Powering Led's off firewire?

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Techx

Supreme [H]ardness
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Say I wanted to wire up some led's to a usb cable, which has max 5v, you could light up maybe, 1 or 2 leds if that right? So I was thinking about firewire which most people don't even use, I googled it up and found these specs on it: (8 to 40 v, 1.5 a max.). Much more power going through there that could be used for a bunch of leds in parallel, so anyone have experience with modding firewire cables to do something like this or know how it could be done safely?

Thx
 
A quick look on google tells me that the max current on a usb port can be 500mA. A typical 5v LED draws around 25mA, so you can run up to 20 LED's off a usb port if you wired them all in parallel. So they would all get 5v and draw 500mA in total. How many LED's did you want to run?
 
egon_dude said:
A quick look on google tells me that the max current on a usb port can be 500mA. A typical 5v LED draws around 25mA, so you can run up to 20 LED's off a usb port if you wired them all in parallel. So they would all get 5v and draw 500mA in total. How many LED's did you want to run?
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ohh i didnt think of it that way, i thought if you had 5v and 2 leds that require 3.5v each, you ran outta volts lol, so its amps you go by?
 
This is a pretty good idea. You could build a board with various positioned LED's, and a contoller chip. Run the whole thing on USB. Some simple programming would allow you to blink patterns shapes, and words.
 
Techx said:
ohh i didnt think of it that way, i thought if you had 5v and 2 leds that require 3.5v each, you ran outta volts lol, so its amps you go by?
Click to expand...
lol... thats what i thought when i did a keyboard mod running two 2.1v leds off a keybaord serial port which also uses 5 volts and i thought that was ok but it burnt out like three leds (including another one i did that was kinda stupid but i rather not go into details now)... later i researched and found out i fed 5volts to 2.1v leds

so i redid the mod with a RESISTOR and now it works great with two bright red leds shining over my keys for late night gaming

i am so stupid sometimes.... :(
 
Hehe yeah youve got to remember to either get the right voltage LEDs or use the correct value resistor in series with the LED to drop the voltage down to the correct level.
So you can work out what resistor you need:

First calculate the voltage drop you need (say for a 2.1volt LED)=
5v-2.1v= 2.9v

The LED will typically need around 25mA, so by using v=ir (voltage=current x resistance) we can get:

r=v/i

r= 2.9v / 0.025A

= 116 ohms. So use the value of resistor that is closest to this figure, obviously after using the specs for your LEDs though.

Matt.
 
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