# resistor wattage?

Discussion in 'General Electronics Chat' started by daviddoria, Jan 7, 2003.

1. ### daviddoriaNew Member

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I called DigiKey to order some components... so i was like
"i need a 47k resistor."
and she was like
"1% or 5%"
i knew what that was, so i said 5% (usually doesn't make a difference does it?)
then she said
"surface mount or through hole"
i guessed through hole
then she said "what wattage?"
i was like ummm i'll call you back

if i'm running a circuit on 3v (battery) then do i need 1W? 1/4W? whats the deal with that?

i'm fairly new, but i'm trying to learn via kits and stuff like that.

any info would be great

PS. i use the same msg board for my site, PHPBB2 is great!

2. ### kinjalgpActive Member

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Its not the voltage but the current that matters for finding wattage of resistor. The formula is P = I*I*R
P = Power in Watts
I = Current through resistor
R = Resistor value
Find out the current through your resistor and calculate the power dissipated in it.

3. ### bogdanfirstNew Member

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usually im most circuits u dont need more than 1/4W....
like kinjal said, it doent matter what voltage u use for the circuit....
all it matters is what current passes trough the resistor; for example if u have a resistor with the value R and the current passing trough it is I then the power disipated trough it would be P=I*I*R.
or if u know what voltage is across it P=U*U/R
for example if u have a resistor of 100ohm and the current trough it is 40mA then P=(40/1000)*(40/1000)*100=0.16W that means that u must use an 0.25W resistor(the maximum power that can disipated on the resistor must be greater than the one it is disipated in your citcuit).
now, on the other hand if on the 100ohm resistor u aply a voltage of 4V, then the power disipated is P=4*4/100=0.16W. now again u will have to use an 1/4W resistor again
now u sai u have a circuit powered by 3V, that means that the maximum voltage across the resistor is 3V(usually is much smaller), taken in reverse if u would use 0.25W resistors, than for that power to be disipated in a resistor its value has to be R=U*U/P=36 Ohms
so as a conclusion, if in the circuit u dont use more than 3V and resistors smaller than 36ohms, use 1/4W types....
hope you are not more confused then u were whe i send u the message

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5. ### daviddoriaNew Member

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wow great responses guys! i like this site already!

6. ### tommyNew Member

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please correct me if im wrong ...doesnt wattage represent the power consumtion to opperate at it intended value

7. ### bogdanfirstNew Member

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no, u see, when current pases trough a conductor it heats...and the power disipated on that conductor is p=U*U/R or P=I*I*R and that energy lost on the conductor is transformed in heat, electromagnetic field and other types of energy.....
on a resistor most of the power is transformed in heat, so if u have a 1W resistor and the power disipated on it is lets say 1.5W then it overheats and might be destroied.....
also, from the second fromula, P=I*I*R means that u can find the maximum current that can pass trough a resistor, and a current larger could also destroy the resistor.....
if the current going trough the resistor is smaller than the maximum current its no problem...
it is normal for a resistor to heat in the circuit, but as long as it doesnt overheat.....

8. ### tommyNew Member

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ok so if you have a 1 watt resistor ........then you need to make sure this resistor doesnt get more than 1 watt so you dont get heat in the circuit......................i am one of those that may have to read what you wrote a few times to understand....................another question i have been trying to figure...................a lot of research i have been pouring over regarding the skin effect...........could this happen in all circuits or is this common agomg inductors that are not consistant and breaks down the tesla field.............please help me to understand...........

9. ### charcoalNew Member

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the skin effect only becomes a real problem at ultra high frequencies or high frequencies coupled with high currents. Thats one of the reasons why power lines only use 50/60 cycles instead of more efficient switched mode frequencies.
The only circuits i can think of where you might have to deal with skin effect to any degree are UHF or GHz frequency radio circuits. (anything above that and you usually have to use wave pipes intead of wiring).
high end RF and microwave circuit design is a great deal different to ordinary circuit design and you need do devote more care to the exact dimensions and positioning of traces. Also in these circuits inductors tend towards being flat rectangular areas of copper on the board itself as opposed to wire coils.

I can't really help much more unless i know what field of electronics you need to know about the skin effect in (VHF/UHF/uWave/Tesla etc)

10. ### tommyNew Member

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cross overs ......for speaker cabs

11. ### charcoalNew Member

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assuming that you don't want a maximum frequency of over 20kHz through your system then i can't imagine the skin effect troubling you (unless you're building speaker systems for open air rock concerts). as long as you use decent quality braided speaker cables for long runs and use wide flat PCB traces on the crossover board itself (my KEF speakers had traces about 3/16ths of an inch wide on the x-over board!).

the only time you'd really have to worry about the skin effect in audio systems is if you were using digital audio at any point. thats why digital audio uses (or used to use) coaxial cable, it copes with high frequecnies and the skin effect better.