USB voltage??

Seanie M

What is the voltage of USB?

I ask because I got a laptop led light, but I want to modify it for my cd case so I can see my discs better when I am DJing. I have a small battery pack I can modify to take the flexible arm (both standard and jewel batteries)... just wanna make sure I have the voltage right for the led.

Seanie.

Ciaran500

5v

Seanie M

Cheers Ciaran!



S.

Seanie M

Another thing - there are 4 wires in the cable: red, white, black, and green. Which 2 carry the voltage +/-?

Seanie.

Mutant_Fruit

red and black is what you should connect to (iirc). But you'll probably need some kind of resistor connected in series between otherwise the led will explode.

Seanie M

Mutant_Fruit wrote:

red and black is what you should connect to (iirc). But you'll probably need some kind of resistor connected in series between otherwise the led will explode.

Cheers Fruit, but can you elaborate on it for me? What rating resistor? how/where to bridge?

S.

Mutant_Fruit

Firstly, let me just make sure i understand 100% what you're doing. You're connecting a led (which is located on a flexible arm) to a USB port, which will power said LED. If so, here's how you should do it.

If i remember correctly, the max current allowed to flow through a LED is about 25mA. So, with a 5v source you'll need at least a 150 Ohm resistor. So you should get a resistor with the resistance as close to that as you can get, but it cannot be a smaller resistance than that! Or else the led might burn out.

As for setting it up, you should attach it as follows... Usb Voltage Out (red) -> one end of resistor <- pass through resistor --> other end of resistor -> led -> USB voltage in (black).

Its different if you have different colour leds, so if you're not using red, let me know.

EDIT: If you're using several leds, you'll have to connect them in parallel as compared to in series, otherwise they won't all light.

Civilian_Target

A variable resistor would be a better plan....

Mutant_Fruit

As good as a variable resistor would be, its harder to know exactly what resistance its set to, so i wouldn't recommend it. Plus, one day you'll want to make it a bit brighter, and you'll reduce the resistance that liitle bit to much, and melt the leds. i know thats what i'd do Just to see what happens...

Seanie M

Cheers you guys, you're a great help!

Fruit, its a white laptop LED that was on sale in Aldi last week. You simply plug it into a USB socket and hey presto, you have a laptop nightlight! I got it to modify it so that I can see my cd's in the flightcase during gigs. The idea is that I would have the power supply, with a USB out socket on it, probably held by a magnet to the back of the case. Then, I would plug in the laptop LED into it, and simply bend the arm as needed to read the contents of my cd's in paper wallets in 3 rows in the flight case.

What you have described sounds fine. So, it would work with the white LED?

Seanie.

Mutant_Fruit

Ah right, i think i get you now. You want to be able to run it off a battery pack as opposed to a USB socket? If thats the case, you have a few options. You can either use a few AA batteries or a 9v battery (the rectangular shaped one).

If you use the 9v, you'll need a 250 Ohm resistor, but you'll get a longer battery life than from AA batteries.

Basically you need 4volts to run a single white led. So if you're running it off a battery pack, you'll need a 4v battery. Or, if you have a higher volttage source, you'll need resistors. For the 9v battery, there will be 5v excess. So to work out the resistance you divide the excess voltage by 0.02, the max current allowed through the led. That gives 250 ohm resistance. If you use different combinations of batteries, work out the resistance you need yourself

Seanie M

Mutant_Fruit wrote:

If you use different combinations of batteries, work out the resistance you need yourself

Spot on Fruit! Thanks for the help! Happy New Year!



Seanie.

carbsy

I doubt very much there's just a basic LED in this device Seanie - you got a link for it?

Capt'n Midnight

If the LED came from a torch that took three batteries ( ~ 4.5V) then it would also have a resistor in series. You could put a normal diode in series with it to drop the 5.1V to 4.5 ish. IIRC White LED's need nearly twice the voltage of Red ones. 15 ohms like the one in my torch sounds low..

If you have a multimeter, you could put a 1K resistor in series with the LED and measure the voltage drop across the LED - say 3V this means you have to drop the remainder 2V across a resistor - for 25mA that would be 80ohm

Mutant_Fruit

a white led needs 4volts, as far as i know. A google would probably verify that.