What do 1/4, 1/2, 1 or 5 watts values mean on a resistor?

I know that this is such a basic question but what do those values mean when selecting a resistor? 

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The value is the maximum wattage the part can dissipate without damage. The value specified is usually for room temp (25C), and the rating will need to be de-rated for higher temperatures.

Some types of resistors may need heatsinking to meet the stated power specs. The large 5, 10, 25, and 50 watt resistors with aluminum cases usually need a fairly large heat sink to achiew their ratings.

So, if P=I^2*R or P=V^2/R for the resistor in your application is less than the rated power (taking into consideration any derating at higher temps) then your part will be fine.

Suppose if I want to purchase the 15 milli Ohm resistor , when I am thinking about the Power Dissipation.

In my circuit,

Max expected current is 5A and maximum possible power dissipation,

Pd = ILoad (max)^2 * Rsh = 5A * 5A * 15 mOhm = 375 mW

Does it mean that the resistor I choose that should be more than 375mW ?

Because, Can I choose if the resistor 15mohm with 1W and 1% . In this case, should I consider this resistor or Can I use this resistor for my circuit with above given requirements?

Please help!

P=I x R means watts = Volts x Amps.

If you are not sure which value to pick, post your situation here and we may be able to recommend a value for you.
blkhawk (author)  Awesome-aniac3 years ago
Let say a circuit where the voltage is 20 and 1 A current. So P=20, what should be the Watt value of the resistor?
20 Watts.
Actually, 25 watts or higher (I keep putting my foot in my mouth).
blkhawk (author)  Awesome-aniac3 years ago
Shouldn't I also take into consideration other components in the circuit?
The watt rating of a resistor is the MAX watts that it can handle. When selecting a proper resistor, you must select one that can handle more watts than you will be putting through it.

For example, if you know that .4 watts max will be passing through the resistor, select a 1/2 to 99 billion watt rated resistor. The wattage rating does not affect how the circuit operates, it is just used so that the resistor doesn't overheat.
Not correct: V = IR, means voltage = current x resistance (one volt = one amp-ohm). I think you meant P = I x V.
Yes, I did mean Watts = Volts x Amps, but it does NOT equal P=I x R. Thanks.