- Information, tutorial about the basics of the different types of resistors including fixed and variable resistors, carbon, film, carbon composition, metal film, etc
There are many different types of resistor available for use within electronic circuits. These different resistor types have somewhat different properties dependent upon their construction and manufacture. This makes the different types of resistor suitable for different applications.
Over the years the resistor types used in mass electronics production have changed. Years ago, all the resistors used had leads and were relatively large, and by today's standards they offered a low level of performance. Today, the resistor types used are much smaller and offer much higher levels of performance.
Fixed & variable 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.
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.
- 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 ppm / °Celcius. The carbon film is protected either by a conformal epoxy coating or a ceramic tube.
- Metal film / metal oxide: 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 film deposited on a ceramic rod. Metals such as nickel alloy, or a metal oxide such as tin oxide are deposited onto the ceramic rod. The resistance of the component is adjusted in two ways. First the thickness of the deposited layer is controlled during the initial manufacturing stages. Then it can be more accurately 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 ±15 parts per million / °K, 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%, ±2% being standard, and with ±1% versions available. They also exhibit a much lower noise level than carbon types of resistor.
- 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..
- Thin film: Thin film technology is used for most of the surface mount types of resistor. As these are used in their billions these days, this makes this form of resistor technology one of the most widely used.
Leaded and non-leaded resistor types
One of the key differentiators for resistors, and many other forms of component these days is the way in which they are connected. As a result of the mass production techniques sued and the widespread use of printed circuit boards, the form of connection used for components, especially those to be incorporated into mass produced items changed.
The two main forms of resistor type according to their connection method are:
- Leaded resistors: This type of resistor has been used since the very first electronic components have been in use. Typically components were connected to terminal posts of one form or another and leads from the resistor element were needed. As time progressed, printed circuit boards were used, and the leads were inserted through holes in the boards and typically soldered on the reverse side where the tracks were to be found.
Typical leaded carbon resistor
- Surface mount resistors: These resistor types have been used increasingly since the introduction of surface mount technology. Typically this type of resistor is manufactured using thin film technology. A full range of values can be obtained.
Typical SMD resistors on a PCB
By Ian Poole
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