Resistors can provide for a number of means, often being employed for reducing current flow, adjusting signal levels, dividing voltages, terminating transmission lines, and much more. They are also quite common to aircraft construction, allowing for electrical systems to be managed for safety and efficiency. In this blog, we will discuss some of the main types of aircraft resistors that are used and how they are coded, allowing you to best understand their benefit to aviation applications.
The fixed resistor is a fairly simplistic type, often used to limit the amount of current flow within a circuit. Generally, these resistors will have set characteristics such as their materials and size that determine their resistance value, precision, and heat dissipation capabilities. Carbon composition resistors are constructed from a mixture of carbon or graphite that is paired with a filler to bind the materials together. This mixture is then formed into a rod which is fitted with leads on each end before a protective coating is added to the component.
Alongside the carbon composition resistor, other various aircraft resistors may be used, such as carbon film, metal oxide, metal film, and metal glaze resistors. When constructing a film resistor in particular, such components rely on the deposition of a resistive material on a ceramic rod or substrate. Depending on the type of film resistor, the resistive material may come in the form of graphite, nickel-chromium, metal and glass, or metal and an insulating oxide. Variable resistors are another type that differ from fixed resistors, such types featuring the ability to change resistance. Generally, variable resistors come in the form of rheostats and potentiometers.
As resistors of the same type may differ in their ohmic values or may not be able to reach specific standards, such components are commonly value marked with color coding. The resistor color coding system consists of color groups, numbers, and tolerance values. Generally, a specific color will be tied to a number, and with those details, a tolerance value can be attained. For example, a yellow marking is tied to the number 4, and such colors denote that the resistor has a tolerance of 4%. Meanwhile, a white color code is tied to the number 9, featuring a tolerance of 9%. These colors are often implemented as bands on the end of a resistor, not to be confused with the general color of the resistor’s body.
In many cases, a resistor will feature upwards of three or four marks on their body, those of which can be different colors. Generally, the first color band that is closest to the end refers to the first digit of the resistance value. Meanwhile, the second color indicates the second digit. With the third band, the number of zeros following the initial two digits may be determined. If the third band is gold, then the two digits are multiplied by 10. If the third banner is silver, then the digits should be multiplied by 1%. In the case that a fourth band is present, then the tolerance percentage can be multiplied by the number tied to the specific color. If there is no fourth band, then the tolerance is typically 20%.
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