Wangdu Downloads: Fundamentals of resistors

Detailed information about resistors…

Resistor – the resistor is an element which opposes to the flow of electric current. Resistance is the res

property of the material by which it opposes to the flow of current, the current may be either AC or DC; it offers equal resistance to both. Basically their are two types of resistors : fixed and variable resistors. The fixed resistors have a given fixed value. It does not change by any physical means. A fixed resistor has a specific value printed on its body, either in the form of color codes or in numerical. The general symbols of a fixed resistor is given here. Any fixed resistor is always denoted by “R” and if there are more than one resistors then they are shown as R₁, R₂, R₃ . . . . and so on. Variable resistor has a variable value over a fixed range. Its resistance can be changed by adjusting the knob attached to the shaft of the variable resistor. A variable resistor is also subdivided into four main types : all these types have the same function, only size and shape differs. Symbols of fixed and variable resistor are given below. dsv

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.

Classification of variable resistors – they are the variable resistors made up of carbon or wire wound type material. They are especially used for controlling voltage and current in the circuit. According to the quality they are useful for industrial, commercial and military grades circuits. There are also some modern types of variable resistors, which are known as ganged type (often called as tandem type also). They are in pairs which are connected internally to vary the values at a time. It is the most popular type of resistor. It is widely used in the control of volume, bass, treble brightness and contrast controls of tape recorders and TV sets. It has three terminals and its
resistance is fixed between the two outer terminals. The middle terminal is known as the wiper. Between the two adjacent points the resistance can be changed by turning the position of the shaft. This shaft can be attached to a control knob for convenience. It is made-up of carbon material and it has two main types : the linear type and logarithmic type. In linear type [known as LIN] the resistance changes linearly and uniformly. However in logarithmic type [known as LOG] the resistance changes on a log scale. Generally the LIN type resistors are used for controlling of volume and treble. In the control of bass, brightness and contrast the LOG types are used. In general the classification of potentiometers is as follows –

Classification of Potentiometers

Rheostat is an important device, in the high voltage and high current adjustments. They are made up of resistive wires like Nichrome, Tungsten and such high resistive materials. It has three terminals – the two end terminals are the end points of the complete wire and the middle terminals is connected to the wiper which is rested tightly on the naked portion of the wire. Its internal resistance, is decided by the length of the wire used. The basic disadvantage of this device over carbon variable resistor is that it has Ohmic resistance as well as inductive reactance (XL), since the resistive wire is wound in coil fashion. Hence it is not suitable where high frequency current is to be controlled. It is therefore specially used for limiting DC current.