is there some combination of voltage rails on an ATX power supply that could be utilized to provide 28VDC @ 8.3A?

I'm trying to find an inexpensive power supply solution for a LUXIM Li-Fi Plasma Lighting source. The RF generator requires 28VDC at 250W is there some combination of voltage rails on an ATX power supply that could be utilized to provide 28VDC @ 8.3A? is there a way that the voltage could be slightly boosted or cut by a couple of volts?

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It's possible if you know enough about electronics to follow these instructions. Make sure your supply is at least 232.4 watts and can put out 8.3 amps or more on the 12V supply - the one we'll be boosting. The specs are usually on the sticker on the side of the unit. First you would have to check the voltage rating on the capacitors and diodes. The capacitors probably aren't going to handle 28V so they'll need to be replaced. Boosting the 12V supply is going to boost the other voltages as well, so all of the other large filter capacitors on the secondary side can just be removed since they won't be used. Then you have to find where the feedback optocoupler(s) is and there is usually a TL431 voltage regulator that looks like a small transistor (TO92). The divider resistors going to the TL431 have to then be changed out to trick it into putting out a higher voltage. Then hopefully you'll be able to get 28V, but probably not from every power supply. Worse comes to worse, you'll have to remove the secondaries on the bigger transformer and wind a new one to get the higher voltage. The core halves are extremely brittle and can be difficult to remove without breaking. What I do is set the transformer upside down on on a thin, flat piece of steel so that the top core half makes good contact with the steel. Then put the whole thing on the stove top on low heat until the epoxy softens enough that you can pull the core halves out, usually about 900ºF. Save all spacers if there is any to reuse when it goes back together. The bottom half can be particularly difficult to remove and it usually requires direct heating which is tricky because the pins are in the way. You'll probably break a few before you get it right, so practice on a few until you're confident to try the one you plan to use. Then you can get the old secondaries off and wind the new one. Leave the (usually yellow Kapton) tape on that covers the primary or cover it with electrical tape for good isolation between primary and secondary. Use several strands of thin (#21-28AWG) magnet wire or the equivalent Litz wire to wind 2.33 (for 28V) times the number of turns as the original 12V winding(s). Don't change the primary or it could possibly saturate and blow up the switching transistor(s). Then cover it up with electrical tape or use the original tape if it wasn't torn or too badly warped. Now with the new secondary, you will probably need to put in higher voltage rated diodes. Schottky diodes are best or ultrafast will also work. All of these modifications are on the secondary side. None of the primary (high voltage) side stuff should be touched. It's also a good idea to find where the control circuitry gets its power from because it might use something which you're trying to boost, so you might have more modification here too.
rich_moe8 years ago
Larger ATX power supplies (450W and up) have the nice little addition of having an adjustment in them for the + voltages. If you wired 2 +12VDC lines from 2 separate power supplies and one +5 Volt line, you would have +29VDC, and could adjust it down to whatever you need for voltage. Amperage would not be a problem, as a single power supply in the 450W range can supply upwards of 18 Amperes. If you go find a used computer parts store, one power supply can run for as little as 5.00 USD.
a problem. the supplies have their voltages relative to earth and not floating you have to isolate the circuit inside 2 of the supplies from earth but it can be electrically unsafe (power supplies are class 1 and need earth for safety)
Thanks, sometimes I forget the important details. Everyone, please heed 11010010110's advice.
but if you use an rcd and leave one properly earthed then its ok i think
jhvh.one (author)  rich_moe8 years ago
it's certainly worth a shot, i've got a spare power supply that i don't mind potentially damaging
110100101108 years ago
get 24 V DC (not 28 but i think it is usefull) open the supply find the 2 live 12 V AC wires on the transformer from them the current goes thru 2 diodes - toroid - few capacitors and maybe chokes - out connect 2 more diodes (identical to existing ones) from those 2 wires but in the oposite direction. this gives you a -12 V rail with high current capability together they give 24 V
frollard8 years ago
At best if you combined the +12 with -12 rails (if I'm not crazy) you'd only get 24 volts.

You can grab a DC-DC boost converter (buck puck, or bucky transformer) but for 250 watts it might be expensive.

Also, a specific ~28 volt psu could be expensive... http://www.surplussales.com/PowerSupplies/PowerS-4.html
jhvh.one (author)  frollard8 years ago
right, but i was thinking more along the lines of combining the 2 12v rails with the 5v rail, which would give me 29v, right? i'm not sure how i would go about designing it, but certainly there is some sort of buck converter that would take the 29VDC input and output 28VDC. I'm no electrical engineer, but i am handy with the soldering iron...hoping i can save a few bucks. I've tried to source a ready-made PSU to meet my specs but everything i've found is $100 and up.
I'd still suggest making your own transformer - much easier to calculate. You could fry a psu by connecting the rails in series (if they are common ground, it would be like a direct short circuit for one of the rails...
jhvh.one (author)  frollard8 years ago
thanks for the advice!
OR - you could wind your own transformer out of an old microwave transformer core, that could easily handle 250 watts - just need to search a tutorial for custom transformers - then you'd just need a rectifier, and some caps to smooth the voltage.