Types of Resistor: Fixed and Variable Resistors

All resistors are classified into two major classes. A fixed resistor exhibits essentially constant value of resistance, whereas a variable resistor exhibits variable resistance. In this post, you'll learn about the difference between fixed and variable resistors.



Let's start with the summarized table of data:

Fixed Resistors

Carbon Resistors

Carbon type resistor is a fixed resistor which contains an insulating body surrounding the carbon. The value of resistance is varied by changing the amount of carbon, a lower proportion of carbon results in higher resistance and vice verse. Carbon resistors are cheaper as well as their manufacturing process is very easy.

Features of Carbon Resistors

Feature	Detail
Designed for Resistance	1 Ω-100 MΩ
Designed for Power	0.125 W - 2 W

Carbon film

A thin carbon film surrounds the ceramic carrier acting as a resistive material. 

Features of Carbon Film Resistors

Feature	Detail
Designed for Resistance	1 Ω-10 GΩ
Designed for Power	0.625 W - 2 W
Tolerence value	2% / 5% / 10% / 20%
Practical applications	HV applications, X-rays, Laser

Metal film

They comprise a thin metallic film surrounding the resistive element. They are reliable and are have excellent tolerance values. However, they are expensive.

Features of Metal Film Resistors

Feature	Detail
Designed for Resistance	1 Ω-10 MΩ
Designed for Power	35 W
Tolerence value	0.1% / 0.25% / 0.5% / 1% / 2%
Practical applications	Low noise applications, X-rays, Laser, Bridges such as Wein, Wheatstone

Variable Resistors

Potentiometer and Rheostat

The class of variable resistors whose value of resistance can be varied by using a circular knob. Although both potentiometer and rheostat are same devices, the names potentiometer is given to configuration when a variable resistor is set to provide a variable voltage on the output. Whereas the name rheostat is given to the device which provides variable current in the circuit.

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Potentiometer	Rheostat
Provides variable voltage	Provides variable current

Thermistor

The thermistor is a variable resistor whose value of resistor changes with the change in temperature. Thermistors are of two types. NTC and PTC.

NTC Thermistor	PTC Thermistor
Resistance decreases when temperature increases	Resistance increases when temperature increases