There are several ways in which fixed resistors (units whose resistance does not change, or cannot be adjusted) are manufactured. Here are the most common types.
The cheapest method of making a resistor is to mix up powdered carbon (a fair electrical conductor) with some non conductive substance, press the resulting clay like stuff into a cylindrical shape, and insert wire leads in the ends following figure. The resistance of the final product depends on the ratio of carbon to the nonconducting material, and also on the physical distance between the wire leads. This results in a carbon-composition resistor.
Construction of a carbon-composition resistor.
Carbon-composition resistors can be manufactured in a wide range of resistance values. This kind of resistor also has the advantage of being nonreactive, meaning that it introduces almost pure resistance into the circuit, and not much capacitance or inductance. This makes carbon composition resistors useful in radio receivers and transmitters.
Carbon-composition resistors dissipate power according to how big, physically, they are. Most of the carbon-composition resistors you see in electronics stores can handle 1⁄ 4 W or 1⁄ 2W. There are 1⁄ 8-W units available for miniaturized, low-power circuitry, and 1- or 2-W units for circuits where some electrical ruggedness is needed. Occasionally you’ll see a carbon-composition resistor with a much higher power rating, but these are rare.
Another way to get resistance is to use a length of wire that isn’t a good conductor. The wire can be wound around a cylindrical form as a coil following figure. The resistance is determined by how well the wire metal conducts, by its diameter or gauge, and by its stretched-out length. This type of component is called a wirewound resistor.
Construction of a wirewound resistor.
Wirewound resistors can be manufactured to have values within a very close range. They are precision components. Also, wirewound resistors can be made to handle large amounts of power. A disadvantage of wirewound resistors, in some applications, is that they act like inductors. This makes them unsuitable for use in most radio-frequency circuits. Wirewound resistors usually have low to moderate values of resistance.
Carbon, resistive wire, or some mixture of ceramic and metal can be applied to a cylindrical form as a film, or thin layer, in order to obtain a specific resistance. This type of component is called a carbon- film resistor or metal-film resistor. Superficially, it looks like a carbon-composition resistor, but the construction is different following figure.
Construction of a film-type resistor.