What are fixed resistors?
The electrical behavior of a resistor obeys Ohm’s law for a constant resistance; however, some resistances are sensitive to heat, light or other variables. Resistors can have a fixed resistance value, or they can be made variable or adjustable within a certain range
The fixed resistance is an electrical component designed to introduce a known resistance value in a circuit. Resistors are often made of carbon chips or thin films of carbon or other resistant materials.
Types of fixed value resistor
Carbon Composition Resistors:
The carbon composition resistors consist of a solid cylindrical resistive element with embedded wire cables or metal caps to which the lead wires are attached. The body of the resistance is protected with paint or plastic.
The resistive element is made of a mixture of finely ground carbon (powder) and an insulating material (usually ceramic). A resin keeps the mixture together. The conductive path is from particle to particle, each of which touches another along the trajectory.
Carbon Film Resistors:
A carbon film is deposited on an insulating substrate and a helix is cut to create a long and narrow resistive path. Variable shapes, together with carbon resistivity (ranging between 90 and 400 nΩ m) can provide a variety of resistances
Carbon film resistors have a power range of 0.125 W at 5 W at 70 ° C. Available resistors range from 1 ohm to 10 M
Metal Film Resistors:
The introduction of metal film technologies brought significant reductions in the size of the resistance and noise. The metallic film resistors are manufactured by the evaporation or sputtering of a layer of nickel chromium on a ceramic substrate. The thickness of the layer depends on the value and varies from 10 Angstroms to 500 Angstroms in thickness. The thicker this layer is (the lower the value), the less noise is inserted.
There are resistances that resemble the types of metallic films, but they are made of metal oxides such as tin oxide. This results in a higher operating temperature and greater stability / reliability than the metal film. They are used in applications with high resistance demands.
The winding resistors are made of alloys similar to those used in the sheet resistors, which are described below. As a result, the only noise insertion caused by these devices comes from the tabs used to connect the thin wire to the thick outer wires. Due to the very high surface temperature, these resistances can withstand temperatures up to +450 ° C
The types with aluminum housing are designed to be connected to a heat sink to dissipate heat; the rated power depends on use with a suitable heat sink, for example, a resistor with a power of 50 W will overheat a fraction of the power dissipation if it is not used with a heat sink. Large winding resistors can have a nominal capacity of 1,000 watts or more.
The applications of winding resistors are similar to those of composition resistors, with the exception of high frequency. A typical noise index is -38 dB. The high frequency of wire resistances is substantially worse than that of a composition resistance which is the main objection
The main resistance element of a sheet resistance is a special alloy sheet several micrometers thick. The aluminum resistors have had the best precision and stability of all the available resistors. One of the important parameters that influence stability is the resistance temperature coefficient (TCR). The TCR of sheet resistances is extremely low and has been further improved over the years. A range of ultra-precision sheet resistors offers a TCR of 0.14 ppm / ° C, tolerance ± 0.005%,