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The 12V LED Molex Mod - Page 2

post #11 of 29
Quote:
Originally Posted by tt_shark View Post
what should i put for desired LED current?

also what setup did you use the 100 ohm resistors in?
I redited my post. You'll want around a 60-80 ohm resistor.
post #12 of 29
Yea, can someone give us a good explanation on using the resistors. That would be so nice.
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post #13 of 29
Quote:
Originally Posted by pheoxs View Post
Well for one, you need a resistor in there, LED's theoretically have no resistance, so without a resistor they won't work at all. The size of the resistor will determine the amount of amperage that passes through the LED's.

LED's only need a resistor to lower your input voltage to its rated voltage. They will work without a resistor if you feed them their spec voltage. All they are is a diode that just happens to emit light from its PN junction.
    
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post #14 of 29
Quote:
Originally Posted by justin146 View Post
LED's only need a resistor to lower your input voltage to its rated voltage. They will work without a resistor..
This is the truth....

You may be running the voltage backwards, leds only accept voltage one way to work(unlike an incandescent bulb). If you wired one of them backwards or connected the input voltage on the wrong side they will not work.
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post #15 of 29
Thread Starter 
should i go by 12V or my psu rated 12.5V?

if 12.5 then i need 93 ohms or so.

does anyone know which cables to attach to on the molex? just the red and first black?
    
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post #16 of 29
So does he only need one 60 ohm resistor 3 leds in series ?
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post #17 of 29
Quote:
Originally Posted by 3xtr3m3 View Post
Yea, can someone give us a good explanation on using the resistors. That would be so nice.
Well, from Ohm's law, V=IR, the voltage passing through the circuit is equal to the amperage times the resistance.

LED's theoretically have zero resistance, and since he has 3, 3.7V LED's. He is using up 11.1V from the 12V rail, leaving 0.9V of forward voltage.

Now since: V=IR, I = V / R

but 0.9V / 0 just doesn't work, too much current actually passes through the LED's and they will not light up. By adding a resistor, you limit the amount of current passing through the circuit.

In his example, adding a 60 ohm resistor will make 15mA (milli-amps) pass through the LED's, while a 100 ohm resistor will only allow 9 mA to pass through.

If that makes any sense...

Edit:

Quote:
Originally Posted by justin146 View Post
LED's only need a resistor to lower your input voltage to its rated voltage. They will work without a resistor if you feed them their spec voltage. All they are is a diode that just happens to emit light from its PN junction.
This may be true, I've only ever used LED's with 12V circuits so I've never run across using the rated voltage...

Quote:
Originally Posted by 3xtr3m3 View Post
So does he only need one 60 ohm resistor 3 leds in series ?

The size determines the amperage, different LED's have different amperage ratings, but yeah, most are right around 15mA's, so a 60 ohm resistor should work fine.

it'll go,

Positive --> resistor --> resistors connected in series --> negative.

The LED's only go one way as mentioned before, if one is backwards they won't light up.

Quote:
Originally Posted by tt_shark View Post
should i go by 12V or my psu rated 12.5V?

if 12.5 then i need 93 ohms or so.

does anyone know which cables to attach to on the molex? just the red and first black?

Yellow is the 12V rail and then any black is the negative.

I guess go by the rating on the psu ... to little current won't overly make a big difference, a 100 ohm resistor will work fine and the LED's will still be bright.
post #18 of 29
Quote:
Originally Posted by pheoxs View Post

but 0.9V / 0 just doesn't work, too much current actually passes through the LED's and they will not light up. By adding a resistor, you limit the amount of current passing through the circuit. ...
They will light, but lifespan will be greatly reduced. Just try 4 in series instead of 3...

If you are using a resistor, it doesnt matter if the resistor is on the positive or negative side- it will drop voltage and work either way.
    
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post #19 of 29
Quote:
does anyone know which cables to attach to on the molex? just the red and first black?
yellow is the 12v and red is 5v...so use te yellow and any black one.
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post #20 of 29
Thread Starter 
Quote:
Originally Posted by Outfitter540 View Post
This is the truth....

You may be running the voltage backwards, leds only accept voltage one way to work(unlike an incandescent bulb). If you wired one of them backwards or connected the input voltage on the wrong side they will not work.
nah i connected it exactly like the diagram i drew.
    
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