Definition of Resistor:

The property of a substance or material which opposes the flow of electric current is called Resistance or Resistance is the ability of a circuit which opposes the flow of electric current. The component which has this property is called a resistor. The unit of resistance is Ohm (Ω). The resistor is a nonpolarized component that has the ability to flow the current in both directions. Resistors are used in electric or electronic circuits to limit current and drop the voltage according to the desired value.

Symbol of Resistor:

Resistor Symbol

Types of Resistor:

There are various kinds of resistors that are available in different shapes, sizes, and materials but resistors are basically classified into two types explained in detail as:

1. Linear Resistors:

Those resistors never change their resistance with a change in temperature, applied voltage, and use. In other words, a resistor, which current value is directly proportional to the applied voltage is known as a linear resistor. Generally, there are two types of linear resistors that have the property of linearity called fixed and variable resistors.

A) Fixed Resistor:

Fixed resistors are those resistors which never change their properties are called fixed value resistors. Fixed resistors are further classified into four types listed below:

a. Carbon Composite Resistor:

A fixed resistor is a mixture of granulated or powdered carbon or graphite, insulation filler, or resin binder. The ratio of the insulation material determines the actual resistance of the resistor. The insulating powder is a binder made in the shape of rods and there are two metal caps on both ends of the rod. A coating of plastic covers the rods with different color codes which represent the value of resistance. Such resistors are available in a range from 1 ohm to 25 mega ohms with different watts from 0.25 watt to up to 5 Watts. 

Carbon Composed Resistor

b. Wire-wound Resistor:

Wire wound resistors are made of wrapped resistive wire on an insulating core or rod. Generally Tungsten, Nickle chromium alloy, manganin, Nichrome. The insulating core is made of porcelain, Bakelite, press bond paper, or ceramic clay material.

The manganin wire wound resistors are very expensive that’s why they are used in sensitive testing equipment like Wheatstone bridge. These wire resistors are available in the range of 2 watts to 100 watts more and in the resistance range of 1Ω up to 200kΩ or more. The operating temperature range of such resistors is up to 350°C.

Wire-wound Resistor

c. Thick Film Resistors:

The production method of Thick film resistors is the same as thin-film resistors, but the difference is that there is a thick film instead of a thin film or layer of resistive material around. That’s why it is called Thick film resistors. There are two additional types of thick film resistors.

i. Metal Oxide Resistors:

By oxidizing, a thick film of Tin Chloride on a heated glass rod (substrate) is the simple method to make a Metal oxide Resistor. These resistors are available in a wide range of resistance with high-temperature stability. In addition, the level of operating noise is very low and can be used at high voltages.

Metal Oxide Resistors.

ii. Cermet Film Resistor:

In the cermet oxide resistors, the internal area contains ceramic insulation materials. A carbon or metal alloy film or layer is wrapped around the resistor and then fix it in a ceramic metal (which is known as Cermet). They are made in a square or rectangular shape and leads and pins are under the resistors for easy installation in printed circuit boards. They provide a stable operation in high temperatures because their values do not change with changes in temperature.

Cermet Film Resistor

iii. Fusible Resistors:

            These kinds of resistors are the same as a wire wound resistor. When a circuit power rating increases than the specified value, then this resistor is fused, i.e. it breaks or opens the circuit. That’s why it is called Fusible resistors. Fusible restores perform double jobs means they limit the current as well as it can be used as a fuse.

            They are used widely in TV Sets, Amplifiers, and other expensive electronic circuits. Generally, the ohmic value of fusible resistors is less than 10 Ohms.

Fusible Resistors

d. Thin Film Resistors:

Thin film resistors are made of a ceramic rod with resistive material. A very thin conducting materials coating overlaid on an insulating rod made of high-quality ceramic material or glass. There are two further types of thin-film resistors.

i. Carbon Film Resistors:

Carbon Film resistors contain an insulating material rod or core made of high-grade ceramic material which is called the substrate. A very thin resistive carbon layer or film overlaid around the rod. These kinds of resistors are widely used in electronic circuits because of negligible noise and wide operating range and stability as compared to solid carbon resistors.

Carbon Film Resistors

ii. Metal Film Resistors:

Metal film resistors are the same in construction as Carbon film resistors, but the main difference is that there is metal (or a mixture of the metal oxides, Nickel Chromium, or a mixture of metals and glass which is called metal glaze which is used as resistive film) instead of carbon. Metal film resistors are very tiny, cheap, and reliable in operation. Their temperature coefficient is very low (±2 ppm/°C) and used where stability and low noise level are important.

Metal Film Resistors

B) Variable Resistor:

Resistors those resistances can be varied properly with the help of a dial, knob, selector switch, or screw is called a variable resistor. Variable resistors have a sliding arm, which is connected to the shaft. The value of resistance can be varied by rotating the arm. Such resistors are used in different electrical Lab., TVs and radio equipment are for volume control and tone control resistance. Variable resistors are further classified into the following types given below;

a. Potentiometer

Potentiometers are three-terminal resistors used to control voltage levels in circuits. The outer terminals of the resistor have fixed resistance while the third terminal is connected with a movable wiper for resistance variation. The value of resistance can be changed by rotating the wiper which is connected to the control shaft.

            Potentiometers are also used for voltage dividing in circuits. These resistors are called variable composition resistors. They are available up to 10 Mega Ohms.


b. Trimmers:

Potentiometers with additional screws for better efficiency and operation are known as Trimmers. Resistance can be changed by rotating the screw’s position with the screwdriver. Trimmers are made from carbon films, cermet’s, carbon composition, and wire terminals.  These trimmers are available in the range from 50Ω up to 5MΩ. The power rating of Trimmers can be are from 1/3 watt to 3/4 watt.



Rheostats are three-terminal variable resistors used for the current limiting purpose by hand or manual operation in circuits. Rheostats are also known as tapped resistors or variable wire wound resistors. Nichrome wire is wrapped around a ceramic core which is assembled in a protective casing.


2. Nonlinear Resistors:

Those resistors, which change their resistance with a change in temperature, applied voltage are called Non-linear resistors. There are four types of resistors that have properties of nonlinearity are photo-resistors, thermistors, varistors, and surface mount resistors.

a. Thermistors:

Thermistors are two-terminal device that is very sensitive to temperature. In other words, Thermistor is a type of variable resistor which notices the change in temperature. Thermistors are made from cobalt, Nickel, Strontium, and the metal oxides of Manganese. The Resistance of Thermistors is inversely proportional to the temperature, i.e. resistance increases when temperature decrease and vice versa.

            Thermistors have a negative temperature coefficient (NTC) but there is also a PTC (Positive Temperature Coefficient) which is made from pid barium Titanate (BATiO3) semiconductor materials and their resistance increases when temperature increases.


b. Varistors:

                  Varistors are voltage-dependent Resistors (VDR), used to eliminate the high voltage transients. These are special types of variable resistors used to protect circuits from destructive voltage spikes is called varistors. 

            When voltage increases (due to lighting or line faults) across a connected sensitive device or system, then it reduces the level of voltage to a secure level i.e. it changes the level of voltages.


c. SMD Resistor:

Surface Mount Device (SMD) resistors also known as Surface Mount Technology resistors are very small surface-mount type resistors used in various printed circuit boards. These tiny chips are mounted on the surface represent a value code on it. Because they cover very small areas so used in very small size devices like USB or Flash Drives etc.

SMD Resistor

d. Photo Resistors (LDR):

Photo Resistor or LDR (Light Dependent Resistors) is a resistor whose resistance changes with change in light intensity. This resistor, which resistance values change with the incident light on their surface, is called Photo Resistor or Photo Conductive Cell or LDR. The material which is used to make these kinds of resistors is called photoconductors, e.g. cadmium sulfide, lead sulfide, etc.

Photo Resistors (LDR)