# ATX PSU (300W) Conversion: Converter/Charger

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#### CaptainSlappy

##### New Member
Hello all! Just joined up, and got (like everyone else), one of those ideas I can't seem to turn into reality.

TO whit:

Take an RV. Now, RV's come with your standard 110v to 12v converters.
These converters do TWO things at once (and the good ones do more), such as-

1.- Convert 110v AC to 12v DC for running 12v pumps, and 12v lighting (constant usage, meaning it pumps that 12v on that rail constantly).

2.- Charge a 12v Lead Battery, but the good ones come with "float" charging, not just constant charge (which in the old ones, destroyed the batteries eventually). Not to mention the new ones desulfate, saving \$! (Pulse charge DC)

My problem is this- I have a few ATX PSU's hanging around, gathering dust.
Technically, I know good and well with the proper setup (though I do not know the how), they can be used to do either one, or the other at once (converting for constant power use, like lighting), or charging (by charging the battery, who then runs the lights, like a buffer if you will).

My question is, can an ATX PSU be setup to do BOTH at once?

If not, not a problem, as I understood (after 3 hours of Googling, with not one bit of it applying directly to my situation, if I do the simplest conversion, the ATX will charge the battery, but straight out of the box, it will NOT "float" charge (thereby eventually destroying the battery).

If I go the other way, it might be possible to have "constant" power to my
12v lighting (if I can figure out how many amps 7-12v 1154 bulbs run), and INCIDENTLY charge my 12v battery along the way (but VERY slowly).

I DO, however, know that if you run a ballast resistor, it might (?) keep the ATX.......(A) from burning up and (B) from destroying a battery especially if there is no load CURRENTLY running on the circuit (meaning, it is idle).

Sorry for the long read, but I was just curious.

The simple answer seems to be: Convert ATX PSU to a simple battery charger, somehow, with the "float" mode. (Though not really simple is it?)

My only concern is that by doing that, the ATX PSU won't be able to handle charging AND running 12v lighting at the same time (in my case, I only need my 12v battery to run lighting ONLY).

And yes, a 1.5A trickle charger tries, but ain't got the cajones.....
It went bye-bye! (I didn't catch the 1.5A part, but she lasted for 2 months)

Just throw some theory at me, I got time and patience, and while Google did help, slightly, it never helped with an ATX "float" charger AND acting as a converter for constant use at the same time. I did see some audio amp ones, but they were putting out close to 15-20 amps, a bit more than I need, methinks........

#### ccurtis

##### Well-Known Member
If I go the other way, it might be possible to have "constant" power to my
12v lighting (if I can figure out how many amps 7-12v 1154 bulbs run), and INCIDENTLY charge my 12v battery along the way (but VERY slowly).

That is the way UPS systems generally work. If the charging system is set up properly, the battery will last a long time. And, when it is not powering up any lights, it can also go into the "pulse" mode.

#### tcmtech

##### Banned
YOU could run two ATX units. One set up at the float charge voltage and the other set for peak charging. By using a window toggling circuit like this one pictured below.
You would have it turn on the peak charger if the battery voltage got pulled down below a certain point and then charge up to the peak voltage and shut off.

But I expect the battery charging purists will be here shortly to overwhelm you with ideas too!

Just beware of the " super ultra multistage microprocessor monitored battery charger posts "

You may have to sell your RV just to buy the gear and go back to school to learn how to understand whats required to build it!

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##### Banned
There's one very fatal flaw with this idea.
The float charge for a 12 volt battery is between 13.5 and 13.8 volts. You would have to modify the ATX supply's 12 volt regulator to this voltage.
Also the 12 volt rail of an ATX supply is very low current, probably not enough to run more than 3 or 4 12 volt lights. The real current that comes from an ATX is from the 5 volt line. Look at the side of your ATX supplies and read their allowed current out from each rail.

#### tcmtech

##### Banned
Actualy tweeking the ATX type power supplies is not that hard. you just need to find the voltage sensing feedback circuit and bump it up some. I have been able to get the 5 volt refrence feed back tweeked up to 7 volts and the 12 volt followed up to about 16 volts. Some have a high voltage shut down monitor IC so you may have to cut it out of the circuit to get a higher than stock voltage too!
I have several 300 - 350 watt units and the 12 volt outputs are rated between 15 and 21 amps depending on manufacture. With out any other load on them I have run them way higher than the rated amps without any problems so far.
The 12 volt output is the least monitored of the power supply circuits and thus tends to be the most forgiving towards being abused.
All of the ones I have disected for parts have the 12 volt output on the transformer in sereis with the 5 volt and 3.3 volt. The transformers I have taken apart had the same size wire for all three outputs so in theory the 3.3 winding is carrying the combined amps of all three outputs. And thus the 12 volt should be able to take far more amps if the rectifiers and filter capacitors are able to handle it.

#### MikeMl

##### Well-Known Member
I just bought an older travel trailer and am dealing with the same issue. I bought a 15V adjustable 40A switching supply on ebay. I tricked the supply into putting out two different preset voltages by switching some additional resistance across the voltage setting pot inside the supply. The two different output voltages I wanted are 13.2V for floating, and 14.5V for recharging.

I haven't built it yet, but I plan on using an external controller to switch the supply between the two different settings to implement this charging algorithm. I have already tested my supply and if connected to a very discharged battery, it limits its own output current to ~45A, about the same as an automotive alternator. As the battery comes up, the charging current begins to taper. I plan to automatically switch to the Float mode when the charging current drops below a couple of A.

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##### Banned
12 to 20 amps on the 12 volt line? Man I must only ever get cheap ATX supplies, none of the one's I've had are over 5 amps.

#### CaptainSlappy

##### New Member
Now see Gents, THAT is what I am talking about....

Good ideas, and the pointing out of fatal flaws, like catching myself on fire!
While always amusing, it isn't......not really.

Hehehe.

I do have a few PSU's hanging around, and while I had taken note that the 5v line carried the highest amps, I wasn't for sure........now I am.

Excellent point, TCM. It might be time to go not to school, but definitely back to basics, and work from the ground up on one of these. The "super duper multiplexing phase shifting uber-charger" isn't a bad idea......

If I lived on Romulas, which I do not, and all the Vulcans took the last train from OKC just yesterday no less.......

Scea- I am definitely keeping in mind what one SINGLE ATX PSU can do, and what it cannot do as far as V/Amps will allow. (And knowing the neat thing is they can also burn up......quite fast, smokey too!)

Interesting point, TCM. I will go back to The Google and see what she says on jacking up an ATX into wonderland, and try to pull some straight info on it.
As I saw earlier, the auto guys running these meat-eating amplifiers did some.....interesting things and were able to charge a battery AND run the amps at the same time, but it is not quite what I was shooting for (but would, in the end, live with).

EXCELLENT post Mike. Thanks for that.

Actually, I have the left over converter from this trailer (original 1976 issue), but MINUS the battery charger module, which Sgt. Schultz from Hogan's Heroes says, "It hears nothing, it sees nothing, it go KAPUT!".

BUT........the transformer is still kicking away like mad. 5A on one side, 3A on the other, 8A combined, ~ 15v output.
HOWEVER........since I am limited in knowledge, and my other trickle charger (1.5A) transformer died (but the board was good), I did the not smartest thing ever, and ran that TC board off of the the old Converter transformer.

Oddly........it works. But not quite right. Meaning, the TC would say, "Hooked Up, Not Hooked Up, Charging, Fully Charged". At the moment, when running on a battery to charge, it says, "Well, I am fairly certain I am not hooked up, but seriously, I will charge this battery nonetheless".

So much for Frankenstein........

In other words, that PCB has all the parts I need (I think), to either rework the old MASSIVE freaking transformer on the old Converter to a Converter/Charger combo (with built in float), or I can cheat and use that TC PCB for an...........ATX supply.

I am not sure I am industrious enough to rip that TC PCB apart to build a specialized "float/charge/convert". However, on a side note, remember that I did the Doc Frankenfurter, and while the TC PCB DOES WORK......it does NOT tell me anything (and hence, I am worried it go BOOM! one day).

It does handle that 5A pretty damned cool, doesn't even get warm.
That is, of course, assuming it IS charging the battery............
(My multimeter comes with a needle, if you get my drift, not fancy and OLD!)

Thanks gentlemen!

#### MikeMl

##### Well-Known Member
Here is an earlier post which spells out the charging formula.

#### Hero999

##### Banned
You aught to can that post.