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Fixed resistors come in a variety of different shapes, sizes and forms. Axial lead resistors have the value of resistance printed on them or as a colour code. Surface mount resistors have a numerical code indicating a value. All resistors have a tolerance value.
There are four types of fixed resistor:
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.
This type is general purpose resistor. It uses a ceramic core coated with a metal oxide film. These resistors are mechanically and electrically stable and readable during high temperature operation. They contain a special paint on their outer surfaces making them resistant to flames, solvents, heat, and humidity. Typical resistances range from 1 Ohm to 200 kOhm, with typical tolerances of +/- 5 percent.
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.
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.