The majority of the resistors used in the electronic circuits are of the type fixed resistors. The fixed resistors are of the type resistors which has the fixed resistance value. It is not possible to change/vary the resistance of the fixed resistor.
The classification and applications of resistors are discussed in the following article.
A fixed resistor can be defined as the resistor whose resistance does not change with the change in voltage or temperature. Fixed resistors are available in various sizes and shapes.
There are three types of Fixed Resistors.
- Carbon composition resistors
- Film type resistors
- Wire wound resistors
Carbon Composition Resistors
Carbon composition resistors are the most common type of composition resistors. Their resistive element is manufactured from a mixture of finely ground carbon dust or graphite (similar to pencil lead) and a non-conducting ceramic (clay) powder to bind it all together.
The ratio of carbon dust to ceramic (conductor to insulator) determines the overall resistive value of the mixture and the higher the ratio of carbon, the lower the overall resistance.
The mixture is moulded into a cylindrical shape with metal wires or leads are attached to each end to provide the electrical connection as shown, before being coated with an outer insulating material and colour coded markings to denote its resistive value.
- They are used in High frequency applications.
- They are used in power control circuit.
- They are used in DC power supplies.
- They are used in the general-purpose electronics equipment.
Film Type Resistors
The generic term “Film Resistor” consist of Metal Film, Carbon Film and Metal Oxide Film resistor types, which are generally made by depositing pure metals, such as nickel, or an oxide film, such as tin-oxide, onto an insulating ceramic rod or substrate.
The resistive value of the resistor is controlled by increasing the desired thickness of the deposited film giving them the names of either “thick-film resistors” or “thin-film resistors”.
Thin and thick film resistors are the most common types in the market. They are categorized by a resistive layer on a ceramic base. Although they might look very similar, their properties and manufacturing processes are very different.
The naming originates from the different layer thicknesses. Thin film has a thickness in the order of 0.1 μm (micrometer) or smaller, while thick film is thousands times thicker.
However, the main difference is the method used to apply the resistive film onto the substrate. Thin film resistors have a metallic film that is vacuum deposited on an insulating substrate. Thick film resistors are produced by firing a special paste onto the substrate.
The paste is a mixture of glass and metal oxides. Once deposited, a laser is used to cut a high precision spiral helix groove type pattern into this film.
The cutting of the film has the effect of increasing the conductive or resistive path, a bit like taking a long length of straight wire and forming it into a coil.
Thin film resistor is more accurate, has a better temperature coefficient. It is more stable. Therefore, it competes with other technologies that feature high precision, such as wirewound or bulk metal foil.
On the other hand, thick film is preferred for applications where these high accuracy and high temperature coefficient are not critical since prices are much lower.
- These resistors are used radio amplifiers.
- They are used in oscillators.
- They are used in Digital multimeters.
Wirewound Type Resistors
Another type of resistor, called a Wirewound Resistor, is made by winding a thin metal alloy wire (Nichrome) or similar wire onto an insulating ceramic former in the form of a spiral helix similar to the film resistor above.
These types of resistors are generally only available in very low ohmic high precision values (from 0.01 to 100kΩ).
The non-inductive resistance wire is wound around a ceramic or porcelain tube covered with mica to prevent the alloy wires from moving when hot.
- They are used in high power circuits.
- They are used in in the electrical heating elements.