- an overview of the different types of resistor that are available and how these different resistor types can be used.
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. Further details of variable resistor can be found on the variable resistors page accessible through the "Related Articles" list which can be found on the left hand side of this page below the main menu.
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 parts per million per degree Celcius. The carbon film is protected either by a conformal epoxy coating or a ceramic tube.
- 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 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 degree Celcius, giving it a far superior performance tot hat 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.
- 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.
Resistor power dissipation
Although the resistance is the key parameter for any resistor, another parameter which is very important is the amount of power it can dissipate. When current passes through a resistor power is dissipated and this manifests itself in the form of heat. In turn this cases the temperature of the resistor to rise, and if too much current passes through the resistor, the temperature rise can be too great and it can cause the resistance to change, or in extreme cases it can cause damage to the resistor.
The power dissipated in a resistor is easy to calculate. The basic equation for power is:
W = V x I
W = power in watts
V = voltage in volts
I = current in amps
It is often easier to combine this equation with Ohm's Law to create a more useful equation which calculates the power dissipated from a knowledge of the resistance and the voltage across it:
W = V2 / R
R = resistance in ohms.
All resistors have a power dissipation rating specification. This is the maximum power that they are designed to dissipate. The resistor type should be chosen so that this power level is never exceeded in operation. In fact good design practice dictates that the maximum power dissipation should be well inside this. Many electronics design companies operate a practice where they state that the maximum actual dissipation should never exceed around 60% of the rating of the particular type of resistor. By doing this, the reliability of the circuit is improved.
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 resistor are important. Voltage withstand, power dissipation and the actual type of resistor itself all have an impact on the performance. Which a variety of resistor types available, it is necessary to choose the correct one for any particular application. In this way the best performance can be assured.
Practical Electronics Handbook
Ian Sinclair, John Dunton