Resistors

The resistor is a passive electrical component that creates resistance in the flow of electric current. In almost all electrical networks and electronic circuits they can be found. The resistance is measured in ohms (Ω). An ohm is the resistance that occurs when a current of one ampere (A) passes through a resistor with a one volt (V) drop across its terminals. The current is proportional to the voltage across the terminal ends. This ratio is represented by Ohm’s law:

R = V/I

Resistors are used for many purposes. A few examples include limiting electric current, voltage division, heat generation, matching and loading circuits, gain control, and setting time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude. They can be used as electric brakes to dissipate kinetic energy from trains, or be smaller than a square millimeter for electronics.

Three Band Resistor Color Codes

The three band color code is very rarely used.

The first band from the left indicates the first significant figure of the resistance.
The second band indicates the second significant number.
The third band indicates the multiplier.
The tolerance for three band resistors is generally 20%.

The color code table corresponding to three band resistors is shown below.

Four Band Resistor Color Codes

Four band color code is the most common representation in resistors.

The first two bands from the left are used to indicate the first and second significant digits of resistance.
The third band is used to indicate the multiplier.
The fourth band is used to indicate tolerance.
There is a significant gap between third and fourth bands. This gap helps in resolving the reading direction.

The color code table for four band resistors is as shown below.

Five Band Resistor Color Codes

High precision resistors have an extra band which is used to indicate the third significant value of the resistance. The rest of the bands indicate the same things as four band color code.

The first three bands are used to indicate the first three significant values of resistance.
Fourth and fifth bands are used to indicate multiplier and tolerance respectively.
There is an exception when the fourth band is either Gold or Silver. In this case, the first two bands indicate the two significant digits of resistance.
Third band is used to indicate multiplier, fourth band is used for tolerance and fifth band is used to indicate the temperature coefficient with units of ppm/K.

The color code table for five band resistors is shown below.

Six Band ResistorColor Code

In case of high precision resistors, there is an extra band to indicate the temperature coefficient.
The rest of the bands are same as five band resistors.
The most common color used for sixth band is black which represents 100ppm/K.
This indicates that for a change of 100C in temperature, there can be a change of 0.1% in the value of resistance.
Generally the sixth band represents temperature coefficient. But in some cases it may represent reliability and failure rate.

The color code table for six band resistors is shown below

Resistor Formula

Below are the main formula associated with calculating resistor values, in both series and parallell