my mate is asking me to make a small light bar type thing using these LEDs for when he's playing gigs
http://www.ultraleds.co.uk/410nm-p-1948.html

just ordered 15 of them, was thinking about power supplies, thinking of DC transformers and all the bits needed for them, got me thinking though, he uses a laptop for some stuff in the band, why not use the usb port power?!

usb i think is 5v, these leds are 3.5v. i'll be using them in parallel, so i'll need a resistor, but what size? not planning on using all 15, but maybe 10, keeping 5 for 'damage' lol

and how would you wire up a usb plug just for power? which pins are used?

[Edited on 22/9/10 by blakep82]

google "USB pin-out" and here's the result...

HTH


Vince

They are 20 mA, so ten will draw 200 mA (0.2A).

V=IR so the resistance of the 10 LEDs in parallel will be 3.5/0.2 = 12.5 ohms.

Since you want to split the 5v so that you have 3.5 across the LEDs and 1.5 across the resistor, the resistances will have to be in the same proportion. i.e. R = 1.5/3.5 * 12.5 = 5.35 Ohms.

Total current draw I = V/R = 5/(5.35+12.5) = 280mA which is within the spec of USB if I recall.

Wikipedia will tell you which pins are 0v and 5v.

Question begs though, what on earth is he going to do with 10 tiny little ultraviolet LEDs?

I should add - test it on a 5v power supply first just in case I've got it wrong or you wire it up wrong!

quote:

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Originally posted by matt_claydon
I should add - test it on a 5v power supply first just in case I've got it wrong or you wire it up wrong!

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most USB power ports on laptops know if you are asking too much current from them. I.e. they are short circuit protected. I sometimes get a little warning on my pc.

Matt

thanks everyone i found the pin out thingy after i asked the question on here, but thanks for the pointer

thank you for all the calculations Matt 5.35 ohms seems very small though? its been many years since i did this at school, and at the time thought i'd never use it, but to get the resistance of the LEDs in parallel, don't you have to use 1/R=(1/12.5)x10 for the 10 leds to get the overall resistance? i can't remember got a feeling i'm going to watch 10 LEDs go pop again

regarding why he wants to to do this lol, i forget what they're called, but there's some style of ww2 goggles wodded with all sorts of things, he wanted LEDs inside thelenses shining out, but i siad the reflection in the glass would hurt his eyes, and by the time you include put in circuit boards etc, he won't see out, so instead we're thinking the outside of the goggles will be fluorescent something, and the LEDs will shine up from his keyboards lighting up the goggles, as for the lenses, undecided, he wanted them to light up too, but i said some kind of silver reflective window tint film would be a better idea, it'll reflect the uv and stop his eyes getting burned lol
its a sort of electro band

Actually it would be (5 - 3.5)V / 200mA = 7.5 Ohm.

But I think it would be better to use one resist0r of 75 ohm for every led, so 10 resistors in total. Otherwise if one Led fails all the other Leds would get a little bit brighter whcih will result in failing the next Led etc.

[Edited on 22/9/10 by ruudbeckers]

I've used THIS calculator before, it'll even draw a diagram for you.

HTH

Neil

awesome thanks everyone. that link's pretty cool neil.
going to have a go at it next week i think!

You need to use one 75 ohm resistor per LED. The forward voltages of LED's are not perfectly matched so if you try connecting all the LEDs in parallel with just one 7.5 ohm series resistor for all of them, then the LED with the lowest forward voltage will hog the majority of the current and will quickly fail.