Encyclopedia of Electrical Engineering
Encyclopedia of Electrical Engineering


What is resistor?

Resistor is a passive two-terminal element to resist electric current in electric circuits. In almost all electrical networks and electronic circuits they can be found. The resistance is measured in ohms.
An ohm is the resistance that occurs when a current of one ampere passes through a resistor with a one volt drop across its terminals.
Fig.No.1: Two-terminal Passive element
The current is proportional to the voltage across the terminal ends. This ratio is represented by Ohms law: formula with ohm's law: $$R=V/I$$
Resistors are used for many purposes. A few examples include delimit electric current, voltage division, heat generation, matching and loading circuits, control gain, and fix time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude. They can be used to as electric brakes to dissipate kinetic energy from trains, or be smaller than a square millimeter for electronic.

Resistor Symbol

Fig.No.2: Resistor Symbol

What are the Resistor Types

There are two main types of resistors: Fixed resistor and Variable resistor
What is a fixed resistor?
Fixed value resistors have a defined ohmic resistance and are not adjustable. Fixed resistors are the most commonly used resistors and in general one of the most used electronic components. Fixed resistors are available in axial leaded and surface mount packages as well as more customized packages depending on their application. While axial leaded resistors used to be the most used resistors, nowadays the advantages of surface mount devices make the SMD resistors the most popular.
Fig.No.3: Carbon film
Fig.No.4: Metal film
Fig.No.5: Surface mount

What is a variable resistor?

A variable resistor is a resistor of which the electric resistance value can be adjusted. A variable resistor is in essence an electro-mechanical transducer and normally works by sliding a contact (wiper) over a resistive element. When a variable resistor is used as a potential divider by using 3 terminals it is called a potentiometer. When only two terminals are used, it functions as a variable resistance and is called a rheostat. Electronically controlled variable resistors exist, which can be controlled electronically instead of by mechanical action. These resistors are called digital potentiometers.

Types of variable resistors

A simple potentiometerThe potentiometer is the most common variable resistor. It functions as a potential divider and is used to generate a voltage signal depending on the position of the potentiometer. This signal can be used for a very wide variety of applications including: Amplifier gain control(audio volume), measurement of distance or angles, tuning of circuits and much more. When variable resistors are used to tune or calibrate a circuit or application, trimmer potentiometers or trimpots are used, this are mostly small potentiometers mounted on the circuit board, which can be adjusted using a screwdriver.
A rheostat, you can see the wiper which moves over a wirewound resistor.Rheostats are very similar in construction to potentiometers, but are not used as a potential divider, but as a variable resistance. They use only 2 terminals instead of the 3 terminals potentiometers use. One connection is made at one end of the resistive element, the other at the wiper of the variable resistor. In the past rheostats were used as power control devices in series with the load, such as a light bulb. Nowadays rheostats are not used as power control anymore as this is an inefficient method. For power control, rheostats are replaced by more efficient switching electronics. Preset variable resistors, wired as rheostats are used in circuits to perform tuning or calibration.
Digital resistor
DIP8 digital resistor packageA digital variable resistor is a type of variable resistor where the change of resistance is not performed by mechanical movement but by electronic signals. They can change resistance in discrete steps and are often controlled by digital protocols such as I2C or by simple up/down signals.