Resistors are one of the most widely used components in electronic circuits - there are many different types of resistor available having different properties and used in different ways in different circuits.
Resistors of all types are used in vast quantities in manufacturing electronic equipment. In fact the resistor is probably the most common type of electronic component used in electrical and electronic circuits.
There is a large number of different types of resistor that can be bought and used. The properties of these different resistors vary, and it helps to obtain the right type of resistor for any given design to ensure that the best performance is obtained.
Selection of fixed leaded resistors or various types
Although many resistors will work in a variety of applications the type of resistor can be important in some cases. Accordingly it is necessary to know about the different resistor types, and in which applications each type of resistor can be used.
What is a resistor?
Resistors are used in virtually all electronic circuits and many electrical ones. Resistors, as their name indicates resist the flow of electricity, and this function is key to the operation most circuits.
Note on Resistance:
Resistance is one of the key factors used in electricla and electronic circuits. Resistance is the proprty of materials to resist the flow of electricity, and it is governed by Ohm's Law.
There are two main circuit symbols used for resistors. The oldest one is still widely used in North America and consists of a jagged line representing the wire used in a resistor.
The other resistor circuit symbol is a small rectangle, and this is often termed the international resistor symbol and it is more widely used in Europe and Asia.
Resistor circuit symbols
The unit or resistance is the Ohm, which is denoted by the Greek letter Omega: Ω and resistor values may be seen quoted in terms of Ohms - Ω, thousands of Ohms or kilohms - kΩ and millions of Ohms, megohms, MΩ.
When written on circuits values like 10k may be seen meaning 10 kilohm, or 10 kΩ. The Omega sign is often omitted and the decimal point replaced by the multipler: e.g. 1R5 would be 1.5 Ohms, 100R is 100Ω, 4k7 is 4.7 kΩ, 2M2 is 2.2MΩ and so forth.
There are many different types of resistor. Some are for special applications like being used as variable resistors, and others are used for surge limitation whilst others provide a variable resistance with temperature. All of these characteristics can be used.
However for fixed resistors there are different characteristics that need to be taken into account.
Even though the actual resistance of the component is of paramount importance, other characteristics also need to be taken into consideration. Power dissipation, noise, inductance, thermal stability and a number of other characteristics may all have a bearing on the operation of the circuit in which the resistor is used.
Different materials and different structures within the resistor can have a major impact. Accordingly when selecting a resistor to be used, these characteristics must also be taken into consideration.
Basic distinction of resistor types
The first major categories into which the different types of resistor can be fitted is into whether they are fixed or variable. These different resistor types are used for different applications:
Fixed resistors: Fixed resistors are by far the most widely used type of resistor. They are used in electronics circuits to set the right conditions in a circuit. Their values are determined during the design phase of the circuit, and they should never need to be changed to "adjust" the circuit. There are many different types of resistor which can be used in different circumstances and these different types of resistor are described in further detail below.
Variable resistors: These resistors consist of a fixed resistor element and a slider which taps onto the main resistor element. This gives three connections to the component: two connected to the fixed element, and the third is the slider. In this way the component acts as a variable potential divider if all three connections are used. It is possible to connect to the slider and one end to provide a resistor with variable resistance. Carbon film preset potentiometer
Variable resistors and potentiometers are widely used for all forms of control:- everything from volume controls on radios and sliders in audio mixers to a host of areas where a a variable resistance is required. Potentiometer & variable resistor
Strictly a potentiometer is a component where there is a fixed resistor which has a slider to provide a potential division from he voltage at the top. A variable resistor is effectively the same, but with the slider linked to one end of the resistor so that it provides a true variable resistance.
There are a number of different types of fixed resistor:
Carbon composition: The carbon composition resistor is a type of resistor that was once very common - it was the main type of resistor, but are now seldom used because newer forms of resistor provide better performance, they are smaller and also cheaper.
Carbon composition resistors are formed by mixing carbon granules with a binder which was then made into a small rod. This type of resistor was large by today's standards and suffered from a large negative temperature coefficient.
The resistors also suffered from a large and erratic irreversible changes in resistance as a result of heat or age. In addition to this the granular nature of the carbon and binder lead to high levels of noise being generated when current flowed.
Carbon film: This type of resistor was introduced during he early days of transistor technology when power levels tended to be lower. Carbon film resistor
The carbon film resistor is formed by "cracking" a hydrocarbon onto a ceramic former. The resulting deposited film had its resistance set by cutting a helix into the film. This made these resistors highly inductive and of little use for many RF applications. They exhibited a temperature coefficient of between -100 and -900 parts per million per degree Celcius. The carbon film is protected either by a conformal epoxy coating or a ceramic tube.
Metal oxide film resistor: This type of resistor is now the most widely used form of resistor. Rather than using a carbon film, this resistor type uses a metal oxide film deposited on a ceramic rod. As with the carbon film, the the resistance can be adjusted by cutting a helical grove in the film. Again the film is protected using a conformal epoxy coating. This type of resistor has a temperature coefficient of around + or - 15 parts per million per °Celcius, giving it a far superior performance to that of any carbon based resistor. Additionally this type of resistor can be supplied to a much closer tolerance, 5% or even 2% being standard, with 1% versions available. They also exhibit a much lower noise level than carbon types of resistor, however it has mainly been superseded but he metal film resistor.
Metal film resistor: The metal film resistor is very similar to the metal oxide film resistor. Visually it is very similar and the performance is also comparable. Instead of using a metal oxide film, this type of resistor uses a metal film as the name indicates. Metals such as nickel alloy may be used. Leaded metal film resistor
The metal film resistor is the type that is most widely used when a leaded resistor is needed.
Wire wound resistor: This resistor type is generally reserved for high power applications. These resistors are made by winding wire with a higher than normal resistance (resistance wire) on a former.
The more expensive varieties are wound on a ceramic former and they may be covered by a vitreous or silicone enamel. This resistor type is suited to high powers and exhibits a high level of reliability at high powers along with a comparatively low level of temperature coefficient, although this will depend on a number of factors including the former, wire used, etc. As wire wound resistors are often intended for high power applications, some varieties are designed so that they can be mounted onto a heatsink to ensure that the power is dissipated into metalwork so it can be carried away.
In view of their wound nature, they are not suitable for operation above low frequencies, although by winding parts of the resistance wire in different directions the inductance can be reduced somewhat.
Surface mount resistors: Surface mount technology, SMT is now the major format used for electronic components. They are easier to use in automated manufacturing, and they are able to provide very high levels of performance. SMT resistors utilise similar technologies to other forms, but in a surface mount format.
Whilst the majority of resistors are standard fixed resistors or variable resistors, there is a number of other resistor types that are used in some more niche or specialised applications.
Light dependent resistor / photoresistor: Light dependent resistors or photoresistors change their resistance with the level of light. They are used in a number of sensor applications and provide a very cost effective solution in many instances.
Typical leaded light dependent resistor
Light dependent resistors have a lag in the time taken to respond to light changes, but they are cheap and easy to use.
Thermistor: As the name indicates, thermistors are heat sensitive resistors. The resistance of the thermistor varying with temperature. Some have a negative temperature coefficient, NTC thermistors, whist others have a positive temperature coefficient, PTC thermistors.
Varistor: Varistors are available in a number of forms. Essentially these electronic components vary their resistance with the applied voltage and as a result they find uses for spike and surge protection. Often they may be seen described as Movistors, which is a contraction of the words Metal Oxide Varistor.
Selection of leaded varistors
Varistors are the devices that are widely used in surge or transient protected mains extension leads and used to protect computers. It should be remembered that each time the varistor receives a spike its properties change slightly.
Although resistors may be thought of as simple electronics components to use, there are a number of parameters that need to be considered when choosing the correct resistor type. Parameters apart from just the resistance are important. Voltage withstand, power dissipation and the actual type of resistor itself all have an impact on the performance. With a variety of resistor types available, it is necessary to choose the correct type for any particular application. In this way the best performance can be assured.
Written by Ian Poole . Experienced electronics engineer and author.
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21 November 2024
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Read more about . . . . Carbon Composition Resistors.
Written by Ian Poole .
