An element used to reduce supply voltages to some desired value or to limit current.
A resistor is a small component of two leads. A wide variety of resistors is used in the electronics industry today.
Carbon composition resistor
The carbon composition resistor is the basic mass-produced resistor of the electronics industry. Billions are used each year. They have low failure rates when properly used. Power ratings available are 1/8, 1/4,1/2, and 2 W in voltage ratings of 250, 350 and 500 V. standard tolerances are 5 % and 10 %. Figure 1 shows these resistors in the wattage ratings mentioned.
Fig.1: Carbon Composition Resistors arranged in their standard Wattage Ratings
The resistive element is a combination of carbon particles and a binding resin; the properties are varied to provide the desired resistance. Attached to the end of the resistive element are metal caps with axial leads of tinned copper wire for soldering the resistor into a circuit. The unit is enclosed in a plastic case to prevent moisture and other harmful elements from entering. Figure 2 shows the construction.
Fig.2: Internal construction of typical carbon composition resistor
Carbon composition resistors are primarily suitable where performance requirements are not demanding and where the lowest possible cost can be achieved. They are extensively used in entertainment electronics, though better resistors such as metal glaze are often used in critical circuits.
Carbon Film Resistor
Carbon film resistors are made by placing ceramic rods in a methane-filled flask and heating it until, by a grass-cracking process, a carbon film is deposited on the cores. A helix-grinding process forms the resistive path.
Compared with carbon composition, deposited carbon offers a major improvement in lower current noise and particularly in close tolerance. A typical carbon film resistor is shown in the following figure.
Fig.:3: Carbon film resistor
Metal film resistor
Metal film resistors are made by the deposition of vaporized metal in a vacuum on a ceramic core rod. The resistive path is helix-ground in the same way as for the carbon-film resistor. The metal film resistance range is not as wide as the carbon range, going from about 10 Ω to 10 M Ω.
Where the metal film excels in tolerance and temperature coefficient (TC). For example, 1 % tolerance is standard, and 0.1 % is available. Metal film resistors have the greatest reliability of any type.
The metal film is definitely the superior resistor for numerous high-grade applications, such as low-level stages of certain instruments. However, the cost is 20 to 40 times the cost of carbon composition. A typical metal film resistor is shown in the following figure.
Fig.:4: Metal film resistor
Power resistor | Wire Wound Resistor
Wire wound resistors are substantially different from all other types in that no film or resistive coating is used. A drawn wire with precisely controlled characteristics is wound around a ceramic core form. Different wire alloys are used to provide different resistance ranges. Wire-wound resistors are characterized by highest stability, highest power rating, and special TC or resistor fuse functions. And only a wire wound can withstand up to 250 W.
However, certain drawbacks to wire-wound resistors place them out of the running for most nonspecialized requirements. In low-power, low cost, or high density, limited-space applications, other types are usually preferred.
Figure 5 shows the typical power wire-wound resistor. The complete unit is coated with an insulating material such as baked enamel. Power wire-wound resistors are not color-coded. Their resistance values, wattages, and type numbers are stamped on with a special high-temperature paint that does not burn off.
Fig.5: Power Wire-Wound resistor
The ceramic resistors are made by firing certain metals blended with ceramics on a ceramic substrate. The type of mix and its thickness determines the amount of resistance. Ceramic resistors have precise resistance values and great stability at extreme temperatures. They are often produced as small rectangles with leads to attach to printed circuit boards. As with other resistors types, they are available with several different resistance values all in one package. Figure 6 shows a dual inline package (DIP), which is commonly used with PC boards.
Fig.6: Dual In-Line Package (DIP): Typical Resistance Network
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