What Is a Resistor? Symbol, Types, Unit, Applications

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element to limit the flow of electric current. It is used in electronic circuits for voltage division, current reduction, noise suppression, and filtering.

But the resistor is much more than that. So, if you’re new to electronics or just want to learn more about what a resistor is, then this blog post is for you!

what is a resistor

What does the resistor do in the electronics circuit?

A resistor is an electronic component that controls the flow of current in a circuit and offers resistance to electric current. Resistors prevent voltage spikes, power surges, and interference from getting to sensitive electronics like digital devices.


Resistor symbol and unit

The unit for resistance is ohms (symbol Ω).

resistor symbol and unit

Resistor Characteristics

Resistors are electronic components that limit the flow of an electric current to a prescribed amount. The simplest resistors have two terminals, one called the “common terminal” or “ground terminal” and one called the “wiper terminal.” Resistors are wire-based components, but other geometric shapes have also been used.

I hope you now better understand what a resistor is.

The two most common geometric shapes are a block called a “chip resistor” and a button called a “carbon-composition resistor”.

Resistors have colored bands around their bodies to indicate their resistance values.

Resistor color code

Resistors will have a color code to represent their electrical value. It’s based on a coding standard that was originally devised in the 1950s by the Joint Electron Components Industries Association.

The code comprises three colored bands, which signify – from left to right – significant digits, the number of zeros, and the tolerance band.

Here is a table of resistor color codes.

resistor color code

You can also use a resistor color code calculator.

Resistor types

Resistor types are available in many sizes, shapes, power ratings, and voltage limits. Knowing the type of resistor is important when selecting resistors for a circuit because you need to know how it will react under certain conditions.

Carbon-Composition Resistor

A carbon-composition resistor is one of the most common forms of resistor today. It has excellent temperature stability, and low noise characteristics and can be used over various frequencies.

Carbon-composition resistors are not designed for high-power dissipation applications.

Carbon composition resistor

Metal Film Resistor

A metal film resistor consists primarily of an evaporated coating on aluminum that acts as the resistance material with additional layers that provide insulation protection from heat and a current-carrying coating to complete the package. Depending on the type, a metal film resistor may be designed for high accuracy or power applications.

metal film resistor

Carbon Film Resistor

This resistor is similar in construction to a metal film resistor, except that it contains additional layers of insulating materials between the resistance element and conductive coatings to provide additional protection from heat and current levels. Depending on the type, a carbon film resistor may be designed for high accuracy or power applications.

carbon film resistor

Wire wound Resistor

This is a universal term given to any resistor that has a resistance element made of wire instead of thin film as described above. Wire wound resistors are usually used when a resistor must carry or dissipate high power levels.

wire wound resistor

High Voltage Variable Resistor

This resistor has a resistance element made of carbon instead of thin film and is used in applications that require high voltage insulation and high stability at elevated temperatures.

High voltage variable resistor


A potentiometer can be thought of as two variable resistors connected back-to-back. The resistance between the two outside leads will vary as the wiper is moved along the track until the maximum and minimum limits are reached.



This resistor has a positive temperature coefficient, which causes its resistance to increase with rising temperature. In most cases, it is used for its negative temperature coefficient of resistance, where it has a resistance that decreases with rising temperature.



This resistor is designed to protect circuits from high voltage transients by providing a very high resistance initially, then reducing to a lower value at higher voltages. The varistor will continue dissipating the applied electrical energy as heat until it is destroyed.


SMD Resistors

They are small, do not require mounting surfaces to install, and can be used in very high-density grid arrangements. The disadvantage of SMD resistors is that they have less heat-dissipating surface area than through-hole resistors, reducing their power-handling capability.

SMD Resistors are typically made of ceramic materials.

SMD Resistors are usually much smaller than through-hole resistors because they don’t need mounting plates or holes in the circuit board to install them. They also take up less space on the circuit board, allowing for higher circuit densities.

The disadvantage of using SMD resistors is that they have much less heat dissipation surface area than through-holes, reducing their power handling capability. They are also harder to manufacture and solder than through-hole resistors due to their very fine lead wires.

SMD Resistors were first introduced in the late 1980s. Since then, smaller, more precise resistor technologies like Metal Glaze (MoGL) Resistor Networks and Chip Resistor Arrays (CRAs) have been developed, leading to further reductions in the size of SMD Resistors.

Today SMD Resistor technology is the most widely used resistor technology; it’s fast becoming the dominant technology. Through-hole resistors are quickly disappearing into history as today they are solely for niche applications like car audio, stage lighting equipment, and “classic” instruments.

Resistor applications

Resistors are used in the circuit boards of radios, TVs, telephones, calculators, instruments, and batteries. 

There are many different types of resistors, each with its applications. Some examples of using resistors:

  • Device protection: This can protect devices from damage by limiting the current that passes through them.
  • Voltage regulation: can be used to regulate the voltage in a circuit.
  • Temperature control: This can be used to control the temperature of a device by dissipating heat.
  • Signal attenuation: This can be used to attenuate or reduce the strength of a signal.

Resistors are also used in many common household items. Some examples of home devices:

  • Light bulbs: A resistor is used in a light bulb to regulate current flow and create a consistent brightness.
  • Ovens: An oven uses a resistor to limit the current that passes through the heating element. This helps to prevent the element from overheating and damaging the oven.
  • Toasters: A resistor is used in a toaster to limit the current that passes through the heating element. This helps to prevent the element from overheating and damaging the toaster.
  • Coffee makers: A coffee maker uses a resistor to limit the current that passes through the heating element. This helps to prevent the element from overheating and damaging the coffee maker.

Resistors are an important component in digital electronics and are used in a variety of applications. They are available in various tolerance levels, wattages, and resistance values.

How to use resistors in a circuit

There are two ways they can be used in an electrical circuit.

  • Resistors in series are resistors where the circuit current has to flow through each resistor. They are connected serially, with one resistor placed next to the other. When two or more resistors are connected in series, the total resistance of the circuit increases according to the rule:

Rtotal = R1 + R2 + ………Rn

Resistors in series
  • In parallel, resistors are connected across different branches of an electric circuit. They are also known as parallel-connected resistors. When two or more resistors are connected in parallel, they divide the total current flowing through the circuit without changing its voltage.

To find the equivalent resistance of resistors in parallel, use this formula:

1/Req = 1/R1 + 1/R2 + ……..1/rn

Resistors in parallel

The voltage across each resistor has to be the same. For example, if four 100 Ohm resistors are connected in parallel, all four will have an equivalent resistance of 25 Ohms.

The current going through the circuit will remain as if a single resistor was used. The voltage across each 100 Ohm resistor is pinched by a factor of four, so instead of having 400 Volts across each resistor, they now only have 25 Volts.

Ohm law

Ohm’s law is the most basic of all the laws of electric circuits. It states that “The current which passes through a conductor between two points is directly proportional to the voltage difference between the two points and inversely proportional to the resistance between them.”

V = I x R or V/I = R


V = Voltage (volts)

I = Current (amps)

R = Resistance (Ohms)

There are 3 variants of Ohm’s law with multiple applications. The first variant can calculate the voltage drop across a known resistance.

The second variant can be used to calculate the resistance of a known voltage drop.

And in the third variant, you can calculate the current.

Ohm law

Video Tutorial about what is a resistor

Learn more about resistors.


Thanks for reading! I hope you learned what is a resistor and how it control the flow of current. If you’re having a hard time learning electronics, don’t worry. We have lots of other blog posts and videos to teach you the basics of electronics engineering.

Alex Klein Author


Alex Klein is an electrical engineer with more than 15 years of expertise. He is the host of the Electro University YouTube channel, which has thousands of subscribers.