# Computer PSU to drive LEDs?



## blasterman (Sep 5, 2008)

I'm working on several projects to drive quite a large number of 1-3watt LEDs for commercial lighting, and the constant snag I run into is powering them. The down conversion from line level AC to stable 12volt keeps adding to to a significant cost to the project.

Then is caught me as I slapped myself in the forehead watching a friend brag about all the LEDs he installed in his rather fugly computer: why not use a computer PSU since they are already a well known 12 volt regulated supply? (at least the good ones). If nothing else is running on the 5volt rails with significant load it looks like most decent PSU brands can easily handle sustained current loads at 12volt for at least half their commercial rating. 150watt at 12volt could run a heck of a lot of Crees at 700ma, and be absurdly cheap given we typically throw them away with the computer case. Using additional current regulation, (if it's needed) on the 12volt side is what I also need advice on.

"So, why not use Xitanium"

(1) Expensive
(2) Little competition
(3) #2 explains #1
(4) I can't do what everybody else is doing and prefer to learn by fire :candle: 

Thoughts, ridicule, advice?


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## TigerhawkT3 (Sep 5, 2008)

The 40W dimmable Xitaniums are a bit costly ($80 or so, IIRC), but the simpler ones are $20-30 each. They also look a lot nicer than a computer's PSU. 

DX has a few AC-DC drivers, and they're dirt cheap, but they don't come with an enclosure or anything.

What exactly do you mean by "commercial lighting"?


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## snarfer (Sep 5, 2008)

Why not use a laptop type power brick instead? I don't know your exact application, but higher voltage - (typically 19 volts or higher) is generally more useful for my purposes. I am using constant current boost driver to boost to as high as 90 volts for driving fairly large numbers of HB LEDs in series configuration. 12 volt supply would really detract from efficiency. 24 volts is ideal.

Personally I don't get what's so great about the Xitanium anyway. 

Another possibility worth investigating is running LEDs with rectified output of fluorescent ballast. I did some initial experiments and it was successful in that current regulation was accurate at approximately 350 mA with a 48 watt ballast. However I had some problems getting the ballast to start. Once started everything was fine.

Tell us a little more about the configuration and maybe some other possibilities will arise. Do you need to dim the LEDs? What wattage? How many per fixture? How much current?

One other thing, do keep in mind that if you are planning on distributing power over any sort of distance line loss will be an issue, especially at 12 volts. So you might have to use some fatter wires, depending on your application.


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## TorchBoy (Sep 5, 2008)

snarfer said:


> Why not use a laptop type power brick instead?


Um :thinking: Because they don't get thrown out in old PC cases? :shrug:

The regulated output of a PC PSU might be quite nice to work with.


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## 2xTrinity (Sep 5, 2008)

> Why not use a laptop type power brick instead? I don't know your exact application, but higher voltage - (typically 19 volts or higher) is generally more useful for my purposes. I am using constant current boost driver to boost to as high as 90 volts for driving fairly large numbers of HB LEDs in series configuration. 12 volt supply would really detract from efficiency. 24 volts is ideal.


If you're running 90VDC then dropping to 12V is silly, indeed. You'd be better off rectifying 120VAC into 170VDC, then using a constant-current buck driver to get 90V. A couple of the DX drivers appear to do just this.

12VDC IMO makes the most sense for driving groups of three white LEDs in series. For quick and easy operation, a power resistor can be added in series with each string to limit current. If you wanted to, you could also ground the strings through power mosfets, and use that to adjust brightness using either PWM (apply a pulse wave to the gate to turn it fully on and off, relying on the resistors to set the current), or linear operation (bias the mosfet so its resistance increases in a controlled fashion).

You could also use an audio taper pot and a voltage divider to meter out gate voltage to the transistors and adjust brighness that way, or if you're really clever, install some feedback using photodiodes or photocells to control brightness based on ambient light.


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## KeithInAsia (Sep 5, 2008)

blasterman said:


> (4) I can't do what everybody else is doing and prefer to learn by fire.


 
I beat you to it.... seriously though... the PC power supply has a lot of advantages and I think you'll find that they works well if you can fit 12 volts into your design. In some cases 5 volts works well too depending on what you're trying to accomplish.

I'm working on a combination of controller/drivers that use PC powers supplies but these units are not quite ready for mass consumption.


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## snarfer (Sep 5, 2008)

> If you're running 90VDC then dropping to 12V is silly, indeed. You'd be better off rectifying 120VAC into 170VDC, then using a constant-current buck driver to get 90V.



It has to be possible to run my fixtures off of either battery or mains. That's why they are designed that way (to run off of 14-40 volt power). But this is off topic.

As for the idea of rectifying 120VAC to 170VAC and then using a constant current buck driver to get 90V, that would make perfect sense for a DIY solution to run off of US power. Which would not for my purposes be the case. Anyway I don't want to get in a pointless off-topic discussion.

I only brought it up because the OP said he is driving a "large number" of LEDs for "commercial lighting". Could this "large number" of LEDs be in series? We don't know. He hasn't told us. There are some possible configurations in which it would be more efficient to have a higher voltage power supply.



> 12VDC IMO makes the most sense for driving groups of three white LEDs in series.



If that is what the OP is doing. He hasn't told us. He could be running groups of 20 for all we know. 

Anyway just my opinion but I think that old ATX type computer PSUs are pretty fugly. They are being thrown out for a reason after all. Some of those old PSUs can have efficiency as low as 50%. There are plenty of other types of regulated supplies out there to buy surplus/used as an alternative. People throw those away too!


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## blasterman (Sep 5, 2008)

Thanks for the help, guys. Lots of ideas here - glad it seems I'm looking at a plausible solution. To answer some questions:



> Why not use a laptop type power brick instead?


 
Simple - More expensive than the Xitanium option.



> 12VDC IMO makes the most sense for driving groups of three white LEDs in series


 
I need to stay below 24watt for safety reason. There is also significant difference in the way 12/24 volt 
can be legally wired in a commercial environment -vs- 120volt. For instance, with track lighting, you can use 
exposed open rails at 12/24 volt. 120volt on the other hand requires conduit, etc. A little bit easier to
work with cheap zip cord than commercial conduit 

I'll likely stick with groups of 3-4 power LEDs in series, and then group them in parallel. Just the most logical way to do it, although this isn't a requirement. However, I'm trying to build in some extra reliability in my series/groups so that if one LED pops it doesn't take down the rest *without* using additional current regulation as a safety net. Losing one 3Watt LED in a group of three has more potential for causing
problems than losing one in a groups four, correct? 



> and adjust brighness that way


 
Neat idea, but dimming is not a requirement. Always found the concept of 'dimming' to be an oxymoron when it comes
to LED discussions regarding household/commercial lighting, IMHO. My reasoning being if you have that much extra lumens to afford dimming, then you are wasting light in the first place.



> He could be running groups of 20 for all we know.


 
As above, groups that can be run on 12-24 volt, depending on if I can hit a sweet spot between redundancy and illumination.

The more LEDs I can run wioth reduced complexity and keep costs down with the project the bigger it will grow over time.



> Some of those old PSUs can have efficiency as low as 50%.


 
Oh yea, Agreed. I wouldn't use a cheap, 150watt PSU for doing anything other than testing. 

However, newer, name brand PSUs are now tested in various computer forums and are rather brutally tested in terms
of efficiency ratings. 80% efficiency is now the lower norm I beleive. Regardless, 250watts of clean, fan cooled 12Volt power
that's UL listed and sitting up in the ceiling would seem to give me more flexibility in terms of budget and availability 
than typical, high current, AC/DC LED drivers.


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## TorchBoy (Sep 5, 2008)

2xTrinity said:


> You'd be better off rectifying 120VAC into 170VDC, then using a constant-current buck driver to get 90V. A couple of the DX drivers appear to do just this.


Do tell. The closest I can find is this one, max 42V DC. (There's also a stroppier one that's sold out.)


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## Mash (Sep 8, 2008)

After a quick read of the thread, Ill put in my 2 cents!
PC PSUs can be great depending on circumstances.
The best way in my experiments, was to have a max of three LEDs in series (so you stay below the 12V max voltage), regulated either by a LM317 chip, or a resistor as mentioned above. Then you can parralel lots of these '3 up' legs, onto the 12V rail.
One problem with some PSUs is that they dont supply a constant wattage, they adjust their output according to load placed, so you might have to have a dummy load to get any juice out of them, if the LED circuits dont load up the PSU enough. this can be irritating and confusing when you are experimenting!
However once you play around a bit you realise that getting 12V input is not the hard part, since you can get really nice and stable (like MEANWELL) PSUs pretty cheap anyway. The trick and the hard part is the current regulating mechanism to the LEDs themselves. Resistors etc are a waste of energy and efficiency. Soooo in the end sometimes a proper LED driver like a xitanium or VARILED (the one im using) ends up being cheaper, less hassle, and much more easier and user friendly to use.


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## TorchBoy (Sep 8, 2008)

Mash said:


> Resistors etc are a waste of energy and efficiency. Soooo in the end sometimes a proper LED driver like a xitanium or VARILED (the one im using) ends up being cheaper, less hassle, and much more easier and user friendly to use.


:shakehead I don't think you get it. Most people running LEDs from a mains-power source are probably not going to be worried about a handful of percent difference in efficiency. The Xitanium drivers I've seen are very expensive. Resistors, throw-away power supplies, and mains power are very inexpensive, possibly even free. Resistors only have two connections and are simply connected in series. Therefore resistors are much cheaper, less hassle, and much more easier and user friendly to use.


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## Mash (Sep 8, 2008)

All above is true Torchboy! 
I suppose you have to balance your cost vs convenience vs efficiency etc as per all situations.
I mentioned efficiency since op stated battery power as a neccessary option for this rig.
However as a chronic salvager and recycler myself, I have no arguments against reutilising free equipment! ;-)


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