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What's the best method to calculate the wattage of a resistor & cap in any circuit ?

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dineshdeshmuk

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What's the best method to calculate the wattage of a resistor & cap in any circuit ?

What's the best method to calculate the wattage of a resistor & cap in any circuit ?
 

FoxyRick

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

For a resistor, P = VI = I²R = V²/R. Take your pick, it's basic stuff, then select a resistor with at least that rating. If you have AC, then use the RMS values. Don't forget to de-rate the power rating of your chosen resistor if it is in a hot enclosure (usually above 70C, check the datasheet)

Capacitors are a little more complicated, see here:

http://www.johansondielectrics.com/...ac-power-ratings-for-dc-rated-capacitors.html
 

dineshdeshmuk

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

The voltage of the resistor would be the input voltage of the circuit , how to calculate the voltage of the resistor to calculate the wattage of it ?
 

rfredel

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

Hello dineshdeshmuk,

the voltage to calculate the wattage is the voltage drop across the resistor. Also you need the current.

For example:

You need a resistor to connect a LED to 12VDC. The LED has a forward voltage of 2V and a current of 20mA.
So you have to calculate:

R = (12V - 2V) / 0,02A = 500Ω

P = (12V - 2V) * 0,02A = 0,2W

Regards

Rainer
 

dineshdeshmuk

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

how did you know that forward voltage drop is 2v and current is 20mA ?
 

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

how did you know that forward voltage drop is 2v and current is 20mA ?
Those figures are approximate for an LED (not white or blue) - one would get the information from the LED's datasheet. Every LED's datahseet will have a current/forward voltage pair, it is the most basic specification for the LED. White and blue LED's are a different type of semiconductor material so have a much different voltage (more around 3.5V to 4V)

Even from the datasheet the numbers are an approximation though, based on the mean values that they have tested their LEDs at. That is because the manufacturing process cannot produce LEDs perfectly the same all the time.
 

dineshdeshmuk

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

it means Kirchhof's laws and the OHms law are very important for the calculation of the resistor and capacitors values and their respective wattages ????

- - - Updated - - -

Those figures are approximate for an LED (not white or blue) - one would get the information from the LED's datasheet. Every LED's datahseet will have a current/forward voltage pair, it is the most basic specification for the LED. White and blue LED's are a different type of semiconductor material so have a much different voltage (more around 3.5V to 4V)

Even from the datasheet the numbers are an approximation though, based on the mean values that they have tested their LEDs at. That is because the manufacturing process cannot produce LEDs perfectly the same all the time.

But the 20mA current is obvious for all the types of LEDs ??? Is it so ?
 

rfredel

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

Hello dineshdeshmuk,

it means Kirchhof's laws and the OHms law are very important for the calculation of the resistor and capacitors values and their respective wattages ????
Yes, Kirchhof's and Ohm's law are very important for electronic calculation.

20mA is a average value for normal LED. For low power or SMD LED the average current is 5mA.

The forward voltage of red LED is around 2V, for IR LED around 1,5V and for colored LED about 4V.

But for the correct voltage and current ask the datasheet.

Regards

Rainer
 

dineshdeshmuk

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

can you provide me some examples including some difficult and complicated examples and kindly can do explain to me with their calculations of the wattages and the values of the capacitors and resistors included in those circuits examples ?
 

FoxyRick

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Re: What's the best method to calculate the wattage of a resistor & cap in any circui

dineshdeshmuk, please allow me to draw your attention to one of the rules that you can read if you click the 'Rules' link at the top of the page:

Do not contact users asking for help or to advertise
Don't make requests for help in private using personal messages, create a thread in the forum so that other members can benefit from the posted answers.
DO NOT ever send private messages to ANY user offering to sell services or products UNLESS the user ASKS you to contact them.
If any user see this behaviour of "stealth" advertising / spam, please report the post.
You have now done so several times to me, even when I have not been involved in your threads. I have already asked you not to do this in return PM, to which you have not responded. An apology would have been appropriate.

I am happy to provide help on the forum. Not by PM. You seem in need of some very basic instruction in electronic theory and yet are asking questions such as how to choose cable for 50kW loads and repair highly technical equipment. Many of your questions could be answered in a few minutes searching or reading. I find it rather confusing, and worrying.

Please do not PM me with any more questions. I am not your personal tutor. If you persist in sending me PM requests for personal help, I will refer the matter to a moderator.
 

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