A potentiometer ( Poti for short , also potentiometer according to the new German spelling ) is an electrical resistance component whose resistance values can be changed mechanically (by turning or moving). It has at least three connections and is mainly used as a continuously adjustable voltage divider . The potentiometer was invented in 1841 by the German physicist Johann Christian Poggendorff .
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Basic structure and function
A potentiometer is made of a resistive material (on or between an electrically non-conductive support) and (mostly) a sliding contact , usually called a wiper . The fixed resistor has a connection at each of its two ends, as does the wiper, which (usually) result in three connections. A variable resistance can be picked up via the slider.
The component itself, as long as it has at least three connections, is always called (the - never "the") potentiometer, regardless of what it is used for or how it is used in the circuit, regardless of how it is connected and regardless of whether all three connections are used, i.e. it is used as a voltage divider, or whether only two connections are used and it is therefore only used as a variable resistor.
Potentiometers are often used to control electronic devices, such as setting an amplifier, e.g. B. the volume setting of a sound amplifier, z. B. in a radio or television set. Other solutions (e.g. keys or incremental encoders ) are increasingly being used in this function . The reasons for this are, on the one hand, the advancing digitization of many electronic functions, and on the other hand, the fact that mechanical potentiometers do not work without wear due to the abrasion of the resistance material by the grinder.
Depending on the external wiring, the result is a
- adjustable resistance,
- adjustable voltage tap (voltage divider) or
- if there is a significant load at the output - a so-called loaded voltage divider.
However, the potentiometer circuit that gives it its name is only used in the circuit as a voltage divider .
A partial voltage (potential difference) is tapped via the middle connection .
The calculation is carried out according to a series connection (also called series connection).
Potentiometers exist in numerous designs, which are explained below.
Wire potentiometers have a toroidal , helical winding made of resistance wire on an insulating body (mostly made of ceramic material ), therefore also called ceramic potentiometers. This design is preferred when a high power dissipation has to be implemented in the component, and thus a higher temperature arises. These components are also called rheostats , especially in the English-speaking world ; Strictly speaking, a rheostat is a variable resistor with two connections , while a potentiometer (as mentioned above) is a voltage divider that has three connections.
allow more than one turn of the rotary wheel.
For this purpose, the wire is housed in the form of a double helix on a coiled insulating body. Hence also called a spiral potentiometer. This helix, as well as the drive of the wiper like a nut on a screw, allow several turns (e.g. five or ten turns, e.g. called a ten-turn potentiometer ).
Use e.g. B. in transmitters and receivers in order to be able to set the frequency (exactly !!).
A special design is the combination of two such potentiometers to form a tandem spiral potentiometer. See photo.
With film potentiometers, the resistance material consists of a carbon layer , a metal layer, a cermet layer or a conductive plastic (conductive plastic). The resistance carrier of film potentiometers is usually in the shape of a segment of a circle, but it can also be stretched in slide potentiometers (slide controls) .
There are film potentiometers with a resistance ratio that is linearly dependent on the angle of rotation and those with an approximately logarithmic characteristic. The latter are particularly advantageous if the setting range covers several orders of magnitude (e.g. volume control ).
Potentiometers with reversed logarithmic characteristics (named: -log. Or neg. Log. ) Are also available and are suitable, for example, for setting the frequency of an astable multivibrator .
In addition to different setting characteristics, there are also controllers with intermediate taps. Intermediate taps are common
- 25% and 50%, as well
- 25%, 50% and 75%.
Rotary potentiometers with a small angle of rotation are used in joysticks .
Trimmers are intended for adjusting a circuit and usually only have a screwdriver slot. They are set once and, if necessary, fixed with varnish.
Multi-thread trimmers (so-called spindle trimmers ) are also used in the industrial sector and in measurement technology . In these, either a linear movement is generated with a spindle drive or a rotary movement with a worm drive (i.e. like a nut on a screw).
In a tandem potentiometer (also called multiple potentiometer), two potentiometers ( electrically separated from each other) are attached one behind the other on an axis.
There are two types of actuation:
- With a rotary wheel: that operates both potentiometers at the same time or
- with two rotating elements that allow separate adjustment. These sit in front of each other, with a smaller knob in front of a larger rotary wheel. The larger rotary wheel has a 6 mm shaft and the smaller one has a 4 mm shaft. The thinner shaft is guided through the thicker, hollow shaft. See image.
Tandem potentiometers are used, for example, to set the volume of both channels of a stereo amplifier at the same time and are then called stereo potentiometers.
But there were also versions with two different tasks on one axis, for example: volume and sound, whose potentiometers then also had different nominal resistances.
Rotary potentiometer with switch
There are also rotary potentiometers with a switch, usually for switching the device on / off.
Motorized potentiometers were first used in very expensive studio mixing consoles . Each controller in these consoles has a motor. So you can save the settings of all sliders and restore them using the control program. Motorized potentiometers are currently also found in less expensive digital mixing consoles, but in much fewer numbers.
High-end hi-fi amplifiers often have motor potentiometers for remote- controlled volume adjustment, in order to shorten the length of the audio lines and thereby reduce interference.
Digital or electronic potentiometers consist of individual resistors connected in series (e.g. 100) and field-effect transistors as electronic switches. This arrangement is combined with a digital control circuit to form an integrated circuit . Such digital potentiometers are used both as trimming potentiometers (they keep their set value for life) or for setting via buttons, an incremental encoder or a microcontroller . Accordingly, you have a volatile and / or a non-volatile memory for the "loop setting".
There are also ICs where the resistors are in OGR arrangement. This saves a lot of resistance for a slightly increased control logic. This is quite useful for large resistance ranges with small stride lengths. There are also various other similar arrangements (e.g. the very popular power of two arrangement, in which the desired resistance value only needs to be specified as a binary number).
See foil potentiometer .
Rotating or sliding version
Potentiometers are available as a control element with a shaft for a rotary control or as a slide potentiometer (e.g. on mixing consoles and in recording studios).
Use as an actual value transmitter
Particularly precise potentiometers are also used for distance or angle measurement (position encoder, angle encoder), see potentiometer encoder .
Execution of connections
Potentiometers are available with different connection types such as:
- Soldering lugs ,
- Connector ,
- oberflächenverlötbares component on the printed circuit board (engl .: SMD S urface M ounted D evice ) or
- through PCB holes and soldered on the opposite side as push-through mounting .
The sliding track of a potentiometer can be equipped with taps so that it can be configured for different characteristics. As a result, such a potentiometer has more than the usual three connections.
Certain versions of aurally correct volume equalization (loudness correction) also work with such taps.
The function between angle or distance and resistance in potentiometers can be linear or non-linear.
Linear potentiometers have no identification or the identification: lin , 1 or B (formerly A), for example "10 k 1" for a 10 kOhm potentiometer with a linear resistance track.
The characteristics of non-linear potentiometers can be: positive logarithmic or negative logarithmic, exponential, or S-shaped.
In positive logarithmic potentiometer the resistance curve / characteristic is logarithmic rising .
Positive logarithmic potentiometers are labeled log , "pos.log", "+ log.", A (formerly C), audio or 2 after the resistance value, for example "10 K 2" . In some cases, however, logarithmic potentiometers are also marked with B , for example in the audio sector.
Negative logarithmic potentiometers have a falling characteristic and are marked with a "-" or "neg." in front of the value or marked with F or 3 after the value.
In addition to its nominal resistance value (resistance between the end connections) and its tolerance, the following features are characteristic of a potentiometer:
- Nominal load capacity (power loss); it is dependent on the angle of rotation
- with linear potentiometers the linearity
- with tandem potentiometers their synchronization
- mechanical parameters: angle of rotation or actuation distance, shaft diameter
- mechanical service life (achievable number of actuations)
- the maximum current that can be transmitted by the wiper without damaging the resistor material or the sliding contact
- Maximum dielectric strength
Different use of the designation
A so-called “active potentiometer” is sometimes required to control dimmable fluorescent tubes . It is not a potentiometer in the above / actual sense, but a controllable voltage source that can typically be set to values between 0 V and 10 V.
- sound-au.com , extensive information site on all types and properties of potentiometers (English)