Potentiometer

A potentiometer 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 .

A measuring instrument called a potentiometer is essentially a voltage divider used to measure electric potential (voltage); 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. A mechanism-driven potentiometer can be used as a position transducer , for example, in a joystick . Potentiometers are rarely used to directly control significant power (more than a watt ), because the power dissipated in the potentiometer will be equal to the power at the controlled load.

Glossary

There are several terms in the electronics industry to describe certain types of potentiometers:

  • 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 that is repeatedly adjusted by means of a small thumbwheel
  • Trimpot or Trimmer Pot : A trimmer potentiometer is usually a one-time or repeated adjustment for “fine-tuning” an electrical signal.

Construction

The potentiometer consists of a resistive element , a sliding contact (wiper) that moves along the element, making good electrical contact with a portion of it, electrical terminals at each end of the element, a mechanism that moves the wiper from one end to the other. leads to. , and the housing containing the element and wiper.

Many cheap potentiometers are manufactured with a resistive element (B in the cutaway drawing) formed in an arc of a circle, usually a little less than a full turn, and a wiper (C) on this element when rotated. Sliding, which leads to electrical contact. The resistive element may be flat or angled. Each end of the resistive element is connected to a terminal (E, G) on the case. The wiper is connected to the third terminal (F), usually between the other two. On a panel potentiometer, the wiper is usually the center terminal of three. For single-turn potentiometers, this wiper typically travels under one revolution around the contact. The only point of entry for contamination is the narrow space between the shaft and its rotating housing.

Another type is the linear slider potentiometer, which consists of a wiper that slides along a linear element instead of rotating. Contamination can potentially enter anywhere along the slot in which the slider moves, making effective sealing more difficult and compromising long-term reliability. One advantage of a slider potentiometer is that the position of the slider gives a visual indication of its setting. While the setting of the rotary potentiometer can be seen from the position of the marking on the knob, an array of sliders can give a visual impression of the settings on the mixing console in the form of graphic equalizers or faders .

The resistive element of cheap potentiometers is often made of graphite . Other materials used include resistance wires, carbon particles in plastics, and a ceramic/metal mixture called cermet . Conductive track potentiometers use conductive polymer resistant pastes that contain hard-wearing resins and polymers, solvents and lubricants in addition to carbon that provide conductive properties.

Multiturn potentiometers are also operated by rotating a shaft, but by several turns instead of less than one full turn. Some multiturn potentiometers have a linear resistive element with a sliding contact that is moved by a lead screw; Others have a helical resistive element and a wiper that spins through 10, 20 or more complete revolutions, rotating with the helix as it rotates. Multiturn potentiometers, both user-accessible and preset, allow finer adjustments; Rotating through the same angle changes the setting usually by one-tenth as much as for a simple rotary potentiometer.

A string potentiometer is a multi-turn potentiometer that is driven by an attached reel of wire against a spring, which enables it to convert a linear position to a variable resistance.

User-accessible rotary potentiometers can be fitted with a switch that usually operates at the opposite-clockwise extreme of rotation. Before digital electronics became the norm such a component was used to allow radio and television receivers and other devices to be switched on at minimal volume with an audible click, then increased in volume by turning a knob. Several resistance elements with their sliding contacts on the same shaft can be connected together, for example, in stereo audio amplifiers for volume control. In other applications, such as household lighting dimmers , the typical usage pattern is best satisfied if the potentiometer remains set to its current position, so the switch is operated by a push action, alternatively by an axial press of the knob.

Others are enclosed within the equipment and are intended to be adjusted to calibrate the equipment during manufacture or repair, and not otherwise touched. They are generally much smaller physically than user-accessible potentiometers, and may need to be operated by a screwdriver rather than having a knob. They are commonly called “preset potentiometers” or “trim [ming] pots”. Some presets can be accessed by a small screwdriver poked through a hole in the case to allow servicing without destroying it.

Resistance-Position Relationship: “Cone”

The relationship between the position of the slider and the resistance, known as the “taper” or “law”, is controlled by the manufacturer. In principle any relation is possible, but linear or logarithmic (aka “audio taper”) potentiometers are sufficient for most purposes.

A letter code can be used to identify which taper is used, but letter code definitions are not standardized. Potentiometers made in Asia and the United States are usually marked with an “A” for logarithmic taper or “B” for linear taper; “C” for the rarely seen reverse logarithmic taper. Others, notably those from Europe, may be marked with “A” for linear taper, “C” or “B” for logarithmic taper, or “F” for reverse logarithmic taper. [2]The code used also varies between different manufacturers. When a percentage is referenced along a non-linear taper, it is related to the resistance value at the midpoint of the shaft rotation. Therefore a 10% log taper will measure 10% of the total resistance at the midpoint of the rotation; ie a 10% log taper on a 10 kOhm potentiometer would yield 1 kOhm at the mid point. The higher the percentage, the sharper the log curve.

Linear Taper Potentiometer

linear taper potentiometer ( describes the electrical characteristic of the linear device, not the geometry of the resistive element) consists of a resistive element of constant cross-section, resulting in a device where the contact between the (wiper) and an end terminal The resistance is proportional to the distance between them . Linear taper potentiometers [4] are used when the division ratio of the potentiometer must be proportional to the angle of shaft rotation (or slider position), for example, used to adjust the center of the display on analog cathode-ray oscilloscopes . controls to be carried out. Precision potentiometers have a precise relationship between resistance and slider position.

Logarithmic potentiometer

A logarithmic taper potentiometer is a potentiometer with a bias across the resistive element. Basically it means that the center position of the potentiometer is not half of the total value of the potentiometer. The resistive element is designed to follow a logarithmic taper, aka a mathematical exponent or “square” profile. A logarithmic taper potentiometer is constructed with a resistive element that either “tapers” from one end to the other, or is made of a material whose resistivity varies from one end to the other. This results in a device where the output voltage is a logarithmic function of the slider position.

Most (cheap) “log” potentiometers are not precisely logarithmic, but use two regions of different resistance (but constant resistivity) to estimate the 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 be simulated (not very accurately) with a linear and an external resistor. True logarithmic potentiometers are significantly more expensive. Logarithmic taper potentiometers are often used to measure volume or signal level in audio systems, according to the Weber–Fechner law , because the human perception of audio volume is logarithmic.

Riostat

The most common way to continuously vary the resistance in a circuit is to use a rheostat . [6] It is basically used to adjust the magnitude of current in a circuit by changing the length. The word rheostat was coined about 1845 by Sir Charles Wheatstone , from Greek rheos meaning “stream”, and – – states (from αι histanai , “to cause to stand, to set” ) ) means “setter, regulating device”, [7] [8] [9] which is a two-terminal variable resistor. The word “rheostat” is becoming obsolete, [10] with the generic term “potentiometer” in its place. Three-terminal potentiometers are often used for low-power applications (less than about 1 watt), with one terminal unconnected or connected to a wiper.

Where the rheostat must be rated for high power (more than about 1 watt), it can be constructed with a resistance wire wound around a semicircular insulator, with the wiper slipping from one turn of the wire to the other. Sometimes a rheostat is made from resistance wire wound onto a heat-resistant cylinder, consisting of a slider made of several metal fingers that hold lightly over a small section of turns of the resistance wire. The “fingers” can be moved by a sliding knob along the wire of the resistance wire thereby changing the “tapping” point. Wire-wound rheostats with ratings up to several thousand watts are used for DC motor drives, electric welding controls, or in applications such as the control of generators. The rating of the rheostat is given with the absolute resistance value and the allowable power dissipation is proportional to the fraction of the total device resistance in the circuit. Application of carbon-pile rheostats for testing automobile batteries and power suppliesThe load is done in the form of a bank .

Digital potentiometer

A digital potentiometer (often called a digipot) is an electronic component that mimics the functions of an analog potentiometer. By means of a digital input signal, the resistance between the two terminals can be adjusted in the same way as an analog potentiometer. There are two main functional types: volatile, which lose their assigned state when power is removed, and are typically designed to initialize at the minimum position, and non-volatile, which are storage mechanisms similar to flash memory or EEPROM. Maintain your set position by using ,

The use of a Digipot is far more complex than that of a simple mechanical potentiometer, and there are several limitations to observe; Yet they are widely used, often for factory adjustments and calibration of instruments, particularly where the limits of mechanical potentiometers are problematic. A digipot is generally immune to the effects of medium-long-term mechanical vibration or environmental pollution, to the same extent as other semiconductor devices, and against unauthorized tampering by protecting access to its programming inputs by various means electronically. can be secured.

In devices that have microprocessors, FPGAs, or other functional logic that can store settings and reload them on a “potentiometer” each time the device is powered on, a multiplier DAC may be used in place of a digipot. and it can provide higher setting resolution, less drift with temperature, and greater operational flexibility.

Membrane potentiometer

A membrane potentiometer uses a conductive membrane that is deformed by a sliding element to contact a resistive voltage divider. Linearity can range from 0.50% to 5% depending on the material, design and manufacturing process. The repeat accuracy is typically between 0.1 mm and 1.0 mm, with theoretically infinite resolution. The service life of this type of potentiometer is typically 1 million to 20 million cycles depending on the material used during manufacture and the actuation method; Contact and contactless (magnetic) methods are available (to sense the position). There are many different material variations available such as PET, FR4 and Kapton. Membrane potentiometer manufacturers offer linear, rotary and application-specific variations. Linear versions can range in length from 9mm to 1000mm and rotary versions can range from 20 to 450mm in diameter, Each of which has a height of 0.5 mm. Membrane potentiometer can be used for position sensing.[1 1]

For touch-screen devices that use resistive technology, a two-dimensional membrane potentiometer provides the x and y coordinates. The top layer is thin glass that is close to a neighboring inner layer. There is a transparent conductive coating under the top layer; The layer beneath it has a transparent resistive coating on its surface. A finger or stylus deforms the glass to make contact with the underlying layer. The edges of the resistive layer have conductive contacts. Locating the contact point is done by applying voltage to opposite edges, leaving the other two edges temporarily unconnected. The voltage of the top layer provides a coordinate. Disconnecting those two edges, and applying voltage to the other two, formerly unconnected, Provides other coordination. Provides frequent position updates by rapidly alternating between pairs of edges. An analog-to-digital converter provides the output data.

The advantages of such sensors are that only five connections are required to the sensor, and the associated electronics are comparatively simple. The second is that any material that presses the top layer over a small area works well. One disadvantage is that sufficient force must be applied to make contact. The second is that the sensor sometimes requires calibration to match the touch location to the built-in display. (Capacitive sensors do not require calibration or contact force, only the proximity of a finger or other conductive object. However, they are significantly more complex.)

Application

Potentiometers are rarely used to directly control significant amounts of power (a watt or more). Instead they are used to adjust the level of analog signals (e.g. in volume control audio equipment), and as control inputs for electronic circuits. For example, a light dimmer uses a potentiometer to control the switching of the TRIAC and therefore to control the brightness of the lamp indirectly.

Preset potentiometers are widely used throughout electronics where adjustments must be made during manufacturing or servicing.

User-activated potentiometers are widely used as a form of user control, and can control a wide variety of instrument functions. The widespread use of potentiometers in consumer electronics declined in the 1990s, with rotary incremental encoders, up/down push-buttons and other digital controls now more common. However they persist in many applications, such as as volume controls and position sensors.

Audio control

Low-power potentiometers, both slide and rotary, are used to control audio equipment, to control loudness, frequency attenuation, and other characteristics of an audio signal.

The ‘log pot’, i.e., a potentiometer having a resistance, taper, or, “curve” (or law) of a logarithmic (log) form, is used as a volume control in audio power amplifiers, where it Also called an “audio taper pot”, because the amplitude response of the human ear is approximately logarithmic. This ensures that on the volume control marked 0 to 10, for example, 5 sounds thematically half as loud as a setting of 10. There is also an anti-log pot or reverse audio taper which is the opposite of a logarithmic potentiometer. It is almost always used in a ganged configuration with a logarithmic potentiometer, for example, in an audio balance control.

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

In audio systems, the term linear is sometimes applied in a confusing way to describe slide potentiometers due to the straight line nature of the physical sliding motion. The term linear when applied to a potentiometer, regardless of slide or rotary type, describes the linear relationship of the position of the pot versus the measured value of the tap (wiper or electrical output) pin of the pot.

Television

Potentiometers were previously used to control picture brightness, contrast, and color response. A potentiometer was often used to adjust the “vertical hold”, which affected the synchronization between the receiver’s internal sweep circuit (sometimes called a multivibrator) and the received picture signal, as well as the audio-video carrier offset. , among other things like tuning frequency (for push). -button set) and so on. It also helps in frequency modulation of waves.

Speed control

Potentiometers can be used as position feedback devices to create closed-loop controls, as in servomechanisms. This method of speed control is the simplest method of measuring angle or displacement.

Transducer

Potentiometers are also widely used as a part of displacement transducers due to the simplicity of construction and because they can give a large output signal.

Calculation

In analog computers, high-precision potentiometers are used to scale intermediate results by desired constant factors, or to establish initial conditions for a calculation. A motor-driven potentiometer can be used as a function generator, by using a non-linear resistance card to supply approximations of trigonometric functions. For example, shaft rotation can represent an angle, and the voltage division ratio can be proportional to the cosine of the angle.

Operating principle

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

Potentiometer
A potentiometer with a resistive load shows the equivalent fixed resistances for clarity.

The voltage across RL can be calculated by:

{\displaystyle 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}.}

If R is large compared to other resistors (such as the input of an operational amplifier), the output voltage can be approximated by the simple equation :

{\displaystyle V_{\mathrm {L} }={R_{2} \over R_{1}+R_{2}}\cdot V_{s}.}

(divide across by rl and cancel the case with rl as the divisor )

For example, suppose , , , and

{\displaystyle V_{\mathrm {S} }=10\ \mathrm {V} }, {\displaystyle R_{1}=1\ \mathrm {k\Omega } }
R_{2}=2\ {\mathrm  {k\Omega }}, ~~and~~ {\displaystyle R_{\mathrm {L} }=100\ \mathrm {k\Omega } .}

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

{\displaystyle {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 a 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, the divider allows the output voltage to vary from a maximum ( Vs ) . are capable of. As ground (zero volts) the wiper goes from one end of the potentiometer to the other. However, there is always a small amount of contact resistance.

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