Variable resistors (potentiometers)
a summary or overview describing what is a variable resistor or potentiometer
and detailing the different types of variable resistor and how variable resistors are used
Variable resistors, or as they are often called potentiometers are used in many areas of electronics. They are used for volume and gain controls as well as a variety of other applications. Preset variable resistors or potentiometers are also used in circuits that need a small adjustment to be made to set the circuit up after manufacture.
For convenience variable resistors are made by having a fixed resistor with
a variable tapping point. As a result of this arrangement these devices are
often called potentiometers or "pots" for short. This name is derived
from the configuration used for many in the early days of electricity for measuring
voltage. Here the potentiometer consisted of a length of resistance wire with
a tapping point that could be moved along the wire - the same configuration
as that used in these variable resistors.
Types of variable resistor
There are a variety of types of potentiometer that are available and there are a variety of different ways in which variable resistors or potentiometers can be classified. The different types may be important in different applications and therefore it is sometime necessary to choose the correct types.
One of the first ways in which variable resistors can be classified is whether they are preset or adjustable.
1. Adjustable: The type of variable resistor that may be termed adjustable are those that have a spindle and can be used with a knob. These are generally used for functions such as volume or tone controls on radios. They also find many other uses where a value needs to be set by the user.
2. Preset: The preset form of variable resistor are not available to the user of a piece of equipment. They are used when a value needs to be set within a circuit, normally within the production set-up and test stage during its manufacture. Although good design dictates that the fewest number of adjustable components should be used, it is sometimes necessary to make some adjustments to ensure that the circuit operates within its required limits.
Some presets consist of a single turn adjustment. This can be rather course when an accurate setting is required. To overcome this a variety of multi-turn preset pots are available. Generally they have about ten turns to set them from one end of their travel to the other. However there are still choices to be made. Some can be adjusted from the side and others have a top adjustment. In this way all possibilities in equipment can be covered.
In addition to the basic format of the variable resistor described above, the internal construction and in particular the material used for the resistive element is of importance as well. Essentially variable resistors have a resistive track that is set down and the slider moves along making contact with a particular position on the fixed resistor. The material used for the track itself can govern aspects of the performance of the component including the power dissipation capability and noise generated.
1. Carbon composition: carbon composition potentiometers or variable resistors are the most commonly used type. The material used is a mixture of carbon and a filler material, the combination determining the resistivity of the carbon composition film of the potentiometer element.
2. Cermet: Cermet, as the name implies is a composite material consisting of ceramic and a metal material. It is particularly applicable where any high temperatures may be experienced. They also offer low noise levels than those offered by carbon composition types.
3. Conductive plastic: These are made from a form of conductive plastic.
4. Wire wound: Wire wound potentiometers are the most expensive type to produce. As the name implies they a manufactured by winding a "coil" of resistance wire on a semicircular former. The surface of the wire must not be insulated so that the slider is able to make electrical contact with the fixed resistance element. In use it is often possible to feel the slider move from one turn of the wire to the next and this can be used to detect if a potentiometer is actually wire wound. These potentiometers are often used for high power or low resistance components.
Another variable in the type of potentiometers that are available is governed by the relationship between the resistance and the position along the track. There are two main types linear and logarithmic:
1. Linear: For this type of variable resistor, there is a linear relationship between the resistance and the position around the track, i.e. for every degree around its travel he resistance will vary by the same amount. Virtually all preset potentiometers are of the linear type, but not all adjustable ones are.
2. Logarithmic: Although preset pots are almost exclusively linear, many of the non-preset varieties are not linear. Instead they may follow a logarithmic law. In this way they have a relatively small change over the first part of their travel, increasing the further along their travel they are set. The reason for this is that the ear is not linear, and a logarithmic scale on the potentiometer gives a more equal increase in volume along the travel of the control as perceived by the ear. In some instances reverse logarithmic or anti-logarithmic scales can be obtained, although they are not nearly as common as the logarithmic potentiometers
The way in which the resistance varies is normally marked on the potentiometer. Descriptions such as 10k LOG or 5k LIN may be seen representing a 10 kilo-ohm potentiometer with a logarithmic value change or a 5 k linear version respectively.
Apart from the electrical considerations, the mechanical ones are also important. The way in which a potentiometer or variable resistor moves can greatly affect the ergonomics of a piece of electronic equipment. One of the major mechanical considerations is the form of movement that creates the electrical changes in the variable resistor. There are two major types:
1. Rotary: The most common for of variable resistor or potentiometer is a rotary version. This versions of potentiometer uses a rotary motion to move the slider around a track that compromises most of a circle, with contacts at either end of the track in the area where part of the circle is missing. This form is widely used with knobs on a spindle for the actual control, and they are found in many applications from providing adjustments on test equipment through to being used for volume controls on domestic radios.
2. Slider: Slider controls are those variable resistors that slide in a linear fashion, i.e. in a straight line. These controls take up more front panel space, but are much easier to use under some circumstances. For example they are widely used for audio mixers and lighting desks. The advantage of sliders is that it is easier to control them quite precisely and compare the relative positions of a number of sliders. It is also possible to control a number of sliders together.
Potentiometers are used in huge quantities in the manufacture of electronics equipment. These variable resistors or potentiometers provide a way of adjusting electronics circuits so that the correct outputs are obtained. Although their most obvious use must be for volume controls on radios and other electronics equipment used for audio, they also find many applications in other areas of electronics.