Types of Resistors

Resistors perform two basic functions in a circuit: they limit current flow and set voltage levels within a circuit.

We have various kinds of resistors e.g. fixed resistors, variable resistors, digitally adjustable resistors, photoresistors, fusible resistors, etc.

Generally, resistors can be broadly categorized as:

• Fixed resistors
• Variable resistors
• Special purpose resistors

Fixed Resistors

Fixed resistors are those whose value cannot be changed after manufacture. They are classified as follows:

• Composition resistors
• Wire-wound resistors
• Metal-film resistors

Composition Resistors

Composition resistors are composed of carbon particles mixed with a binder. The mixture is moulded into a cylindrical shape and hardened by baking. Leads are attached axially to each end and the assembly is encapsulated in a protective encapsulation coating. Color bands on the outer surface indicate the resistance value and tolerance.

Composition resistors are economical and exhibit low noise levels for resistances above 1 MW. They are normally rated for temperatures around 70°C for power ranging from 1/8 to 2 W. Composition resistors have end to end shunted capacitance that may be noted at frequencies around 100 kHz especially for resistance values of above 0.3 MW.

Wire-wound resistors

These are made by winding wire of nickel-chromium alloy on a ceramic tube, then covering with a vitreous coating. The spiral winding has inductive and capacitive characteristics that make it unsuitable for operation above 50 kHz. The frequency limit can be raised by non-inductive winding so that the magnetic fields produced by the 2 parts of the winding cancel.

Metal-film resistors

These are commonly made of nichrome, tin-oxide, or tantalum nitride. They are either hermetically sealed or use moulded-phenolic cases. Metal film resistors are not as stable as the wire-wound resistors.

Depending on the type of application, fixed resistors are manufactured as precision resistors, semi-precision resistors, standard resistors, general resistors and power resistors.

Precision resistors have low voltage and power coefficients, superb temperature and time stabilities, low noise and very low reactance. These resistors are available in metal-film or wire construction and are normally designed for circuits having close resistance tolerances on values.

Semi-precision resistors are smaller than precision resistors and are mainly used for current-limiting or voltage-dropping functions in circuit applications. They have long-term temperature stability.

General purpose resistors are employed in circuits that do not require tight resistance tolerances or long-term stability. They have a high coefficient of resistance and high noise levels. For general purpose resistors, initial resistance variation may be around 5 % and the variation in the resistance under full rated power may approach 20%.

Power resistors are employed in power supplies, control circuits, and voltage dividers where operational stability of 5% is acceptable. They are available in wire wound and film construction. Film type power resistors have the advantage of being stable at high frequencies and high resistance value than wire-wound resistors for a given size.

Variable Resistors

Potentiometer

A potentiometer is a special form of a variable resistor with three terminals. Two terminals are connected to the opposite sides of the resistive element, and the third connects to a sliding contact that can be adjusted as voltage divider. The variable potentiometer is usually in a circular form with a movable contact attached to the shaft that rotates.

Potentiometers are manufactured as carbon composition, metallic film or wire-wound resistors available in single turn or multi-turn units. The movable contact doesn’t go all the way towards the end of the resistive element. A small resistance called the hop-off resistance is used to prevent burning of the resistive element.

Rheostat

This is a current-setting device in which one terminal is connected to the resistive element and the second terminal is connected to a movable contact to place a selected section of the resistive element in the circuit. Normally rheostats are wire-wound resistors used in speed controls of motors, ovens and heater controls. They are also employed in applications where adjustments on the voltage and current levels are needed such as voltage dividers and bleeder circuits.

Related: Resistance and Resistivity

Special Purpose Resistors

Integrated Circuit Resistors

These are classified into two general categories:

• Semiconductor resistors
• Deposited film resistors

Semiconductor resistors use the bulk resistivity of doped semiconductor regions to acquire the desired resistance value. Semiconductor resistors can further be divided into four types:

• Diffused
• Bulk
• Pinched
• Ion implanted

The diffused semiconductor resistors use resistivity of the diffused region in the semiconductor substrate to introduce a resistance in the circuit. Both the n-type and p-type diffusions are used to form the diffused resistor.

Deposited film resistors are formed by depositing resistance films on an insulating substrate which are etched and patterned to form the desired resistive network. They are classified into thick-film and thin-film resistors depending on the thickness and dimensions of the deposited films.