# PC Power supplies in series?



## mrunlucky07 (Dec 15, 2009)

I am looking at options to drive a bunch of 3.7v crees in series.

I was looking through my junkpiles and had a thought that I am sure has been covered, I just couldn't find the answer.

I have a bunch of old computer power supplies which have +12v and +5v high amp outputs.

Is it possible/feasible to connect several supplies in series to get a higher voltage?

I would like to get up in the 34-48v range so I can run at least 9 leds in series.

Can 3 supplies be run in series to get 36v off the 12v supplies? Or combine both the 12v and 5v outs off of two separate supplies to get 34v?

Thanks!


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## balou (Dec 15, 2009)

Why don't you just connect the LEDs in parallel (or say 3 in series and multiple 3-in-series parallel)? Only one supply needed...

And I don't know if you could connect multiple power supplies in parallel. I think that depends on whether PC power supplies use floating ground.

But as a first answer... don't try it out unless you have an answer with a good reason why it's 100% safe. Until then consider it unsafe.


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## mrunlucky07 (Dec 15, 2009)

balou said:


> Why don't you just connect the LEDs in parallel (or say 3 in series and multiple 3-in-series parallel)? Only one supply needed...
> 
> 
> But as a first answer... don't try it out unless you have an answer with a good reason why it's 100% safe. Until then consider it unsafe.



I am looking to drive 50+ over a fish tank, so I would like to extend each series string.

For now this is an out there option, just wanted to check and see if anyone has tried it. I am on the unsafe wagon for now.

I found this diagram







http://power-topics.blogspot.com/2009/07/operating-power-supplies-in-series.html

Which is what started me thinking about it.


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## The Dane (Dec 15, 2009)

PC supplies can be coupled in series with no ill effect appererntly (just tried 2 with the 5V's in a 1.0 Ohm series resistor. No problems, but start up the supplys first and then plug in the LED's, in order for the supplies to establish regulation first.
Please measure and take to account that regulation Voltage sags as amperage increases.

PS i didnt use diodes! But why be less carefull


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## Conte (Dec 16, 2009)

This might sound silly, but I'd probably check to see if they were ground isolated first, just to make sure the negative of the output is in no way tied to the AC plug ground. 

Some older electronics I've worked on is wired this way. Mostly guitar amps. 

I doubt a computer PSU would be, but it's just my style of due diligents. 

If the ground is isolated, I don't see why this would not work.

The diode idea is a good one, but it be tricky finding ones big enough to take the >10amp load the PSU can put out. You might need multiple smaller ones in parallel. 

You don't need the diodes, if you fire up the PSUs and make sure they are all running before you apply the load you should be fine as long as one unit doesn't fail. 

For this reason, your application, a fish tank, might complicate things. 
I dont' know about you, but I have my fish tank light running on an AC timer. If your using such a timer, then def rig the diodes if you can. 

I've modded my fish tank hood lamp like I mod my flashlights.
It had stock a 20watt 24inch T12 bulb, couldn't even throw to the bottom of the tank. I gutted it and rigged a 24watt 24inch SHO T5 and lined the hood lamp enclosure with silver tin foil tape. Much better. 

Now, if you can afford to build a 50 led lighting array, why not build a custom powersupply to run it ? I'm under the impression that this would be a pricey thing to build ?

And what kind of fish, and how big a tank are you running that you need that much light ? 50 crees would pump quite a bit wouldn't they ?


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## mrunlucky07 (Dec 16, 2009)

Thanks. I should have some time to test a little today. Realistically I will probably buy meanwell led drivers, but thought I would explore the options first.

From what you guys are saying it sounds like I would want to keep a small constant load on the supplies to keep them running 24/7, and then establish a timer between the supplies and leds for daily on/off.

It is for a 110g tall sw tank, half white half RB, the leds will have tight optics and be close together.


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## Conte (Dec 16, 2009)

Ah, yeah, Saltwater tanks do need alot of light.

I dont' think you need to keep the PSU's under load when not in use. I think letting them run idle is better, if there is no circuit path connected to them the power can't budge and will stay out of trouble. 

Running them 24/7 will solve one of the problems provided you don't mind wasting the electricity, you just have to mind the maintenance cycles. 

Have to make sure you keep them clean and the fan spinning, so you'd have to take them apart about once a year to clean out any dust. About 6 months if you smoke, have furry pets, or keep them close to the floor, maybe even less if all 3. Be prepared to replace the fans one day, also. 

Keep them running clean and cool, they'll probably last forever if they are of reasonable quality to begin with. :thumbsup:


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## balou (Dec 16, 2009)

Ok, so PC power supplies seem to have no problems concerning the grounding.

One thing to keep in mind are the different ratings of the power supplies. Not as whole, but on the different power rails. The lowest rated power rail dictates the maximum power you can use. E.g. if your 3 supplies are able to deliver 6, 4, and 3 amps on their 12v rail, then you could only load it with 3 amps. I don't know if I'm being Mr. Obvious, but it's hard to guess sometimes.

Oh and don't bother trying out the -12v rail: it delivers only a few milliamperes of power
edit: and the 5V and 3.3V rails aren't that suitable either. 12V is where you can get loads of amperes


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## Conte (Dec 16, 2009)

> E.g. if your 3 supplies are able to deliver 6, 4, and 3 amps on their 12v rail, then you could only load it with 3 amps. I don't know if I'm being Mr. Obvious, but it's hard to guess sometimes.



Actually. That's some very important information. Probably more important then anything else discussed so far. Thanks for bringing it up, wish I had of thought of it. 



> Oh and don't bother trying out the -12v rail:



It's too bad it's so weak, If you connected your positive to the +12v rail, and your negative to the -12 rail you'd get an easy 24volts. I was going to mention it but I decided not to.


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## Conte (Dec 21, 2009)

Whoa I just had a brain storm . . . . . . . 

If you are running so many LED's you could push the voltage even higher . . . 

If you hooked 25 leds in series, and they can accept 4.8v each, you could basically just plug them straight into the wall without need of a PSU. 

You could run 2 sets of 25 LEDs in series. Well, I'm no led expert. But if you could chain them that long, or be more mathematical in how you chain them, you could pretty much eliminate the need for a PSU all together.

If I had more specs to work with I could crunch some numbers, and even draw you up a schematic of a direct line PSU.


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## balou (Dec 28, 2009)

You'd need a full wave rectifier - else the LEDs could be damaged.
A smoothing cap would be another good idea, to make it flicker less (the 100/120Hz flicker is horrendous I can tell you...).



> If you hooked 25 leds in series, and they can accept 4.8v each, you could basically just plug them straight into the wall without need of a PSU.


It's not primarily about volts, but amperes. You could run a single LED at 20V as long as it doesn't get to much amps.
You'd need a rather big resistor - maybe 5-10W? I'd have to run some calculations.

But thinking of it - Aquariums and mains power? No good idea. Keep it safe, keep it below 50V. There must be a reason only the cheapest chinese christmas light strings (how they are called exactly in english?) run directly on mains.


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## Conte (Dec 30, 2009)

Yeah, you'd need a full wave rectifier for sure. You'd be building yourself a sort of transformerless power supply.

I don't know much about leds but I'm assuming he has some drivers on them. I do know electronics in general tho, and the way he's planning on running it in the first place is not much different. Will just be adding more to the chain.

If I knew all the specs and requirements of the Led, I could likely crunch the number myself.

Fish tank range hoods run on mains power already. 
I assume he still is going to have a hood over his tank separating the light from the water. 

If its done right, it should be perfectly safe. IF its done right.
You've made me think on the other hand. I'm used to working with mains voltage, I could set this up safely. If the other party is not, perhaps they should not try it 

The heapest chinese christmas light strings run that way simply cause its the cheapest way to fill their power requirements, and they are not mission critical. Not really a safety thing.

He'd be hooking up his leds in the same manner, so if one were to fail, the whole string would go out. 

Most things have power supplies cause they want to run them in parallel for reliability and consistent performance, in which case they have to bring the voltage down to match what they are running.


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## Magic Matt (Dec 30, 2009)

mrunlucky07 said:


> I am looking to drive 50+ over a fish tank



That much light may actually kill your fish. Remember fish can't blink, and bright light can cause them to become very stressed. Unlike in their natural environment, they have no way to avoid the strong light, and many breeds of tropical and marine fish avoid strong direct sunlight, preferring shaded or diffused lighting. This is also the reason that tropical and marine lighting tanks use daylight-balanced flourescent tubes that are not very strong light sources. 

Consider the different that you feel exposed to sunlight, compared with somebody shining a projector light straight into your eyes. Now imagine you can't blink or avoid the light.

Please, do make sure you are considering the health of the fish in your plans as much as you're considering the design of your power sources. Both could lead to fatal mistakes.


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