Potentiometer

Several types of potentiometer

Potentiometers meant to be mounted on a PCB for infrequent changes; "trimmer resistors".

The original meaning of the term potentiometer, which is still in use, is an apparatus used to measure the potential (or voltage) in a circuit by tapping off a portion of a known voltage from a resistive slide wire and comparing it with the unknown voltage by means of a voltmeter or galvanometer.

The present popular usage of the term potentiometer (or 'pot' for short) describes an electronic component which has a user-adjustable resistance. Usually, this is a three-terminal resistor with a sliding contact in the center. If all three terminals are used, it can act as a variable voltage divider.

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. 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.

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

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 minimise 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

File:Reochord.jpg

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. Although the term varistor implies a similar function, a varistor is not the same device as a potentiometer. See the varistor article for details. Variable capacitors are sometimes incorrectly referred to as potentiometers, possibly due to their similar appearance. Potentiometers are not used to control high-power devices, such as electric stoves, because of resistive losses, which limit the current-carrying capacity. Typically a pot will be used to regulate a higher power device by controlling the gain of a transistor or other device.

Common applications for pots include volume controls in audio equipment and (crude) light dimmers. In consumer electronics sometimes the on-off switch for the device is combined on the same shaft as the volume control. When the potentiometer reaches the 'minimum volume' end of the track (usually fully CCW) further rotation operates the on/off switch.

Types of potentiometers

Low-power types

A typical single turn potentiometer

A potentiometer is constructed using a flat graphite annulus 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. One popular form of rotary potentiometer is called a string pot. It is a multi-turn potentiometer with an attached reel of resistance 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").

Linear potentiometers

A linear pot has a resistive element of constant cross-section, resulting in a device where the resistance between the 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.

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 pot.

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 around a toroid 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.

Digital control

See digitally-controlled potentiometer.

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.

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. 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 ("type C") 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.

A potentiometer used as a "tone" control knob also acts as a filter, and sometimes linear taper pot is used for the tone control. A capacitor is used to increase the filtering effect. When the potentiometer is set to the shorting position, the capacitor goes into maximum effect, attenuating high frequencies dramatically.

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.

The voltage across $R_{\mathrm {L} }$ 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. 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 $R_{\mathrm {L} }$ 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 $R_{\mathrm {L} }=100\ \mathrm {k\Omega }$. Since the load resistance is large compared to the other resistances, the output voltage $V_{\mathrm {L} }$ 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. There is, however, always a small amount of contact 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.

See also

Linear taper

External links

• Pictures of (original) potentiometers
• Electrical calibration equipment including various potentiometers
• The Secret Life of Pots Dissecting and repairing potentiometers
• Beginners' Guide to Potentiometers
• Making a rheostat
• Linear pot turned into log pot