Resistor help

RIPSTER · Feb 8, 2007

I just bought 4 resistors to go with the LED's that I salvaged from a LED fan lighting strip, I found the LED's to run at 3.2v and I'm trying to run them of the 5v line in my Xbox, My question is what ohm and wattage resistor will I need. An online calculator I used said that I needed a 90ohm (100 rounded up), 1/8th watt resistor this was for LED's rated at 20MA I wasn't sure what mine were the calculator said most standard LED's were 20MA , the resistors I bought are 100ohm 0.6 watt resistor. Seeing as the wattage I bought Isn't what was specified will this have a bad effect on the LED's. How can I tell which side of the LED is positive and which negative, I know that usually one side is rounded and one flat but on these LED's they both look identical, I have a multimeter is there a test which could determine which leg is + and which is -.

thnx

RIPSTER

Automata · Feb 8, 2007

RIPSTER said:
I just bought 4 resistors to go with the LED's that I salvaged from a LED fan lighting strip, I found the LED's to run at 3.2v and I'm trying to run them of the 5v line in my Xbox, My question is what ohm and wattage resistor will I need. An online calculator I used said that I needed a 90ohm (100 rounded up), 1/8th watt resistor this was for LED's rated at 20MA I wasn't sure what mine were the calculator said most standard LED's were 20MA , the resistors I bought are 100ohm 0.6 watt resistor. Seeing as the wattage I bought Isn't what was specified will this have a bad effect on the LED's. How can I tell which side of the LED is positive and which negative, I know that usually one side is rounded and one flat but on these LED's they both look identical, I have a multimeter is there a test which could determine which leg is + and which is -.

thnx

RIPSTER

The way I tell which is + and which is -. Hook it up!

If it lights up, there you go. Since LEDs are diodes, they don't work in reverse....well, they explode in reverse if you really overpower them...found that one out the fun way

Roofles · Feb 8, 2007

Led's have a round side and a flat side when looking down on the LED. The flat side is the negative and the round side the positive.

The wattage rating is how much heat/power the resistor will dissipate before it blows up. In your case, you are ok.

noxqzs · Feb 8, 2007

RIPSTER said:
I just bought 4 resistors to go with the LED's that I salvaged from a LED fan lighting strip, I found the LED's to run at 3.2v and I'm trying to run them of the 5v line in my Xbox, My question is what ohm and wattage resistor will I need. An online calculator I used said that I needed a 90ohm (100 rounded up), 1/8th watt resistor this was for LED's rated at 20MA I wasn't sure what mine were the calculator said most standard LED's were 20MA , the resistors I bought are 100ohm 0.6 watt resistor. Seeing as the wattage I bought Isn't what was specified will this have a bad effect on the LED's. How can I tell which side of the LED is positive and which negative, I know that usually one side is rounded and one flat but on these LED's they both look identical, I have a multimeter is there a test which could determine which leg is + and which is -.

thnx

RIPSTER

the basic formulas you want to use in your application are:

voltage = current x resistance, V=IR

power(Watts) = voltage x current, P = VI

Lets say your diode draws 20mA, and your voltage source is 5v. To find the resistance needed in this circuit, you would use

5v = .020A x R

R = 250ohm

The power consumed by the circuit will be :

P(Watts) = 5V x .02A = .1W

1/8W is also .125W. Using higher rating components should not have adverse effects.

Resistor help

RIPSTER

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Automata

Destroyer of Empires and Use

Roofles

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noxqzs

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