An instrument for the accurate measurement or control of electrical potential. A known potential drop is created in a long (usually coiled) wire by a battery, and a sliding contact is used to tap off any proportion of this drop. Potentiometers are used in electronic circuits, especially as volume controls in transistor radios.
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.
Potentiometer as measuring instrument
The original potentiometer is a type of bridge circuit for measuring voltages. 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). 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.
Microvolt potentiometer
This is a form of the constant resistance potentiometer described above but designed to minimize the effects of contact resistance and thermal emf.
Thermocouple potentiometer
Another development of the standard types was the 'thermocouple potentiometer' especially modified for performing temperature measurements with thermocouples.
Potentiometer as electronic component
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.
Ordinary potentiometers are rarely used to control anything of significant power (even lighting) directly due to resistive losses, but they are frequently used to adjust the level of analog signals (e.g.
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.
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.
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.
Most (cheaper) "log" pots are actually not logarithmic, but use two regions of different, but constant, resistivity to approximate a logarithmic law.
High-power types
A rheostat is essentially a potentiometer, but is usually much larger, designed to handle much higher voltage and current. 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.
Digital control
Digitally controlled potentiometers can be used in analogue signal processing circuits to replace potentiometers.
The same idea can be used to create digital volume controls, attenuators, or other controls under digital control.
The DCP should not be confused with the digital to analogue converter (DAC) which actually creates an analogue signal from a digital one.
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
One of the most common uses for modern low-power potentiometers is as audio control devices.
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.
A potentiometer used in combination with an inductor or capacitor acts as a "tone" control.
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.
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 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 pot to the other.
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.
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