## Definition of Resistor

A resistor offers resistance to the flow of current. The resistance is the measure of opposition to the flow of current in a resistor. More resistance means more opposition to current. The unit of resistance is ohm and it is represented as Ω. When one volt potential difference is applied across a resistor and for that one ampere of current flows through it, the resistance of the resistor is said to be one Ω. Resistor is one of the most essential passive elements in electrical and electronics engineering.

## Resistor

It is some time required to introduce electrical resistance in different circuit to limit the current through it. **Resistor** is an element of circuit which does the same. Such as series connected resistor limits the current flowing through the light emitting diode (LED).

In addition to that **resistors** serve many other purposes in electrical and electronic applications. The most essential requirement of a resistor is that its value of electrical resistance should not vary with temperature for a wide range. That means resistance variation with temperature must be as minimal as possible for a wide range of temperature. In other word the temperature coefficient of resistance of must be minimum for the materials by which a **resistor** is made of.

### High Frequency Effects on Resistor

The effective resistance value of a resistor may be changed when it is subjected to alternating voltage. Not only that the value of resistance varies with variation of frequency of supply voltage. This change of resistance with frequency is called Boella effect. Actually it is not practically possible to make an ideal resistor. Practically it may have some inductor and capacitance in addition to its resistance. Therefore, the value of impedance of the resistor may vary with frequency. That is why a resistor is referred to be used within its useful frequency range. Useful frequency range is defined as the highest frequency limit beyond which the impedance of the resistor crosses its tolerance value.

High frequency effect on resistor depends upon its constructional feature. The impedance of wire wound resistor increases with frequency. On the other hand composition resistor reduces its impedance with increasing frequency. The impedance of film resistors does not change up to 100 MHz and then it decreases. Film resistors have the most stable high-frequency performance.

The high frequency response of a resistor may also depend up to some extend on diameter of the resistor. It is seen that smaller diameter resistor has better frequency response. Because of that the length – diameter ration of high-frequency resistors is between 4:1 to 10:1.

### Voltage Coefficient of Resistance

The resistance of a resistor may also vary sometime with applied voltage across it. This change is expressed as a percentage of the resistance at 10 % of rated voltage. Thus the voltage coefficient of resistance is give by

Where, R_{1} is the resistance at the rated voltage V_{1} and R_{2} is the resistance at 10% of rated voltage V_{2}.

### Noise of a Resistor

Noise in a resistor is caused by its applied voltage, its physical dimensions, and materials by which it is made of. The noise includes Johnson noise, noise due to

Johnson noise is thermal noise. This noise depends upon temperature but does not depend upon frequencies. As this noise is same for all frequencies it is referred as “white noise”. The magnitude of thermal noise is given by,

Where, E_{RMS} is the value of the noise voltage (V) in ^{ – 23} J/K), T is the temperature measured in Kelvin, and Δf is the bandwidth in Hz over which the noise energy is measured.

### Power Rating of Resistor

When current passes through a resistor there would be I^{2}R loss and hence as per Joules law of heating there must be temperature rise in the resistor. A resistor must be operated within a temperature limit so that there should not be any permanent damage due high temperature. The power rating of resistor is defined as the maximum power that a resistor can dissipate in form of heat to maintain the temperature within maximum allowable limit. How much power a resistor will dissipate depends upon material, dimensions, voltage rating, maximum temperature limit of the resistor and ambient temperature.

### Voltage Rating of Resistor

This rating is defined as the maximum voltage that can be applied across a resistor due to which power dissipation will be within its allowable limit.

From above equation it is clear that for limiting P, V must be limited for a particular resistor of resistance R. This V is voltage rating of resistor of power rating P watts and resistance R Ω.

#### Resistor Color Code

There are mainly four color bands provided on the body of resistors and each color indicates unique digit. Such as Black ⇒ 0, Brown ⇒ 1, Red ⇒ 2, Orange ⇒ 3, Yellow ⇒ 4, Green ⇒ 5, Blue ⇒ 6, Violet ⇒ 7 Gray ⇒ 8, White ⇒ 9. The first and second color bands indicate a two digits number. The 3rd color band indicates the power of ten as multiplier. The fourth band indicates the tolerance.

Note: We created a resistor color code calculator to make it easy for you to figure out the color code you need, or the resistance of your resistors (works for 3, 4, and 5 band resistors!).

If

Suppose we have a carbon composition resistor which has four color bands among which first band is blue second band is yellow, third band is red and fourth band is golden. So from the above rule the first digit of the number will be 6 ( as Blue ⇒ 6 ), the second digit of the number will be 4 ( as Yellow ⇒ 4 ) and the multiplier of this ^{2} ( as Red ⇒ 2 ). Hence, electrical resistance value of the resistor will be 64 × 10^{2} Ω. The tolerance of that value may be ± 5 % as the color of fourth band is golden.