Potentiometers
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Wikipedia-Article "Potentiometers"
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 acts as a variable voltage divider. Potentiometers are also widely used as a part of displacement transducers because of the simplicity of construction and because they can give a large output signal.
Originally the term potentiometer was used to describe 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.
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Original potentiometer
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 emf 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]
Modern potentiometer
This is a three terminal resistor where the position of the sliding connection is user adjustable, usually found as a knob but occasionally occuring as a 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. Tuning capacitors are sometimes incorrectly referred to as potentiometers, possibly due to their similar appearance. If the device has only two terminals brought out (or if only the slider and one end of a three-terminal device are used) it is called a rheostat, or variable resistor. This is used to control current. 'Pots' are not used to control high-power devices, such as electric stoves, because as a voltage divider or variable resistor, power is inherently wasted. The pot could actually become hotter than the heating element of the stove. 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 with the volume control such that just past the setting least volume, the device switches off.
Types of potentiometers
Low-power types
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.
Linear potentiometers
A linear law pot ("type A") 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 ("type B") (sometimes called a log taper pot) has a resistive element that either 'tapers' in a logarithmic manner from one end to the other, or is made from a material whose resistivity varies logarithmically from one end to the other. This results in a device where the distance between the wiper and one end terminal is proportional to the logarithm of the resistance between them.
Potentiometers with switch
A Potentiometer with switch ("type C"). Same as a linear potentiometer, except that it has a switch mounted on it´s back or that it has a dead section on one end of the resistive element.
High-power types
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
Audio Control
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 regulary used to adjust loudness, frequency attenuation and other characteristics of audio signals.
A potentiometer used as a loudness control knob acts as a filter, dumping off a certain amount of signal into a ground, thus attenuating the signal that continues on past the control. High frequencies tend to pass into the ground more easily however, so in audio applications, often a higher resistance potentiometer is used to keep the sound "bright".
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
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 RL is determined by the formula:
The parallel lines indicate components in parallel. Expanded fully, the equation becomes:
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:
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 the variable resistor 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.
External links
- Pictures of potentiometers
- Electrical calibration equipment including various potentiometers
- The Secret Life of Pots Dissecting and repairing potentiometers
- Beginners' Guide to Potentiometers