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What is a Resistor?

So, what is a resistor? A resistor is a material that resists, limits or impedes the flow of electrical current. Resistive materials include carbon and various metallic alloys such as nichrome. Resistors are the most common electronic component — DigiKey lists some 550,000 resistors — WOW! They come in all resistance values, tolerances, types, mounting styles, power ratings and physical sizes. Some are also adjustable as in rheostats and potentiometers.

This article is part of a long series of articles on resistors. Here are some of them:

Carbon composition resistors

brightly colored axial leaded carbon composition resistors

The once popular brightly colored axial leaded carbon composition resistors are cylindrically shaped devices inside which are encapsulated small cores of solid carbon. Tolerances generally range from ±5% to ±20%. Power ratings range from ¼W to 2W. Resistance values range from about 1ohm to 22M. They are non-inductive. Unlike the present carbon film devices, they have a high energy rating and are therefore good for pulse applications. They began to be phased out in the 1990’s and only a few select values are currently available.

Wirewound resistors

this particular wirewound axial lead version looks almost identical to the carbon composition resistor

While there will be another article dealing with wirewound resistors, just note that this particular axial lead version looks almost identical to the carbon composition resistor. This is the Ohmite WH /RN series. It is available in a number of power ratings, but the 2W version is the most common. The resistance range is limited to 0.1 to 10K due to its wirewound construction. http://www.ohmite.com/cat/res_wh_wn.pdf

Carbon film resistors

axial leaded carbon film resistors are constructed from a thin film of carbon

Axial leaded carbon film resistors are constructed from a thin film of carbon that is deposited upon a glass cylinder. The thin film of carbon is then laser etched in spiral fashion until the desired resistance is measured. They are slightly inductive, but this becomes an issue only in low resistance applications. The specifications are essentially the same as the carbon composition. Power ratings range from 0.125W to 2W. Years ago, the ½W resistor was the device of choice—then as the ¼W resistor became available, they quickly became the resistor of choice. When the 0.125W device started to come to age, it was quickly supplanted by the new surface mount devices—as a result, the 0.125W devices never really caught on—however, I prefer them for prototyping due to the extremely small size.

Metal film resistors

Precision resistors are generally of the metal film type. They are available in tolerances of 0.1% to 1% and also have a specified temperature coefficient of resistance. They may or may not be banded with color codes. Shown in the photo are the popular military grade (RN) and commercial grade (CN) style axial resistors. They are generally larger than similarly rated carbon resistors because they are rated at a higher temperature.

Thick film & thin film resistors

Thick film resistors are manufactured via a sputtering process in which metal is deposited upon a ceramic substrate—it is the most common and inexpensive type. Thin film technology yields much higher accuracy and is used for precision resistors. Susunu resistor primer

Resistor Networks or Arrays

SIP and DIP resistor arrays are popular

SIP (Single In-line Package) and DIP (Dual In-Line Package) resistor arrays are popular. They are manufactured via the thick film process and contain multiple resistors and complex networks. They come in numerous packages sizes.

For the future

SMD resistors
Wirewound & power resistors

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