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Potentiometer
This article is about the electrical component. For the measuring instrument, see Potentiometer
(measuring instrument).
Potentiometer
A typical single-turn potentiometer
Type
Passive
Electronic symbol
(International)
(US)
A potentiometer /pɵˌtɛnʃiˈɒmɨtər/, informally a pot, is a three-terminal resistor with a sliding or
rotating contact that forms an adjustable voltage divider.[1] If only two terminals are used, one end
and the wiper, it acts as a variable resistor or rheostat.
The measuring instrument called a potentiometer is essentially a voltage divider used for
measuring electric potential (voltage); the component is an implementation of the same principle,
hence its name.
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 transducers, for
example, in a joystick. Potentiometers are rarely used to directly control significant power (more than
a watt), since the power dissipated in the potentiometer would be comparable to the power in the
controlled load.
Nomenclature
There are a number of terms in the electronics industry used to describe certain types of
potentiometers:
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slide pot or slider pot: a potentiometer that is adjusted by sliding the wiper left or right (or up
and down, depending on the installation), usually with a finger or thumb
thumb pot or thumbwheel pot: a small rotating potentiometer meant to be adjusted
infrequently by means of a small thumbwheel
trimpot or trimmer pot: a trimmer potentiometer typically meant to be adjusted once or
infrequently for "fine-tuning" an electrical signal
Potentiometer construction
Single-turn potentiometer with metal casing removed to expose wiper contacts and resistive track
Potentiometers comprise a resistive element, a sliding contact (wiper) that moves along the element,
making good electrical contact with one part of it, electrical terminals at each end of the element, a
mechanism that moves the wiper from one end to the other, and a housing containing the element
and wiper.
Many inexpensive potentiometers are constructed with a resistive element formed into an arc of a
circle usually a little less than a full turn and a wiper sliding on this element when rotated, making
electrical contact. The resistive element, with a terminal at each end, is flat or angled. The wiper is
connected to a third terminal, usually between the other two. 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. The only point of ingress for contamination is the narrow
space between the shaft and the housing it rotates in.
Another type is the linear slider potentiometer, which has a wiper which slides along a linear element
instead of rotating. Contamination can potentially enter anywhere along the slot the slider moves in,
making effective sealing more difficult and compromising long-term reliability. An advantage of the
slider potentiometer is that the slider position gives a visual indication of its setting. While the setting
of a rotary potentiometer can be seen by the position of a marking on the knob, an array of sliders
can give a visual impression of, for example, the effect of a multi-band equaliser (hence the term
"graphic equaliser").
The resistive element of inexpensive potentiometers is often made of graphite. Other materials used
include resistance wire, carbon particles in plastic, and a ceramic/metal mixture called cermet.
Conductive track potentiometers use conductive polymer resistor pastes that contain hard-wearing
resins and polymers, solvents, and lubricant, in addition to the carbon that provides the conductive
properties.
PCB mount trimmer potentiometers, or "trimpots", intended for infrequent adjustment
Electronic symbol for pre-set potentiometer
Others are enclosed within the equipment and are intended to be adjusted to calibrate equipment
during manufacture or repair, and not otherwise touched. They are usually physically much smaller
than user-accessible potentiometers, and may need to be operated by a screwdriver rather than
having a knob. They are usually called "preset potentiometers" or "trim[ming] pots". Some presets
are accessible by a small screwdriver poked through a hole in the case to allow servicing without
dismantling.
Multiturn potentiometers are also operated by rotating a shaft, but by several turns rather than less
than a full turn. Some multiturn potentiometers have a linear resistive element with a sliding contact
moved by a lead screw; others have a helical resistive element and a wiper that turns through 10,
20, or more complete revolutions, moving along the helix as it rotates. Multiturn potentiometers, both
user-accessible and preset, allow finer adjustments; rotation through the same angle changes the
setting by typically a tenth as much as for a simple rotary potentiometer.
A string potentiometer 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.
User-accessible rotary potentiometers can be fitted with a switch which operates usually at the anticlockwise extreme of rotation. Before digital electronics became the norm such a component was
used to allow radio and television receivers and other equipment to be switched on at minimum
volume with an audible click, then the volume increased, by turning a knob. Multiple resistance
elements can be ganged together with their sliding contacts on the same shaft, for example, in
stereo audio amplifiers for volume control. In other applications, such as domestic light dimmers, the
normal usage pattern is best satisfied if the potentiometer remains set at its current position, so the
switch is operated by a push action, alternately on and off, by axial presses of the knob.
Resistance–position relationship: "taper"
Size scaled 10k and 100k pots that combine traditional mountings and knob shafts with newer and smaller
electrical assemblies. Note the "B" designating a linear taper.
The relationship between slider position and resistance, known as the "taper" or "law", is controlled
by the manufacturer. In principle any relationship is possible, but for most
purposes linear or logarithmic (aka "audio taper") potentiometers are sufficient.
A letter code may be used to identify which taper is used, but the letter code definitions are not
standardized. Newer potentiometers will usually be marked with an 'A' for logarithmic taper or a 'B'
for linear taper. Older potentiometers may be marked with an 'A' for linear taper, a 'C' for logarithmic
taper or an 'F' for anti-logarithmic taper. The code used also varies between different manufacturers.
When a percentage is referenced with a non-linear taper, it relates to the resistance value at the
midpoint of the shaft rotation. A 10% log taper would therefore measure 10% of the total resistance
at the midpoint of the rotation; i.e. 10% log taper on a 10K ohm potentiometer would yield 1K at the
midpoint. The higher the percentage the steeper the log curve[2]
Linear taper potentiometer
A linear taper potentiometer (linear describes the electrical characteristic of the device, not the
geometry of the resistive element) 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 taper potentiometers [3] are used when the division ratio of the
potentiometer must be proportional to the angle of shaft rotation (or slider position), for example,
controls used for adjusting the centering of the display on an analog cathode-ray oscilloscope.
Precision potentiometers have an accurate relationship between resistance and slider position.
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 slider position.[4]
Most (cheaper) "log" potentiometers are not accurately logarithmic, but use two regions of different
resistance (but constant resistivity) to approximate a logarithmic law. The two resistive tracks overlap
at approximately 50% of the potentiometer rotation; this gives a stepwise logarithmic taper.[5] 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
See also: Liquid rheostat
Charles Wheatstone's 1843 rheostat with a metal and a wooden cylinder
Charles Wheatstone's 1843 rheostat with a moving whisker
The most common way to vary the resistance in a circuit is to use a rheostat. The
word rheostat was coined about 1845 by Sir Charles Wheatstone, from the
Greek ῥέος rheos meaning "stream", and -στάτης -states (from ἱστάναι histanai, " to set, to cause to
stand") meaning "setter, regulating device",[6][7][8] which is a two-terminal variable resistor. The term
"rheostat" is becoming obsolete,[9] with the general term "potentiometer" replacing it. For low-power
applications (less than about 1 watt) a three-terminal potentiometer is often used, with one terminal
unconnected or connected to the wiper.
Where the rheostat must be rated for higher power (more than about 1 watt), it may be built with a
resistance wire wound around a semicircular insulator, with the wiper 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. Wire-wound rheostats made with ratings up to several
thousand watts are used in applications such as DC motor drives, electric welding controls, or in the
controls for generators. The rating of the rheostat is given with the full resistance value and the
allowable power dissipation is proportional to the fraction of the total device resistance in circuit.
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Electronic Symbol for rheostat
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Electronic Symbol for pre-set rheostat
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A high power wirewound potentiometer.
Digital potentiometer
Main article: Digital potentiometer
A digital potentiometer (often called digipot) 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. There are two main functional types: volatile, which
lose their set position if power is removed, and are usually designed to initialise at the minimum
position, and non-volatile, which retain their set position using a storage mechanism similar to Flash
memory or EEPROM.
Usage of a digipot is far more complex than that of a simple mechanical potentiometer, and there
are many limitations to observe; nevertheless they are widely used, often for factory adjustment and
calibration of equipment, especially where the limitations of mechanical potentiometers are
problematic. A digipot is generally immune to the effects of moderate long-term mechanical vibration
or environmental contamination, to the same extent as other semiconductor devices, and can be
secured electronically against unauthorised tampering by protecting the access to its programming
inputs by various means.
In equipment which has a microprocessor, FPGA or other functional logic which can store settings
and reload them to the "potentiometer" every time the equipment is powered up, a
multiplying DAC can be used in place of a digipot, and this can offer higher setting resolution, less
drift with temperature, and more operational flexibility.
Membrane potentiometers
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, FR4, and Kapton. Membrane potentiometer manufacturers
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 multiple full turns, with each having a
height of 0.5mm. Membrane potentiometers can be used for position sensing.[10]
For touch-screen devices using resistive technology, a two-dimensional membrane potentiometer
provides x and y coordinates. The top layer is thin glass spaced close to a neighboring inner layer.
The underside of the top layer has a transparent conductive coating; the surface of the layer beneath
it has a transparent resistive coating. A finger or stylus deforms the glass to contact the underlying
layer. Edges of the resistive layer have conductive contacts. Locating the contact point is done by
applying a voltage to opposite edges, leaving the other two edges temporarily unconnected. The
voltage of the top layer provides one coordinate. Disconnecting those two edges, and applying
voltage to the other two, formerly unconnected, provides the other coordinate. Alternating rapidly
between pairs of edges provides frequent position updates. An analog-to digital converter provides
output data.
Advantages of such sensors are that only five connections to the sensor are needed, and the
associated electronics is comparatively simple. Another is that any material that depresses the top
layer over a small area works well. A disadvantage is that sufficient force must be applied to make
contact. Another is that the sensor requires occasional calibration to match touch location to the
underlying display. (Capacitive sensors require no calibration or contact force, only proximity of a
finger or other conductive object. However, they are significantly more complex.)
Potentiometer applications
Potentiometers are rarely used to directly control significant amounts of power (more than a watt or
so). Instead they are used to adjust the level of analog signals (for example volume controls
on audio equipment), and as control inputs for electronic circuits. For example, a light dimmer uses a
potentiometer to control the switching of a TRIAC and so indirectly to control the brightness of
lamps.
Preset potentiometers are widely used throughout electronics wherever adjustments must be made
during manufacturing or servicing.
User-actuated 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 rotary encoders, up/down push-buttons, and other digital controls now more
common. However they remain in many applications, such as volume controls and as position
sensors.
Audio control
Linear potentiometers ("faders")
Low-power potentiometers, both linear and rotary, are used to control audio equipment, changing
loudness, frequency attenuation, and other characteristics of audio signals.
The 'log pot' is used as the volume control in audio power amplifiers, where it is also called an "audio
taper pot", because the amplitude response of the human earis approximately logarithmic. It ensures
that on a volume control marked 0 to 10, for example, a setting of 5 sounds subjectively 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) and the received picture signal,
along with other things such as audio-video carrier offset, tuning frequency (for push-button sets)
and so on.
Motion control
Potentiometers can be used as position feedback devices in order to create "closed loop" control,
such as in a servomechanism. This method of motion control used in the DC Motor is the simplest
method of measuring the angle or speed.
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.
Computation
In analog computers, 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, 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
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 potentiometer. This is
their most common use.
The voltage across RL can be calculated by:
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:
(dividing throughout by RL and cancelling terms with RL as denominator)
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
potentiometer to the other. There is, however, always a small amount ofcontact
resistance.
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