What is Resistor?


To build an electronic circuit different components are required, and resistor is one of them. In all electronic circuits they can be found very easily. That means it is one the most important components in electronics.



Definition of Resistor -It is a passive electronic component and contains two terminals. It is used to limit the flow of electric current in a circuit. It is a bi-directional component which means it allows the flow of current in both directions, unlike diode which is a unidirectional component which allows the flow of current only in one direction. A resistor doesn’t have polarity that means it doesn’t have any positive and negative terminals. Resistors are used for many reasons such as voltage division, heat generation, adjusting signal levels and terminate transmission lines etc.

Construction of Resistor- Resistors can be made up of a variety of materials. But most of the resistors nowadays are made from of either a carbon, metal, or metal-oxide film. The first layer on the top is called protective coating or the insulating layer on which the color coding is printed. And just below the insulating layer there is a thin metal film. On the metal film layer a helical groove is cut to control the value of resistance. The resistor has two terminals.



How Resistors Work?

As we know that electric current flow through the metallic wire because there are lots of free electrons inside and move in a random direction when there is no potential difference across the ends of wire. But when potential is applied across the ends of wire electrons start to attract or flow towards to +ve terminal of the battery. Because opposite charge particles attract each other electrons do have negative charge on them. It is called flow of current in a wire because it has large number of free electrons that are responsible for current flow. But in the case of resistor the metal film layer from which the current is supposed to flow is made up of a material in which free electrons are in very less quantity.
And if electrons are less there will be less amount of flow of electrons, that means less current will flow through the resistor. Electrons collide with ions which slow down the flow of electricity and lowers the current and produces heat. That heat is absorbed by the ceramic core because the ceramic works as a heat conductor and can handle thermal stress very well so that the desired resistance levels can be obtained and maintained across the resistor.


Resistance- The measure of opposition applied by resistor to the flow of electric current is known as resistance. Resistance is an important concept in electrical engineering and physics, as it plays a crucial role in the operation of electrical circuits and devices.

Formula of is Resistance:   R =V/I
Where I= current,
V= voltage,
R=resistance

Above formula states that resistance is directly proportional to voltage, and resistance is inversely proportional to current. Which means if the current is held constant, an increase in voltage will result in an increase in resistance. Alternately, an increase in current while holding the voltage constant will result in a decrease in resistance.


Symbol of Resistor







SI Unit of Resistance

The SI unit resistance is the ohm, represented by the symbol Ω(omega). 1Ω is defined as the amount of resistance in a conductor that will cause a current of one ampere to flow through it when a potential difference of one volt is applied across its ends.

In other words, resistance of a resistor is said to be 1 ohm when current of 1 ampere flows through it and having a potential difference of 1 Volt across it.


Factors Affecting Resistance

The resistance of a conductor depends upon four factors:

Material-For example silver is a better conductor than copper. Which means wire made up of silver will have less resistance than wire made of copper.
Length-The longer the wire, higher the resistance.
Cross Section Area-The thicker the wire, smaller is the resistance.
Temperature-Most of the materials have higher resistance at higher temperatures. As temperature increases, the flow of electrons inside a conductor also increases. This means that the collision between the electrons increases, which increases the resistivity inside the conductor.

(Note: In some materials- notably Carbon and Electrolytes the resistance decreases as the temperature rises.)


Resisters in Series

Resistors are said to be connected in series when –ve of the first resistor is connected to the +ve of the second resistor and the –ve of second resistor is connected to the +ve terminal of the third resistor and soon on.




When resistors are connected in series the equivalent resistance (Req) is the sum of individual resistance of each resistor.

Req = R1 + R2 + R3 + ... + Rn

In a series circuit, the current across all resistors remains the same. But the voltage across each resistor in series is different. And the total voltage across the circuit is the sum of the voltages across all the resistors.

Resistors in Parallel-

Resistors are said to be connected in parallel when –ve terminal of first resistor is connected to –ve of the second resistor and –ve of the second resistor is connected to the –ve terminal of the third resistor and so on. And the same connections are made with all positive terminals.

Resistors in parallel.svg

The equivalent resistance of resistors connected in parallel is always less than the smallest individual resistance present in the circuit.

1/Req = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

In parallel circuit, resistors do not have the same current across each of them. The sum of current across each resistor adds up to the total current across circuit. But resistors in a parallel configuration have the same potential difference (voltage) across each of them.

Specific resistance-

Specific resistance or resistivity is a measure of how strongly a material can resist the electric current flow. In other words, it is defined as the resistance given by a material with a unit length and unit area of cross section to the electric current flow.

We know that-
  • Resistance of any conductor is directly proportional to its length i.e. R l.
  • Resistance of any conductor is inversely proportional to the area of cross section.
In the equation of the above image: R = Resistance in ohms
a = Area in meter square
l = Length in meter
Where ρ is known as specific resistivity or resistance of material. Constant of proportionality (ρ) on the place of ∝ after removing it. If the length of the material is in meter and cross section area is square meter,
then SI unit of specific resistance is ohm meter.




How to Calculate Resistor Value?


Resistor Color Code- You have seen different color bands on the resistor protective layer. These color bands are used to indicate the resistance value of a resistor. But the value of a resistor can also be checked via a multimeter. A resistor can have 6 color stripes on it. Different resistors do have different color bands printed on them. If a resistor is 330Ω it will have different color bands printed on it than a resistor of 440Ω.

If you want to calculate the value of a resistor it could be tricky but not that hard. The resistor color code is read from left to right as shown in the below image.

How to find which side is left. It is easy. The tolerance band is always on the right side. Which is separated from other color bands by some distance on the resistor as you can see in the above image. Gold color band is printed separately from brown and red color bands. The space between the last two bands, the last band and second last band, indicates the reading direction. In the above image three color bands are separated from gold color tolerance band to show the reading direction. The arrow shows the left to right reading direction. But sometimes you will find that in a resistor all the color bands are printed on a resistor and there is no gap between bands. So, you can take the metal color of a band as right side because metal color band never printed on the left side of a resistor.

To calculate the value of a resistor it requires a resistor color code table as well.



Resistor Color Code Table





Example 1: 4 Band Resistor

The resistor shown below contains 4 color bands.




1st and 2nd band indicates digits ---- 2, 2

To know the digits, see the resistor color code table above. In which red color indicates 2 and again red indicates 2.

3rd band indicates multiplier which is brown. Look at the table brown color band has a multiplier of 10.

Now multiply the digits with the multiplier.
2, 2x10 = 22x10 = 220Ω

4th band represents tolerance of the resistor ---- ±5%.
Look at the table to know the tolerance, see gold color tolerance section which shows ±5%.



Example 2: 5 Band Resistor

The resistor shown below contains 5 color bands.




1st, 2nd and 3rd band indicates digits ---- 1, 0, 2

4th band indicates the multiplier ---- 0.01
1, 0, 2x0.01 = 102x0.01 = 1.02Ω

5th band indicates the tolerance which is ± 5%.



Example 3: 6 Band resistor

The resistor shown below contains 6 color bands.




1st, 2nd and 3rd bands in the above resistors indicates digits ---- 1, 9, 6

4th band indicates multiplier ---- 10
1, 9, 6x 10 = 196x10Ω = 1960Ω or 1.9

5th band indicates tolerance ---- ± 1%

6th band indicates temperature coefficient ---- which is 50ppm (parts per million) for red color band.