What is the Power Rating of a Resistor : Working & Its Uses

The flow of current throughout a resistor can be done because of the existence of voltage across it, so electrical energy can be lost through the resistor in heat form. The flow of current is higher; the resistor will get heat more which is known as the power rating of the resistor. Generally, resistors are rated by their resistance value and the electrical power can be given in watts. Based on resistor size, the electrical power is dissipated securely. Every resistor includes a maximum power rating which can be determined through its size. Once the surface area of the resistor is high then the power can be dissipated into the ambient air securely. So this article discusses an overview of a power rating of a resistor and its calculation.


What is the Power Rating of a Resistor?

Definition of power rating of a resistor is; the maximum amount of energy that a resistor can dissipate safely. Generally, the power rating is one of the specifications of a resistor, and sometimes, it is also called the resistor wattage rating. Joule’s first law states that the electrical power which is generated is the product of the voltage across the resistor & the flow of current throughout the resistor.

Power Rating of Resistor
Power Rating of Resistor

For instance, if the power rating of a resistor is ½ watts then it is the maximum amount of power that must be fed into the resistor. If the power is increased to a few percent more than the power rating, then the resistor will burn.

How to Determine Power Rating of a Resistor?

The power rating of a resistor can be determined by monitoring its package size. Typical through-hole resistors are available with ½W or ¼W ratings. But power resistors are actually available with power ratings on the resistor. For example; 3W, 5W & 25W power resistors are available with 0.1Ω, 2Ω, 3Ω & 22kΩ values.

The surface mount resistor power ratings can be determined by their size. Both 0603& 0402 size resistors are generally rated with 1/16W whereas 0805 sizes can take 1/10W.

The power rating of a resistor formula can be determined by using the electrical power formula given;

P = V * I

Where ‘V’ is voltage, and ‘I’ is current.

From Ohm’s Law V = I × R

Where ‘R’ is resistance

So P = P = V * I can become as P = I^2 × R => P = V^2 / R

How to Measure Power Rating of a Resistor?

The power rating for a resistor can be measured by using the standard equations which are given below.
Power P = V * I

P = I^2 × R

P = V^2 / R

The power rating can be rated in watts or W and it is the units of power. Usually, if the resistor size is larger, then the resistor can handle more power. When the wattage of the resistor enhances then the cost will be increased. Generally, this rating ranges from 1/8th watt to kilowatts.

Electrical Power Units

The electrical power units with values, symbols, and abbreviations are given in the following table.

Unit

Symbol Value

Abbreviation

milliwatt

mW

1/1,000watt 10^-3W

kilowatt

kW 1,000watts

10^3W

megawatt mW 1,000,000watts

10^6W

Example Problems

The resistor power rating example problems are given as;

Example-1: If a fixed resistor has 10V voltage across its terminals and the flow of current through it is 40mA then what is the maximum wattage or power rating of a resistor in watts?

We know the values of V = 10V and I = 40mA

The formula for power rating within watts is P = V*I.

P = 10 x 0.04 = 400mW or 0.4 Watts.

Example-2: When the current flows throughout a 1.8KΩ resistor which is rated at 0.5 Watts then calculate the maximum safe current flowing through the resistor.

We know the values of R = 1.8 KΩ and P = 0.5 Watts

According to the standard power equation P = I^2R.

I^2 = P/R => I = √P/R

Substitute the power and resistance values in the above equation then we can get the flow of current through the resistor.

I = √0.5/1800 => 0.016A or 16mA.

So, the maximum safe current flowing through the resistor is 0.016A or 16mA.

Chart

The size of any type of resistor mainly depends on its power or wattage rating. When the resistor size is high, then it handles with a high wattage value. The following resistor power rating chart will provide information regarding the rating of various resistors & their sizes.

Resistor Size
Resistor Size

Power Rating in Watts

Diameter in mm Length in mm Lead length in mm

Lead Diameter in mm

1/8

1.8 3 28 0.45

1/4

2.5 6.5 28 0.6

1/2

3.2 8.5 28 0.6

1

5 11 28

0.8

2 5.5 15 35

1

3 6 17 35

1.1

5 7.5 24 38

1.2

  • When1/8 Watt Resistor is connected to a 9V source, and then it will be 1.8 x 3 x 28mm. So it handles 13.88 mA.
  • When1/4 Watt Resistor is connected to a 9V source, and then it will be 2.5 x 6.5 x 28mm. So it handles 27.77 mA.
  • When1/2 Watt Resistor is connected to a 9V source, and then it will be 3.2 x 8.5 x 28mm. So it handles 55.55 mA.
  • When1Watt Resistor is connected to a 9V source, and then it will be 5 x 11 x 28mm. So it handles 111.11 mA.
  • When a 2 Watt Resistor is connected to a 9V source, and then it will be 5.5 x 15 x 35mm. So it handles 222.22 mA.
  • When 3 Watt Resistor is connected to a 9V source, and then it will be 6 x 17 x 35mm. So it handles 0.4 A.
  • When5 Watt Resistor is connected to a 9V source, and then it will be 7.5 x 24 x 38mm. So it handles 0.8 A.

Uses of Power Rating in Resistors

The use of power rating in resistors includes the following.

  • The resistor power rating is the specification that specifies the highest amount of power that the resistor can withstand.
  • It is a significant factor to consider while selecting a resistor for a specific application. The main function of a resistor is to resist the flow of current throughout a circuit by dissipating useless power like heat.
  • All resistors have a Maximum Dissipated Power Rating, which means the maximum amount of power it can safely dissipate without damage to itself. Once the maximum rating of the resistor is increased then the resistor will be damaged.

Thus, this is all about an overview of the power rating of a resistor with example problems. Generally, resistors are used in different applications but all types of resistors are not used in all the applications. The selection of resistors can be done based on different parameters like color code and power rating. Resistor color code is used to read parameters like resistance, voltage & tolerance whereas power rating in the circuit is used to avoid the circuit from damage. Here is a question for you, if a resistor has an above 5W power rating then that resistor is known as?

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