controls, attenuators, or other controls under digital control. An attenuator is an Electronic device that reduces the Amplitude or power of a signal without appreciably distorting its Usually such devices feature quite a high degree of accuracy, and find applications in instrumentation, mixing consoles and other precision systems. In professional audio, a mixing console, or audio mixer, also called a sound board or soundboard, is an electronic device for combining

The DCP should not be confused with the digital to analog converter (DAC) which actually creates an analogue signal from a digital one. In Electronics, a digital-to-analog converter ( DAC or D-to-A) is a device for converting a digital (usually binary code to an Analog signal A DCP only controls an existing analogue signal digitally. However, some DACs using resistive R-2R architecture have been functionally used as DCPs where the (varying) analog signal is input to the reference voltage pin of the DAC and the digitally-controlled attenuated output is taken from the output of the DAC.

Applications of potentiometers

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 has declined in the 1990s, with digital controls now more common. However they remain in many application, such as volume controls and as position sensors.

Audio control

Sliding potentiometers ("faders")

One of the most common uses for modern low-power potentiometers is as audio control devices. Both sliding pots (also known as faders) and rotary potentiometers (commonly called knobs) 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 amplifiers, where it is also called an "audio taper pot", because the amplitude response of the human ear is also logarithmic. An audio amplifier is an Electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 Hertz to Amplitude is the magnitude of change in the oscillating variable with each Oscillation, within an oscillating system The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations 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 log pot. It is almost always used in a ganged configuration with a log pot, for instance, in an audio balance control.

Potentiometers used in combination with filter networks act as tone controls or equalizers. An equalization ( EQ) filter is a filter, usually adjustable chiefly meant to compensate for the unequal Frequency response of some other

Television

Potentiometers were formerly used to control picture brightness, contrast, and (in NTSC receivers) color response. NTSC ( National Television System Committee) is the Analog television system used in the United States, Canada, Japan, Mexico A potentiometer was often used to adjust "vertical hold", which affected the synchronization between the receiver's internal sweep circuit (sometimes a multivibrator) and the received picture signal. A multivibrator is an Electronic circuit used to implement a variety of simple two-state systems such as Oscillators Timers and flip-flops It

Transducers

Potentiometers are also very widely used as a part of displacement transducers because of the simplicity of construction and because they can give a large output signal. In Physics, displacement is the vector that specifies the position of a point or a particle in reference to a previous position or to the origin of the chosen A transducer is a device usually electrical, electronic, Electro-mechanical, Electromagnetic, Photonic, or Photovoltaic

Theory of operation

A potentiometer with a resistive load, showing equivalent fixed resistors for clarity.

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 pot. In Electronics, a voltage divider (also known as a potential divider) is a simple Linear circuit that produces an output Voltage ( This is the most common use of pots.

The voltage across $R L$ is determined by the formula:

$V_\mathrm{L} = { R_2 \| R_\mathrm{L} \over R_1 + R_2 \| R_\mathrm{L}} \cdot V_s$

The parallel lines indicate components in parallel. If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. Expanded fully, the equation becomes:

$V_\mathrm{L} = { R_2 R_\mathrm{L} \over R_1 R_\mathrm{L} + R_2 R_\mathrm{L} + R_1 R_2}\cdot V_s$

Although it is not always the case, if $R L$ is large compared to the other resistances (like the input to an operational amplifier), the output voltage can be approximated by the simpler equation:

$V_\mathrm{L} = { R_2 \over R_1 + R_2 }\cdot V_s$

As an example, assume

$V_\mathrm{S} = 10\ \mathrm{V}$ , $R_1 = 1\ \mathrm{k \Omega}$ , $R_2 = 2\ \mathrm{k \Omega}$ , and $R_\mathrm{L} = 100\ \mathrm{k \Omega}$ . An operational amplifier, often called an op-amp, is a DC - coupled high- Gain electronic voltage amplifier with differential

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

${2\ \mathrm{k \Omega} \over 1\ \mathrm{k \Omega} + 2\ \mathrm{k \Omega} } \cdot 10\ \mathrm{V} = {2 \over 3} \cdot 10\ \mathrm{V} \approx 6.667\ \mathrm{V}$

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 ( $V S$ ) to ground (zero volts) as the wiper moves from one end of the pot to the other. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. In Electrical engineering, the term ground or earth has several meanings depending on the specific application areas There is, however, always a small amount of 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

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.

Physics demonstration

The determination of the emf of a cell using a potentiometer is a classic experiment which is sometimes done at A-level in physics. Electromotive force ( emf, \mathcal{E} is a term used to characterize electrical devices such as Voltaic cells thermoelectric devices electrical The A-level, short for Advanced Level, is a General Certificate of Education qualification in England, Northern Ireland and Wales, Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. This experiment has the advantage that the emf of a cell can be measured without drawing any current from the cell, in short the measurement can be done as if by a voltmeter of infinite resistance. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. A voltmeter is an instrument used for measuring the Electrical potential difference between two points in an electric circuit Infinity (symbolically represented with ∞) comes from the Latin infinitas or "unboundedness

If a potentiometer is formed from a length (AB) of uniform resistance wire attached to a DC source such as a Lead-acid battery, then a standard cell whose emf is known (eg 1. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Direct current ( DC) is the unidirectional flow of Electric charge. Lead-acid batteries, invented in 1859 by French Physicist Gaston Planté, are the oldest type of Rechargeable battery. 0183 volts for a weston standard cell)[2][3] can be used to calibrate the potentiometer. The Weston cell, invented by Edward Weston in 1893 is a wet-chemical cell that produces a highly stable Voltage suitable as a laboratory standard Calibration is the process of establishing the relationship between a measuring device and the units of measure

The standard cell is wired in series with a galvanometer between A and a movable probe on the resistance wire, the galvanometer will give a zero reading at point X. A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. Then distance AX is measured. The experiment should be repeated to find point X' where a zero current reading is obtained for the unknown cell at distance AX'.

Then the emf of the unknown cell can be calculated using:

E = E^{standard cell} (AX'/AX)

Early patents

Carbon track potentiometer/rheostat, Thomas Edison, 1872
Mary Hallock-Greenewalt invented a type of nonlinear rheostat for use in her visual-music instrument the Sarabet (US Patent 1,357,773, 1920)

External links

^ IEEE Std. Year 1872 ( MDCCCLXXII) was a Leap year starting on Monday (link will display the full calendar of the Gregorian Calendar (or a Leap year Mary Hallock-Greenewalt (1870 - 1951 was an inventor and pianist who performed with the Philadelphia and Pittsburgh symphonies as a soloist. Year 1920 ( MCMXX) was a Leap year starting on Thursday (link will display 1920 of the Gregorian calendar A potentiometric sensor is a type of chemical sensor that may be used to determine the analytical concentration of some components of the analyte gas or Solution. A digital potentiometer is a digitally-controlled electronic component that mimics the analog functions of a Potentiometer. A potentiometer is a three-terminal Resistor with a sliding contact that forms an adjustable Voltage divider. A trimmer is a miniature adjustable electrical component It is meant to be set correctly when installed in some device and never seen or adjusted by the device's user 100 The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition, IEEE Press, Piscataway New Jersey, 2000 ISBN 0-7381-2601-2

Dictionary

-noun

(electronics) a user-adjustable variable resistor that functions as a voltage divider.
(physics) an instrument that measures a voltage by opposing it with a precise fraction of a known voltage, and without drawing current from the unknown source.

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