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Encyclopedia > Potentiometer

A potentiometer is a variable resistor that can be used as a voltage divider. Image File history File links Mergefrom. ... It has been suggested that this article or section be merged into Potentiometer. ... In electronics, a voltage divider is a simple device designed to create a voltage (Vout) which is proportional to another voltage (Vin). ...


Originally a potentiometer was an instrument to measure the potential (or voltage) in a circuit by tapping off a fraction of a known voltage from a resistive slide wire and comparing it with the unknown voltage by means of a galvanometer. In physics, a potential may refer to the scalar potential or to the vector potential. ... It has been suggested that Tangent galvanometer be merged into this article or section. ...


The present popular usage of the term potentiometer (or 'pot' for short) describes an electrical device which has a user-adjustable resistance. Usually, this is a three-terminal resistor with a sliding contact in the center (the wiper). If all three terminals are used, it can act as a variable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor. Its shortcoming is that of corrosion or wearing of the sliding contact, especially if it is kept in one position. Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... In electronics, a voltage divider is a simple device designed to create a voltage (Vout) which is proportional to another voltage (Vin). ...

Contents

Potentiometer as measuring instrument

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

The original potentiometer 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. The word itself derives from the phrase "voltage potential," and "potential" was used to refer to "strength." The original potentiometers are divided into four main classes: the constant resistance potentiometer, the constant current potentiometer, the microvolt potentiometer and the thermocouple potentiometer. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... 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, thereby enclosing a loop. ...


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). 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. 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. A demonstration electrochemical cell setup resembling the Daniell cell. ... Electric current is the flow (movement) of electric charge. ... It has been suggested that Tangent galvanometer be merged into this article or section. ...


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 10 nV or so.


Thermocouple potentiometer

Another development of the standard types was the 'thermocouple potentiometer' especially modified for performing temperature measurements with thermocouples. [1]


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).

In modern usage, a potentiometer is a potential divider, a three terminal resistor where the position of the sliding connection is user adjustable via a knob or slider. 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. Image File history File linksMetadata Download high resolution version (2068x2048, 489 KB) Construction of the rotational variable resistor. ... Image File history File linksMetadata Download high resolution version (2068x2048, 489 KB) Construction of the rotational variable resistor. ... In electronics, a voltage divider or resistor divider is a design technique used to create a voltage (Vout) which is proportional to another voltage (Vin). ... Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ...


Ordinary potentiometers are rarely used to control anything of significant power (even lighting) directly due to resistive losses, but they are frequently used to adjust the level of analog signals (e.g. volume controls on audio equipment) and as control inputs for electronic circuits (e.g. a typical domestic light dimmer uses a potentiometer to set the point in the cycle at which the triac turns on). Potentiometers used to control high power are normally called rheostats. The horizontal axis shows frequency in Hz Loudness is the quality of a sound that is the primary psychological correlate of physical intensity. ... The term Audio equipment refers to any device designed principally to reproduce broadcast or recorded sounds. ... Triac Schematic Symbol A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. ...


Types of potentiometers

Low-power types

A typical single turn potentiometer

A potentiometer is constructed using a flat graphite annulus (ring) as the resistive element, with a sliding contact (wiper) sliding around this annulus. The wiper is connected to an axle and, via another rotating contact, is brought out as 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. In addition to graphite, other materials may be used for the resistive element. These may be resistance wire or carbon particles in plastic or a ceramic/metal mixture called cermet. One popular form of rotary potentiometer is called a string pot. It is a multi-turn potentiometer with an attached reel of wire turning against a spring. It's very convenient for measuring movement and therefore acts as a position transducer. In a linear slider pot, a sliding control is provided instead of a dial control. The word linear also describes the geometry of the resistive element which is a rectangular strip, (not an annulus as in a rotary potentiometer). Because of their construction, this type of pot has a greater potential for getting contaminated. Potentiometers can be obtained with either linear or logarithmic laws (or "tapers"). picture of a single term potentiometer This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... picture of a single term potentiometer This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... For other uses, see Graphite (disambiguation). ... This article is about the shape. ... General Name, symbol, number carbon, C, 6 Chemical series nonmetals Group, period, block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ... A Cermet is a composite material composed of ceramic (cer) and metallic (met) materials. ... An annulus In mathematics, an annulus (the Latin word for little ring, with plural annuli) is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. ... The word linear comes from the Latin word linearis, which means created by lines. ... Logarithms to various bases: is to base e, is to base 10, and is to base 1. ... In cymbal making, taper refers to the gradual change in thickness from the bell to the rim of the cymbal. ...

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

Image File history File linksMetadata Download high resolution version (732x771, 87 KB) Variable resistors meant to be mounted on a PCB for infrequent changes; trimmer resistors. Source: Taken by User:Omegatron using a Canon Powershot SD110 File links The following pages on the English Wikipedia link to this file (pages... Image File history File linksMetadata Download high resolution version (732x771, 87 KB) Variable resistors meant to be mounted on a PCB for infrequent changes; trimmer resistors. Source: Taken by User:Omegatron using a Canon Powershot SD110 File links The following pages on the English Wikipedia link to this file (pages... Trimmer potentiometers or trimpots. A trimmer is a miniature adjustable electrical component. ...

Linear potentiometers

A linear pot 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. Linear describes the electrical 'law' of the device, not the geometry of the resistive element. In mathematics, two quantities are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio. ...


Logarithmic potentiometers

A log pot 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.


High-power types

A high power toroidal wirewound rheostat.

A rheostat is essentially a potentiometer, but is usually much larger, designed to handle much higher voltage and current. Typically these are constructed as a resistive wire wrapped to form a toroid coil (or most of one) 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. Image File history File linksMetadata Download high resolution version (2272x1704, 1405 KB) Summary An old wirewound potentiometer manufactered by Ohmite for an X-ray tube power supply. ... Image File history File linksMetadata Download high resolution version (2272x1704, 1405 KB) Summary An old wirewound potentiometer manufactered by Ohmite for an X-ray tube power supply. ... A toroid is a doughnut-shaped object whose surface is a torus. ...


Digital control

Digitally Controlled Potentiometers (DCPs) or digipots can be used in analogue signal processing circuits to replace potentiometers. They allow small adjustments to be made to the circuit by software, instead of a mechanical adjustment. Because this type of control is updated only infrequently, it often has a slow serial interface, like I²C. Some types have non-volatile memory to enable them to remember their last settings when the power is switched off. A digipot is a digital potentiometer. ... Signal processing is the processing, amplification and interpretation of signals, and deals with the analysis and manipulation of signals. ... In telecommunications and computer science, serial communications is the process of sending data one bit at one time, sequentially, over a communications channel or computer bus. ... I²C is a multi-master serial computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. ...


The same idea can be used to create Digital Volume Controls, attenuators, or other controls under digital control. Usually such devices feature quite a high degree of accuracy, and find applications in instrumention, mixing desks and other precision systems. An attenuator is a telecommunication device that reduces the amplitude of a signal without appreciably distorting its waveform. ... BBC Local Radio Mark III radio mixing desk In professional audio, a mixing console, mixing desk (Brit. ...


The DCP should not be confused with the digital to analogue converter (DAC) which actually creates an analogue signal from a digital one. 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) analogue 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. In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analogue signal (current, voltage or charges). ...


Applications of potentiometers

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 Newtonian mechanics, displacement is the vector that specifies the position of a point or a particle in reference to an origin or to a previous position. ... A transducer is a device, usually electrical or electronic, that converts one type of energy to another. ...


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. Image File history File linksMetadata Download high resolution version (1778x1302, 260 KB) Fader potentiometers. ... Image File history File linksMetadata Download high resolution version (1778x1302, 260 KB) Fader potentiometers. ...


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. 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. Mission Cyrus 1 Hi Fi integrated audio amplifier An audio amplifier is an electronic amplifier that works with audio frequencies (generally 20 hertz to 20,000 hertz). ... It has been suggested that pulse amplitude be merged into this article or section. ... For other uses, see Ear (disambiguation). ...


A potentiometer used in combination with an inductor or capacitor acts as a "tone" control. The word tone is used in several different fields with different meanings. ...


Theory of operation

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

The 'modern' potentiometer can be used as a potential divider (or 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. This is the most common use of pots. Image File history File links A potentiometer with a resistive load, with equivalent fixed resistors for clarity. ... In electronics, a voltage divider is a simple device designed to create a voltage (Vout) which is proportional to another voltage (Vin). ...


The voltage across RL is determined by the formula:

The parallel lines indicate components in parallel. Expanded fully, the equation becomes: Parallel Lines, released in 1978, was the third album from the band Blondie, and also their most popular and best selling. ... This article or section does not adequately cite its references or sources. ...

Although it is not always the case, if RL is large compared to the other resistances (like the input to an operational amplifier), the output voltage can be approximated by the simpler equation: A 741 operational amplifier in a TO-5 metal can package An operational amplifier, usually referred to as an op-amp for brevity, is a DC-coupled high-gain electronic voltage amplifier with Differential Inputs and, usually, a single output. ...

As an example, assume

, , , and .

Since the load resistance is large compared to the other resistances, the output voltage VL 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 (VS) to ground (zero volts) as the wiper moves from one end of the pot to the other. There is, however, always a small amount of contact resistance. In electricity, current refers to electric current, which is the flow of electric charge. ... It has been suggested that Ground conductor be merged into this article or section. ...


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

  • 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)

“Edison” redirects here. ... Year 1872 (MDCCCLXXII) was a leap year starting on Monday (link will display the full calendar) of the Gregorian Calendar (or a leap year starting on Saturday of the 12-day slower Julian calendar). ... Mary Hallock-Greenewalt (born Beirut, Syria 1871; died 1950). ... 1920 (MCMXX) was a leap year starting on Thursday. ...

See also

  • Potentiometric sensor
  • Digipot
  • Determining emf of primary cells using potentiometer
  • Trimmer
  • String Potentiometer

a potentiometric sensor is a type of chemical sensor based on the measurement of a potential under no current flow. ... A digipot is a digital potentiometer. ... It has been suggested that this article or section be merged into Potentiometer. ... Trimmer potentiometers or trimpots. A trimmer is a miniature adjustable electrical component. ... A string potentiometer is a transducer used to detect and measure linear position and velocity using a flexible cable and spring-loaded spool. ...

External links


  Results from FactBites:
 
Potentiometer as a voltage divider : DC CIRCUITS (640 words)
Potentiometers are variable voltage dividers with a shaft or slide control for setting the division ratio.
Manufactured potentiometers enclose a resistive strip inside a metal or plastic housing, and provide some kind of mechanism for moving a "wiper" across the length of that resistive strip.
Some rotary potentiometers have a spiral resistive strip, and a wiper that moves axially as it rotates, so as to require multiple turns of the shaft to drive the wiper from one end of the potentiometer's range to the other.
Potentiometer (1139 words)
Potentiometers can be used to allow a change in the resistance in a circuit or as a variable voltage divider (in the case of a volume control).
These are general purpose potentiometers and may be used for controlling DC voltage (as it did in the diagram) or to control the levels of the individual bands on an equalizer.
If you are using the potentiometer to control a signal with significant current flow, you'd have to calculate the power dissipation across its resistive element and use a potentiometer of a sufficient power rating.
  More results at FactBites »

 
 

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