Types of Resistor
There is a wide variety of resistors available, but they may be divided into the broad groups of fixed and variable resistors.
A fixed resistor has its total resistance determined during the manufacturing process, and there are no provisions for changing this value. The user orders the total resistance required from a broad selection of standardized values, and other values are available on special order. Of a variety of fixed resistor types available, the composition, film and wirewound resistors are three common types.
In this type of resistor the resistance element consists of a very finely powdered conducting material, such as graphite or carbon, mixed with a powdered insulating material, such as silicon or talc stone, together with a plastic binding material. These materials are pressed into the form of a rod with a wire lead imbedded part way into each end. This conducting core is covered with a layer of Bakelite with good insulating and moisture-resisting properties. The final construction is known as an insulated composition resistor with axial leads.
For any given power rating, all resistors have the same body size, regardless of the total resistance value. The total resistance is not adjusted by changing the length or diameter of the conducting rod, but rather by changing the resistivity by the adjustment of the proportions of the conducting and insulating materials.
Advantages of composition resistors are small size, low cost, good frequency-response characteristics, and available resistance values ranging from 1 Ω to 22 MΩ. They are commonly available with resistance tolerances of 20, 10, and 5% and with wattage ratings of 1/4, 1/2, 1, and 2 W.
Disadvantages are that composition resistors are subject to aging effects and have high voltage and temperature coefficients. In addition, tolerances of less than 5% are not available except at substantially increased cost, and such lower tolerances are difficult to maintain during the normal use of the resistor.
Carbon-film resistors are made by depositing a thin layer of carbon on a ceramic rod or tube. The thickness of the film is varied to give the desired total resistance. Terminals are made by coating the ends with graphite or conductive paint or by plating the ends with metal. Resistances up to several megaohms can be obtained by control of film thickness. Higher resistance values are obtained by cutting a continuous spiral through the film over the full length of the tube. The remaining carbon is thus in the form of a band wound around a ceramic core. Controlling the width of this spiral provides a means of obtaining the desired total resistance. High-quality commercial types are coated with resin over which is placed a baked-on, moisture-resistant coating. Hermetically sealed types have an outer ceramic shell to seal out moisture and air and to give additional mechanical protection.
Advantages of carbon-film resistors are that they have negligible voltage coefficient, very low temperature coefficient, and excellent frequency-response characteristics. They represent a good compromise between composition resistors and precision wirewounds. Resistors of 200 Ω have a temperature coefficient of about 0.025% resistance change per deg. C. For resistors of 10 MΩ this change is in the order of 0.05%.
Metal-film resistors have the same general construction as carbon-film resistors except that the film consists of metal or metal alloy. Through the use of suitable alloys, temperature coefficients lower than those for carbon can be obtained. Available power and tolerance ratings are equivalent to those for carbon-film types.
The general construction of wirewound resistors consists of a metal alloy wire wound around a solid or hollow core made of insulating material (e.g. ceramic). Metal rings with provisions for attaching terminals are crimped around the ends of the ceramic core, and the ends of the windings are connected to these rings. The windings and end rings are next coated with a ceramic paste which is then baked. Coatings which harden without baking are also used. The coating provides electrical insulation, holds the windings in place, and prevents mechanical and moisture damage. In addition, the ceramic coating is a good heat conductor, transmitting heat readily from the resistance wire to the surrounding air.