convert ATX power supply to 12VDC input

[eric©]

Have a need for an ATX power supply with a 12VDC INPUT as opposed to
the typical 120VAC. Looking at commercial off the shelf offerings,
and these things are either absurdly expensive, or intended for the
CarPC market, and are tiny little things with low wattage. I don't
have space issues, so the standard ATX size is fine for me.

I seem to recall seeing a page some years ago that detailed how to
convert a regular supply to DC input, but of course can't find it now
(or am just "remembering" I didn't really see, lol). Seems like if I
just bypass the rectifier section, and possibly change the
transformer, I should be OK, but I don't know enough about power
supply design to be sure of this. Is this just not as easy as it
would seem, or is my Google-Fu seriously lacking tonight?

Eric

[Keith Mc]

My first thought was to tie a cheap 12VDC => 120VAC stepped
sine wave inverter and a cheap standard ATX power supply in series.
BUT,,, As they are BOTH switchers, they may NOT be happy interacting.

Anyone here have any experience with attempting that?

I've had mixed results. On one hand, I've had OK luck with
a cheap car cig lighter plug to 120VAC inverters driving my
Dell laptop's AC power bricks, but they've quickly fried themselves
whenever I tried to drive a lousy little tiny phone charger...

The point is, you can find old ATX supplies for nearly nothing,
and some AC inverters are getting quite cheap as well.
If you CAN find a pair that'll work in tandem, that may be
a lot cheaper total than a true 12VDC ATX supply.

- Keith Mc.

[Krunal Desai]

PicoPSUs weren't horribly expensive the last I looked; I think I have
a $20 unit running a mini-ITX router fed from a DC power brick.

They make different variants for different input voltage ranges as well.

--khd (mobile)

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[eric©]

I've seen the cheap Picos, but not in the 200+watt size I need (at
least not at the $20 mark anyways). Seems the relatively inexpensive
ones top at about 120watts. I'm looking at a minimum need of
200watts, and more likely closer to 350 (several large fans, multiple
hard drives, and a monitor being driven off the supply in addition to
the computer itself). There is the Pico PW-200-M that would work if I
can keep the loads down, and only runs around $65, but I'd really
rather have the extra power available if I end up needing it.

All this said, it's not so much a cost issue, as it is a "can I do it"
issue. And if I can do it cheaper than an off the shelf solution, why
not? I do have a 800 watt Statpower inverter that plays just fine
with ATX power supplies that I'll likely end up using if I can't come
up with a way to convert an existing supply that I have here.

Eric

[Roger S]

You can't "just" change the transformer since your input is DC not AC. The cheapest way might be to get and inverter to drive a regular supply.  But that won't get be very efficient.

[eric©]

Well yeah, that's why I said " Seems like if I

just bypass the rectifier section, and possibly change the

transformer," although thinking about more now, assuming it's a
bridge rectifier, there's no reason why I can't feed it DC as the
output from the rectifier would be DC anyways.

Then when I was looking at the theory of operation of a switched mode
power supply, it suddenly dawns on me - "duh! Transformer! Needs AC
to work!" followed by epic facepalm. I've been looking at this the
wrong way - it's not that I need to change to DC input on the power
supply, I need to drop the entire thing up to the rectified DC output
before it goes into whatever part of the circuit splits it off in
12VDC+, 12VDC-, 3,3VDC, etc and tap my DC input there. But I'm
guessing that with my rather limited knowledge and understanding of
this stuff, I'll quickly be back to the inverter on my DC input idea.

Eric

[Andrew Meyer]

On Wed, Dec 28, 2011 at 00:40, eric© <eric...@gmail.com> wrote:
> I need to drop the entire thing up to the rectified DC output
> before it goes into whatever part of the circuit splits it off in
> 12VDC+, 12VDC-, 3,3VDC, etc and tap my DC input there.

In my experience with ATX supplies (generally older ones, rated
100-200W), each voltage rail was a separate tap off the transformer.
More modern supplies might be different, but this conversion may not
actually be possible, depending on the supply. There is a reason folk
make dedicated DC-DC supplies, after all--if it were easy to convert
an AC one, most folk would do that instead of shelling out for a
premade one.

--
Andrew G. Meyer
agm...@gmail.com
"Both optimists and pessimists contribute to our society. The optimist
invents the airplane and the pessimist the parachute."--Gil Stern

[Benjamin Murdock]

You should be able to run the fans off the unregulated 12v. Then the
120w supply should come closer to the mark. If you use a separate 12v
to 12v regulator for the monitor you should have nothing to worry
about.
- Ben

[Roger S]

Eric, use the fu. LOL. See if you can find schematics for your supplies to determine whether or not there are multiple taps. It's possible that a switching supply could use a single DC rail for multiple outputs. However, if your board requires some - outputs you're screwed.  You would need two separate DC inputs in that case.

[moos3]

A typical supply uses 120 volts dc (rectified from the mains current)
as the input to a transformer having multiple outputs for the big
loads 12v0 5v0 3v3. The primary side has 2 windings, one is for the
power in, and the other powers the regulation circuitry. The
regulation is bootstrapped when the initial inrush of 120VDC induces
emf in the regulators winding.

The whole thing is a carefully balanced symphony of emf and counter-
emf, all centering around a carefully crafted transformer made just
for that exact application. I doubt you will find a drop in
replacement transformer, that you will want to unwind and rewind your
existing one, or that if you did it would have the properties you
expected.

I would recommend;

1.) Roll your own supply based on bucking regulators with each stage
sized to emulate the rated capacities shown on a commercial ATX
supply. Don't forget to regulate the 12v0 also, that stage may need
one of those newfangled regulators that can automatically toggle
between boosting and bucking. (+this is a hack, -this is hard)

2.) Buy an inverter much bigger than 200 watts.(+you can use it for
other stuff too, -inefficiencies,costs as much as #4,not a hack)

3.) Roll your own inverter to boost 12vdc to 120vdc and inject that to
your ATX supply. (+hack, -dangerous voltage
levels,inefficiencies,hard,result is not very different from #2)

4.) Buy 12v0 ATX (+works, -$$,not a hack)

On Dec 27, 5:34 pm, eric© <ericli...@gmail.com> wrote:
> Have a need for an ATX power supply with a 12VDC INPUT...

[Roger S]

How does 120VDC energize a transformer? !  DC does not drive a changing magnetic flux in the windings.

[Chris Chan]

While that was true in older supplies, modern designs have one big fat 12v rail and regulate the 5v and 3.3v off that. This is because today's computers have higher power demands form the 12v rail and 5v and 3.3 have become minor. http://jonnyguru.com is a great resource about power supplies; their reviewsusually state what kind of design the power supply is based off of.

Chris Chan ch...@incien.so
+1 909 278 7753
Sent from my tablet; please excuse my brevity.

[Krunal Desai]

On Dec 28, 2011, at 13:34 , Chris Chan wrote:
> While that was true in older supplies, modern designs have one big fat 12v rail and regulate the 5v and 3.3v off that. This is because today's computers have higher power demands form the 12v rail and 5v and 3.3 have become minor. http://jonnyguru.com is a great resource about power supplies; their reviewsusually state what kind of design the power supply is based off of.

This was also due to a change in the ATX specification; prior to version 2.2 (or 2.3, I don't recall exactly) there was an enforced current-limit per rail (highly frustrating for high-end GPU users). You had to hook up your devices carefully to balance the load over the rails and ensure you didn't trip current-limiting on a particular rail. Prior to the change, some manufacturers ignored the limit, others tried their best and yet others just said "screw it, we're doing single-rail".

That particular requirement has now been deleted (or made optional) so you can get excellent supplies like the Corsair AX/HX/TX/etc that have obnoxiously huge 12V rails. I think my AX750 can deliver something like 65A on the 12V rail.

--khd

[eric©]

Looking over some pages that go over the theory of design of these
power supplies, it looks like the regulation of the 12V rail still
happens before the transformer though, where it's regulating the input
in order to control the output. So I could in theory just tap my
input on the 12V rail, and still maintain regulation on the 5V and
3.3V rails, but likely would lose the regulation on the 12V rail?

As I mentioned earlier, l do have a 800watt inverter, and
realistically that's likely the way I'll end up going but I'm still
curious to see just what I would need to pull off either roll my own
supply, or modifying an existing supply to work. I'm going to look
over those links posted later today when I get more time.

Eric

[Joseph C. Bender]

On 12/28/2011 12:20, Roger S wrote:
> How does 120VDC energize a transformer? ! DC does not drive a changing
> magnetic flux in the windings.
>

Obviously it doesn't. Someone skipped a step. :)

Power comes in, gets rectified to DC, then switching power supply
goodness happens, starting with a high-frequency chopper/inverter. It
lets you get away with using much smaller transformers for a given VA
than you'd be able to normally for 60hz, so that 12v@ 65A per rail power
supply doesn't need to be outboard or mounted on a concrete floor. :)
It's basically a really complicated DC-DC buck converter.

(See Astroncorp.com for big linear 12V supplies, a *one* rail 13.8V @
56A linear supply has a shipping weight of 48lb! The 25A switcher they
sell only weighs 7, so even if you buy a couple, you're still way under
the weight of the linear.)

In the realm of "stupid power supply tricks", it's entirely possible to
connect ATX supplies directly to 120-240VDC on the "AC" outlet. I have
heard of things like hybrid car traction power packs (200+ VDC) being
used to run one.

--
Joseph C. Bender
(248) 971-0747
jcbender at bendorius dot com

[eric©]

Under the same concept of using a bridge rectifier as a reverse
polarity protection device, I've done that before. In thinking along
those lines, what's really involved in just stepping up DC voltage,
like in a device like this -
http://www.hybridconversions.com/12_120V_converter_for_HHo_p/12%20to%20120v%20%20power%20module.htm
?

Not that I'm thinking that's a viable solution, I'm just curious at this point.

Eric

[Roger S]

It's still doing an AC conversion internally. It just doesn't need to synthesize a 60 Hz sine wave on the output.