Print

[Granulated]

So this guy wanted orange LEDs on his xbox...so I thought, Hey, i'll comply.  

I have 3 sets of 10 LEDS in paralell.   (side vents x2 and the jewel)

Metkumods LED calculator says I need a 33 ohm, 3 watt resistor, and recommends a 39 ohm, 5 watt resistor.  

(Leds are 2.1volt, 30mA)

I went to Radioshack and bought 20x 33 ohm 1/2 watt resistors.  

So, if I run 6 of these in parallel, that will be 3 watts, and it should work, right?  

Everything works, but the resistors get RED hot.  

So I said, fine, i'll run 5 watts, and I ran 10 resistors in parallel.  

And I burned my fingers on the resistors again.

What the hell is going on?  I can't put this together if they're gonna start a fire.   I have to drop this xbox off in 2 hours and everything is completed except for the goddamn resistors.

[xman954]

QUOTE

So I said, fine, i'll run 5 watts, and I ran 10 resistors in parallel.

yes thats 3.3 ohms at 5 watts
not 33 ohms

make 3 groups of 3 resistors in parallel then connect the groups in seres
that will give you 33 ohms 4.5 watts

resistors in parallel rt = r1*r2/r1+r2
seres add them


This post has been edited by xman954: Sep 1 2005, 07:09 PM

[Granulated]

Okay, I just made 3 groups of 3 resistors and ran them in series, and the LEDs still work, and the resistors STILL get red-hot.

I'm just gonna go see if I can get some resistors for the 5v source.

EDIT:

For the 5v source, they recommend 15 ohm, 1 watt.  Can I get 2x 15 ohm, 1/2 watt, and run them in parallel?

This post has been edited by Granulated: Sep 1 2005, 07:31 PM

[Granulated]

Okay, after messin with the calculator for a bit, I figured out what i'm going to do.


If I run 5 volts at 20 mA instead of 30,  and run 5 LED's instead of 10, then I can use 1x of the resistors I just bought (33 ohm, 1/2 watt)

[xman954]

30 led at .030 amps = 0.9 amps
12 v - 2.1 v = 9.9 volts
9.9 * 0.9 = 8.9 watts <<<<<< thats why

at 5 volt supply
2.9 * 0.9 = 2.6 watts
R = 3.3 ohms
use (10) 33 ohm resistors in parellel = 3.3 ohm 5 watt

[xman954]

QUOTE(Granulated @ Sep 1 2005, 02:38 PM)

If I run 5 volts at 20 mA instead of 30,  and run 5 LED's instead of 10, then I can use 1x of the resistors I just bought (33 ohm, 1/2 watt)

33 ohms at 2.9 volts will supply 0.088 amps at .255 watts
so 5 leds, each one will get .017 amps and thats sounds good

[Granulated]

Hey, this is workin great!  Thanks a ton!

[jameswalter]

Its always smart to use the 5V and not the 12V.....since the resistors bring that voltage down they do so by disapating heat.  12V-2.1V is almost 10V times whatever your current is.....so the resistors are dissapating 5X more energy than is being used in the LEDs...much better to use the 5V source.

[networkBoy]

or you could run several LEDs in series.
If the LEDs are 2.1 volts at 20mA then you can run 5 of them in series:
5x2.1 = 10.5
then you only need to dissapate 1.5 volts.  
In reality you could likely forgo the resistor all together in that case, though I would tend to suggest a 5 or 10 ohm resistor.  Get 4 1/2 watt 40 ohm resistors and run them in parallel and they will provide 10 ohms at 2 watts.
-nB

[SICKdimension]

QUOTE(Granulated @ Sep 1 2005, 12:25 PM)

For the 5v source, they recommend 15 ohm, 1 watt.  Can I get 2x 15 ohm, 1/2 watt, and run them in parallel?

Wiring resistors in parallel divides the ohms too, not just multiplies the watts. To get 15 ohm 1 watt, you'd need to use two 30 ohm 1/2 watt resistors.

[grimlock]

QUOTE(SICKdimension @ Sep 1 2005, 11:00 PM)

Wiring resistors in parallel divides the ohms too, not just multiplies the watts. To get 15 ohm 1 watt, you'd need to use two 30 ohm 1/2 watt resistors.

I think we pretty much covered that...  
See how annoying a useless post is?