Variable / Adjustable Resistor or Potentiometer

- notes or overview about the adjustable resistor or variable resistor used as a trimmer resistor and the circuit symbol

Variable or adjustable resistors are often needed within electronic circuits to act as a preset control within the circuit. The variable resistor, is also widely referred to as a potentiometer as a result of its configuration.

While it is often considered poor design practice to include unnecessary adjustments in a circuit, the variations in circuit values may mean that a preset, or variable resistor may be required in order to set the circuit to function within its required limits.

Additionally, variable resistors are used for controls - a prime example is the volume control on a radio, television of hi-fi unit.

Variable resistor basics

The variable resistor comprises a fixed resistive element along which a slider passes. The variable or adjust able resistor forms a potential divider in which the overall resistance between the two end points remains the same, but the ratio of the two resistors in the legs changes.

In view of the fact that the variable resistor is effectively a potential divider, it is called a potentiometer.

Variable resistor is effectively a potential divider
Variable resistor is effectively a potential divider

Variable resistor symbol

The variable resistor symbol used in circuit diagrams indicates its construction. Effectively it is a fixed resistor with a slider that can move along the length of the resistive element. In this way it forms a potentiometer as described before.

Variable resistor symbol
Variable resistor symbol for circuit diagrams

The variable resistor symbols depict the current version use din circuit diagrams today and the traditional format that may be seen on older circuit diagrams.

When a true variable resistor with only two connections is needed, it is common practice to connect the slider to the remote end of the variable resistor as shown below.

Variable resistor
Variable resistor element with two connections

Types of adjustable or variable resistors

There are a number of different types of variable resistor that are available on the market. Each of these different types of variable resistor has slightly different properties and is suitable for different applications and situations.

  • Wirewound variable resistors:   Wirewound variable resistors are able to give a high level of performance and as a result they are often the variable resistor of choice for many applications such as audio, etc..

    Wirewound variable resistors are manufactured using very fine resistance wire. This is wound around a former that is almost torroidal. The most commonly used form of resistance wire used is a nickel chrome alloy. Which has some further additives to improve its electrical characteristics.

    Wirewound variable resistors offer a high level of linearity and close tolerance. Some very close tolerance versions may be able to offer linearity tolerances of ±0.1%. These variable resistors are also stable over a wide temperature range.

    There are two main disadvantages with the wirewould variable resistor. The first is that often as the slider moves over the wires, the resistance changes have discrete steps. This may not be a problem in many applications, but it is a point to note. The second is that they are not suitable even for low frequency RF applications as the resistance wire forms a coil and has significant inductance.
  • Cermet variable resistors:   Cermet variable resistors are widely used, particularly for trimmer resistors. The name cermet is derived from the fact that the resistive element is made from CERamic and METal. The resistive element is made from a mixture of fine metal oxides or precious metal particles and glass in a viscous organic material. The resulting paste is applied to the substrate and fired to solidify the mixture.

    Cermet variable resistors are ideal for trimmer resistors because they have a low to medium adjustment life, and they often have temperature coefficients of around ±100ppm/°C.
  • Carbon composition variable resistors:   For the carbon composition variable resistor, a mixture of carbon powder and a binder are moulded under heat into the required shape. In some manufacturing processes the carbon composition element is moulded at the same time as the plastic substrate.

    Carbon composition variable resistors are some of the least expensive types and they are widely used in many areas - they are a good all round general purpose variable resistor.

    The carbon composition variable resistor element can be tailored to give approximately linear, logarithmic or even anti-logarithmic characteristics. The logarithmic forms are particularly useful for audio applications because the hearing characteristics of the ear mean that a logarithmic curve is more useful.

    Carbon composition variable resistors can sometimes exhibit temporary resistance changes of up to around ±10% if operated at very high or low temperatures. Typically there operating temperature range might be expected to be from -55°C to +120°C.

    These variable resistors can also become noisy after use as wear and dirt appear on the track. Often some switch cleaner improves the situation.
  • Conductive plastic variable resistors:   Conductive plastic variable resistors are made using a conductive plastic ink. This ink contains carbon, resin, solvent and other materials specific to the manufacturer. It is applied to the substrate, either by screening or co-moulding. As the ink has a relatively low curing temperature, this enables a variety of substrate materials to be used.

    Conductive plastic variable resistors have a high rotational life as well as providing a low noise output. As a result, they are often used for position sensors in servo-controlled machines, etc.

Summary of characteristics of variable resistor types

The different types of variable resistor have different characteristics and this enables them to be used in different situations. The table below summarises some of the major characteristics of the most commonly used types of variable resistor.

Variable Resistor Type Typical Resistance Ranges Typical Tolerance Typical Power Handling Typical Life
Typical Temperature Coefficient
Wirewound 10Ω - 50kΩ ±5% up to 1 Watt 500 ±50
Cermet 50Ω - 2MΩ ±10% 500mW 200 100
Conductive Plastic 50Ω - 2MΩ ±10% 250mW 100 000 500
Carbon composition 50Ω - 2MΩ ±20% 250mW 1000 ~±10%/°K

By the very nature of this table the characteristics for the various forms of variable resistor can only be taken as a guide - with developments in technology occurring, along with the differing specifications of components from the variety of manufacturers, there is a large variation of performance of components from different sources.

By Ian Poole

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