Resistors are one of the most widely used components in electronic circuits - there are many different types of resistor available having different prioperties.
Resistor Tutorial Includes:
Resistors overview Carbon composition Carbon film Metal oxide film Metal film Wirewound SMD resistor Variable resistors Light dependent resistor Thermistor Varistor Resistor colour codes Resistor specifications Standard resistor values & E series
The unit of resistance is the ohm, and the chief parameter for any resistor is its resistance. However there are a number of other parameters that are also important.
In view of these other resistor parameters there are several different resistor types that are available. In fact choosing the right type of resistor for a given application can be important.
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.
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. Read more about Variable resistors / potentiometers
Fixed resistor types
There are a number of different types of fixed resistor:
- Carbon composition: These types were once very common, but are now seldom used. They 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. Read more about Carbon Composition Resistors.
- Carbon film: This resistor type 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. Read more about Carbon Film Resistors.
- 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. Read more about Metal Oxide Film Resistors.
- 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. Read more about Metal Film Resistors.
- Wire wound: 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. In view of their wound nature, they are not suitable for operation above low frequencies. Read more about Wire Wound Resistors.
- 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. Read more about Surface Mount Resistors.
Other types of resistor
Whilst the majority of resistors are standard fixed resistors or variable resistors, there is a number of other resistor types that are sued 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.Read more about Light Dependent Resistors.
- Varistor: Varistors are available in a number of forms. They vary their resistance with the applied voltage and as a result they find uses for spike and surge protection. Read more about Varistors.
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.