need help with 3w led wiring and high power arduino shield

im having some trouble with my 3w leds. im using an uno and this shield right here from spark the shield itself is awesome....i tried to wire them up as common anode and im using the basic fade sketch from the examples but all i got from them was a flash every minute or so with just a computer power supply hooked up to them.and so i rewired them in series and got absolutely nothing from them. here are the specs on the leds...

white leds are....Forward voltage: 3.2-3.6V Forward current: 750mA. there are 25 of these and im wiring 8 of them together in both configurations.

blue leds are.....3.9V forward current 700 mA like the white there are 8 of them wired together in both configurations.

the power supply im using is from a old dell i had sitting around and the specs for it is

+12v 16A max and +5V 22A max....the total output is 250W the atx connector is only a 20 pin instead of the 24 pin like the shield has on it. the only thing im missing with this is one of the 12V pins from the ps...and im going to bridge the 12V to the 5V pin on the shield which should not be a problem as the transistors are the same on the 5v side of it as the 12V side. im wondering what would be the best way to wire them up and if the supply has enough umph to it to work them all. any help would be great.

Don’t wire LEDs in parallel. However when you flow this advice then you’ll find that 8 x Vf is alway higher then the 12V from the computer. When something does not work as expected, turn of the power, take a step back and do some basic calculations.

The shield does not look like a constant current driver, which is what you need to drive LEDs. YourLED’s might already be damaged if they’ve seen more than their rated max current.

Computer PSU’s have the problem that they often do funny things when no load is applied to them. Not very good choice either.

Before making any more wiring attempts please browse through this section o the forum to brush up on your LED know how :wink:

Headroom…i have been through this forum over and over again…have even posted about it before IN THIS FORUM,and even had a post moved here from general electronics( no one even really answered that post…i know that when i wired all anodes and all cathodes together and built the same circuit that that driver shield (that actually is a constant current driver hence the name power driver shield kit…) is they worked fine for at least 2 weeks until the weak ass diode on the external power supply side of my uno fried…if you look around these are the same circuits in those drivers save for a transformer to step the voltage from 120 ac to 24v @8a with 250w output…those are the specs on all the drivers out there and they all say supply voltage to positive side and signal from pwm ie…either the driver shield or a dimmer or a simple fet circuit to negative cause your switching them on and off on the negative side…and well i have done all the calculations over and over again and they all come out the same…and besides go look at a few online led circuit builders and also build it on like multisim or something to that effect and watch it crash and burn when wired in series cause you would need like a 75 v power supply cranking 10 a or more for that and well good luck finding those…more over you want them to divide the current not the voltage so that that way you can monitor the fluctuations in current not voltage…leds could care less about voltage they like current and a 250w power supply is whats recommended for 3w leds…ask Google it…you will find this out…and instead of offering me no advice on a power supply except to say its no good…try offering you r opinion of a proper power supply then. as far as them being damaged nah they are fine as they light up just fine when i test them with the diode tester on my DMM. and again give you opinion on proper wiring of them as well considering there is over 60 pages in this forum alone and yeah i have not the time to go through them all again…

does anybody else have any opinions here?.....

I think you need some parts like this

Basically a switching regulator with max output current set by external resistor value. Simple on/off transistor circuits do not provide current control that is needed for higher current parts to keep them from going into a heat-generated runaway condition.

drab: worked fine for at least 2 weeks until the weak ass diode on the external power supply side of my uno fried.... wasn't really working "fine", was it? You were doing something very wrong.

With no circuit diagrams or anything there's not much anybody can do to help.

those look pretty nice crossroads thank looks pretty strait forwards as far as wiring them up and the fact they can be dimmed is awesome now i guess ill have to order them and try it out....and fungus actually there is a link to my other post about this that has the diagram in it and at the top of this post is the specs for the leds themselves please read the entire thread before telling me that i need to include that stuff not my first rodeo in here bub

ok so according to the data sheet to run 3 leds ill need 24 v can i tie 2 12v ps together and get that?

Why do you say 24V? I thought you had max 3.9V LEDs, 3 of these in series I would think would run from 12V.

yuppers i got that figured out crossroads....i wired up 4 in series and hookes two sets of 4 up in parallel unsing the shield i mentioned and they work just fine specially with the fade function and im also only driving them at about 75% of their total brightness i thank you very much for your input and i may use those diodes in the future as well