Potentiometers

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Potentiometers are electronic components that vary their resistance depending on a longitudinal or angular displacement.

What is a potentiometer?

A potentiometer is an electromechanical component used to control the flow of electrical current in a circuit. It consists of an adjustable resistor that can vary its resistive value through a moving contact. This moving contact, often called a slider, moves along the resistor, allowing the total resistance between its moving end and its ends to vary. Potentiometers are commonly used in electronic circuits to control the voltage level, the volume of an audio device, the intensity of light, control or detect a position, among other uses. There are different types of potentiometers, including sliding potentiometers and rotary potentiometers, which are chosen depending on the specific needs of the circuit in which they are to be used.

Law of variation of a potentiometer

The function that relates the resistance between the cursor and the fixed terminals of the potentiometer, with the consequent positions that the cursor can occupy in its movement along the entire resistive body of the potentiometer, is what is known as the law of variation of the potentiometer.

We can therefore intuit that a large number of laws can be followed. Of all of them the most important are:

Linear law: If the function is linear, R(B) = K * B (Ω)

Where K is an adjustment constant and B is the path.

The graph that follows is the equation of a line. In a linear potentiometer, the resistance varies uniformly along its length. This means that if you divide the potentiometer into equal parts and measure the resistance between the slider and one end at each point, you will find that the resistance increases or decreases constantly.

Logarithmic law:

If the relationship is of the form B= K * log (R(B) + 1) (Ω)

In which K is an adjustment constant and B is the path.

A logarithmic potentiometer, the resistance changes according to a logarithmic scale. This means that the change in resistance is not linear in relation to the movement of the cursor. Logarithmic potentiometers are commonly used in audio applications, such as volume control, as they better match human perception of sound.

Inverted or negative logarithmic law: When the function is of the form R(B) = K * log (B+1) (Ω)

ley de variación de un potenciómetro

Currently there are only potentiometers that follow either linear or logarithmic laws, depending on whether their variation follows one or another law of variation.

What is the conformity of a potentiometer?

Let it be a variable resistor, with any ideal law of variation and on which we make a measurement in the laboratory to obtain the real law of variation of the variable resistor. We will say that the real law is the more conformable (it will have more conformity), the closer it approaches the ideal.

Quantitatively, conformity is measured by the conformity error, which can be defined as the maximum relative deviation (in resistance) that we can have between the ideal key and the real one. In the particular case that the law of variation is linear, the linearity term is used instead of the conformity term, and therefore we will call the error linearity error.

Displacement and roughness resolution

Displacement resolution of a potentiometer: It is the minimum displacement, in a certain sense, to experience an increase in value. In wound variable resistors it makes a lot of sense since it is limited by the turns. In such a way that the resolution will be equal to:

Rt=(1/N)*100 (%) ; where N is the number of turns of the resistor.

Roughness of a potentiometer: Taking into account the concept of displacement resolution above, in continuous layer resistors, such as all those that are not wound-wound, it does not make sense to talk about displacement resolution, since we could ask ourselves: where will we find the minimum, since since the law of variation is a continuous function, can you always find a smaller value within that same value?

Taking into account that the surface of the resistive layer on which the electrical contact is made is not absolutely flat, but rather has a certain roughness, the resolution term is replaced by the more appropriate roughness term. This is defined as the maximum instantaneous variation with respect to the ideal.

Rheostats or potentiometers

Although both are variable resistors, they receive one name or another depending on how they are connected to the load.

Thus, if they are connected in series we will obtain current variations, and we will call them rheostats and if we connect them in shunt, we will obtain voltage variations and we will call them potentiometers.

However, it should be said that when they are built, the most suitable application is thought of and therefore they are called either potentiometers or rheostats.

Types of potentiometers according to their mounting

Depending on the application, we have two fundamental types of potentiometers:

Adjustment potentiometers:

Adjustment potentiometers or trimmers allow easy adjustment of resistance values through physical manipulation. They are manufactured in single and multi-turn in a variety of styles, sizes and housing technologies. Trimming potentiometers are variable resistors used for all applications where the exact ohmic value required for the design is not defined in advance. They offer easy adjustment (usually performed with a screwdriver) and stability after being assembled on the PCB.

Main parameters:

Single-turn or multi-turn (single-turn potentiometers are cheaper, while multi-turn devices offer more precise turns)
Through or SMD design (according to customer's choice)
Adjustment range: defines the maximum ohmic value needed
Overall dimensions: depending on available PCB space

Panel potentiometers: These are variable resistors used in various applications that require manual adjustments and a moderate number of cycles. They are mounted behind the front panel of computers and usually have buttons on their shafts. They are used in a wide range of markets, such as industrial machinery, avionics and medical equipment. Some key factors that define these potentiometers include resistive element material, dimensions, sealing level, expected life and allowable power. Additional common options such as detents, switches, and custom shafts are also mentioned. In turn, the panel potentiometers can be single-turn or multi-turn if the application requires fine control of the magnitude to be varied.

Applications: Used in a wide range of markets, such as industrial machinery, avionics and medical equipment. Some key factors that define these potentiometers include resistive element material, dimensions, sealing level, expected lifespan, and allowable power.

Types of potentiometer depending on the resistive material

There are four main types of potentiometers based on the resistive material they use:

Carbon film potentiometers: They use this type of film as a resistive material. They are the most common and cheapest, but may have lower precision and stability compared to other types
Wirewound Potentiometers: They employ a resistive wire wound around a core, and the moving contact adjusts the resistance by moving along the wire. They offer high precision and stability, but can be more expensive than carbon film potentiometers. They are usually made for higher powers and are often called rheostats.
Cermet potentiometers: Use a metallic ceramic (cermet) as a resistive material. It offers high stability and precision, as well as a long service life, making it a popular option for applications that require high precision and stability with variable resistance. It is manufactured in one vuelta and in 10 vueltas or multivuelta, and also sealed or sealed. By general rule, multivariate potentiometers that we know can be adjusted for printed circuits or with panels, which can be made of ceramic.
Conductor plastic potentiometers: This type of potentiometer is designed for professional audio applications. Panel models can have an integrated control button and very high immunity to octopus and water (IP67, IP68)

Each type has its own advantages and disadvantages, and the choice of potentiometer type depends on the specific requirements of the application.

Other types of potentiometers

In some cases, more than one variable resistor is incorporated into the same potentiometer body:

Potentiometer with switch: There is an additional switch independent of the resistance of the potentiometer, but which acts together with the same axis of the potentiometer right at one of the two ends of the travel. They were used in domestic equipment to provide the shutdown function and usually volume, but they are still used in light regulators or dimmers.
Tandem potentiometer: By moving the axis of the potentiometer we move two cursors that each act on a different resistive element. We also call it double potentiometer. It is used in audio applications, in order to vary the volume, tone, etc. of the two channels of the amplifier, mixing console, etc. Today it is increasingly out of use since digital techniques do not require it.
Multifunction potentiometers: Sometimes several potentiometers are applied to the same potentiometer axis, each with its own independent control. They were formerly used in car radios and measuring instruments, but today they are no longer used.

What to do if my audio potentiometer produces noise?

That a potentiometer is sealed prevents dust from entering that causes failures in contacts, measurements and even noise in sound systems. These types of potentiometers are very expensive compared to a carbon film potentiometer. And furthermore, only sealed rotary and adjustment potentiometers are manufactured, designed for either hostile environments or to provide a long useful life.

In the event that moving the cursor of a potentiometer causes noise, we can try to see if our potentiometer has a slot on the back and apply contact cleaning spray. If the noise does not disappear we will have to change the potentiometer.

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