Resistor wattage?

[HaGGardSmurf]

Hey guys, I dont know too much about wiring, but I have decided to build a light strip... Basically just some semi clear tubing with a bunch of LED's inside...

I was on LED Calc, and have found out that I need an 82 OHM 1/8 watt resistor. I was wondering if there is any harm in using a 1/4, 1/2, or 1 watt resistor? I dont think it would be bad, as the resistor can just handle more power before failing? But again I am no electrician, so I do not know...

[stuzza]

Hi there haggardsmurf

Well first thing you have to think about is how much current is required for each light emitting diode (l.e.d) usually in my knowledge of electronics the standard l.e.d requires at least 20mA so if your voltage rail that you are tapping off is 12 volts and you need 20mA, you can use ohm's law to calculate the resistance 12 / 20mA = 600 ohms i use a 1 watt resistor prob dont even need that much maybe a quater watt will do, ok matey i hope this will help you oh and the resistor can just fit in series with the plus 12 volt line not accross the supply if you need any further help dont hesitate to ask
stuart

[technoe]

Stuzza is definitely on the right track. But essentially for your purposes the reason you might want to use a lower wattage resistor is safety not longevity. A resistor can never short, PERIOD. (Except for that .00001 pico second before it's permanently open) But what I'm saying is if for whatever reason all of your LEDs at one time shorted out then your current would spike and could cause a fire in one of the components, this is however HIGHLY UNLIKELY, but I wouldn't use more than 1/2 watt resistors.

BTW: Here's a list of Ohm's Law Equations (V=Volts, I=Current R=Resistance W=Power)
V/I=R
V/R=I
IR=V
I²R=W
VI=W
W/V=R

[HaGGardSmurf]

Well, stuzza I used http://www.ledcalc.com" onclick="window.open(this.href);return false; and entered in my power source (5v) and the voltage drop across my LED (3v) and the current across LED (25mA) since the circuit is parallel it doesn't matter how many LED's I enter in.

So the result was I need 80 ohms of resistance in the system, and the closest resistor to that is 82. It also recommends 1/8 watt

I was looking for the resistors on ebay and I find lots of 82 ohm 1/4 watt or 1/2 watt. Would they work? Isnt the wattage of the resistor a measurement of how much power the resistor can handle before it will heat up and melt?

Edit: I said series where I meant parallel

[technoe]

The 1/4 and 1/2 will work, but like I said before it's more of a safety issue than an electronics issue. Also depending on how strict you want to be you might want to check the tolerance level on your resistors, It's a gold or silver band on the end.

Remember that in series current is constant and voltage drops, in parallel voltage is constant and current drops.

[COEBRA]

Thats a sweet diagram !! I had to try and memorize them in class and that sucked !! This is pretty cool

[technoe]

This one is actually very similar to one I had in college.

[HaGGardSmurf]

Alright, I think ill see if I can find some 1/8 if not, 1/4 it is..

[freedog]

With your current and voltage you would have an easier time starting a fire using the jedi mind treatment. Resistors are all cheap components so cost shouldnt matter, but an 1/8 watt 82 ohm is very uncommon. Use the 1/4 watt. It will not do any more or less than the 1/8 except it can withstand a little higher currents before it burns up. Your 1/8 watt will burn up quicker than the 1/4, however if for some reason all this has put the possibility of fire in your head, for a couple cents more each you can buy flame resistant resistors. The color code will be the same but the resistor itself will be blue.

[blufmaster]

i have a quiry?
i have 30w power supply and i want only 6watt power out put with keeping 12v voltage how can i do it? can i add any resister to reduce its wattage
plz reply

[technoe]

You would need to put something in parallel with the power supply to drop the current but keep the voltage. I(Current)=6W/12v OR .5AMPS Rt=12V/.5A or 24Ω. So given this info you would need a current divider circuit. I'll draw one up on Vizio when I get to work.

EDIT: It=30W/12V=2.5A so in the divider circuit you need .5A*12V=6W. One side of the circuit will output this and the other will output 24w. So Rt=Vt/It.. Rt=12V/2.5A so Rt=4.8Ω Which is extremely small. That's basically the resistance of an iron wire. Which don't really exist anymore.

[CoFree]

blufmaster
you hijacked some ones thread
you should have started your own thread
its not a big deal its just kind of rude.