13.8V from an ATX supply?

Discussion in 'General Electronics Chat' started by ksoggs, Jul 4, 2010.

  1. ksoggs

    Thread Starter New Member

    Jul 4, 2010
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    is there any way to get 13.8v out of a 12v ATX power supply.....

    I am a computer technician.... and i have BOXES of power supplies and old computer parts laying around....

    I have been using computer power supplies to power 5 and 12v items for years...

    but i recently have found the need for voltages other than just 5 and 12...

    So i was wondering if there was an easy way to convert a computer supply into more of a variable voltage bench supply....

    most importantly and urgently is the need for 13.6-13.8v.... Is there a way to step up the voltage of an atx supply to get this?

    also... i know the 5v side always has much higher amperage than the 12v... is there any way to modify them to get more amps from the 12? can you run them in series? get like 17v at a mid-range amperage... and then step that back down to 13.8?

    how hard is it to adjust the output voltage on these things? can it be done with a few modifications and extra parts? or is it a major deal?

    I just know that they make wall warts with little switches on them to give different voltages... and those are crazy simple...
    and they make laptop power bricks that go from 12v to 19.5v... and not much to them either...
    sooooo, cant i just combine the 3 things and come up with an ATX based variable voltage bench supply?



    oh and then i guess next question.... is how hard would it be to add current limiting ability to this?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    You could use a DC to DC converter or a BOOST circuit to boost the 12vDC to 13.8vDC
     
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Computer Power Supplies are made for a specific job with low profit margins.

    There is some flex in the current capacity to allow adding hard drives for example but if you have fixed enough computers you know that they don't always even handle that well.

    You might be able to modify a computer supply but make no mistake that you would really be using it to build a new power supply.

    I don't want to get your hopes up but yes they might have an adjustment that would allow you to raise the voltage to around 13.2 Volts. That doesn't mean that the supply would work safely or long when run high.

    These are also mains powered and it is very different and dangerous to start poking into things that have 120V. We don't want to discuss these types of projects in here because often it is not just your safety but the safety of everyone that might read this and start thinking about making their own bench supply from an old computer switching supply. That is a lottery that somebody will lose if we keep playing it.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    As has already been said, computer power supplies are purpose-built by a variety of manufacturers.

    An ATXplus12 might be converted, but it would be very mfgr and model specific, not like the generic "ATX Bench Supply" plans you'll find by searching on Google.

    You'd first have to determine which switching regulator IC was in use, and then look up its' datasheet - and armed with that information, reverse-engineer the whole supply to determine the inner workings.

    It would probably be easier and much more satisfactory to simply build a dedicated charger, using something like the UC3909 like I referenced in the other thread. Or, simply buy a COTS (commercial off-the-shelf) charger. If you tried to convert an ATX supply to a battery charger, you'd have a built-in inefficiency, as the 5v supply needs a load in order to regulate the 12v supply. That's pretty common. Have a look at the UC2842 IC datasheet; there's a "cheapie" flyback converter shown which outputs +12v/-12v and 5v, but it's not really a true regulated supply. A regulated supply provides feedback from the output to the input. In order to do this using an offline mains-powered switcher, you need to couple the feedback via a transformer, optoisolator, or other method to keep the output isolated from mains power.

    [eta]
    If you would like to learn more about switching supplies, visit this site:
    http://www.smps.us/
    Lots of good stuff there, but stay away from the hobbyist circuit section; some are not good and some are just unsafe.
    Here's the computer supply section:
    http://www.smps.us/computer-power-supply.html
     
  5. sceadwian

    New Member

    Jun 1, 2009
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    I don't think it's that complicated Sgt, all you need to do is find out where the feedback network for the voltage regulation is and adjust that portion of the circuit, mind you it will stress the components that much more, and could cause caps and or any number of things to fail in the ATX supply but I've heard of people doing it before.

    There are probably already components on the feedback network that allow trimming of the output voltage.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I'm not saying that it's impossible. However, I really don't feel that it's the type of thing that a newcomer to switching power supplies should take on. Just by opening the supply enclosure, they're exposing themselves to roughly 170VDC if it's powered by 120VAC mains.

    I just popped the tops off of a couple of 250W ATX form factor supplies from different mfgrs; one had a 494 switcher, the other a 3842. They both perform somewhat similar functions, but they're quite different. The interior of the supplies are practically completely different.

    I certainly wouldn't want to spend a few days generating a schematic. Sure, once the reg IC was known, you could find out pretty quickly where the feedback was coming from and change the resistive dividers or whatever - but what our OP wants is to charge batteries with it.

    A current regulation circuit would have to be added to the 12v section, and the fold-back portion removed. But still, unless the other voltages were disabled, they would be essentially wasting a good deal of power - even just at idle current levels. The 5v supply needs a 2A load to regulate 12v on most of them.
     
  7. ksoggs

    Thread Starter New Member

    Jul 4, 2010
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    well the atx supply might be used for float charging a battery when not being used... but more often i would like to a 13.8v power supply to power 12v accessories without the use of a car or battery. like to have a "cigarette lighter" for the garage.
     
  8. sceadwian

    New Member

    Jun 1, 2009
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    If you're just trying to power 12V devices you don't need a 13.8V supply, the straight 12V from an ATX will work perfectly fine. I know of no devices that require the 13.8 volts that an alternator puts out because as soon as the car turns off the voltage will drop bellow that, under load it'll be well under 12, and during starting only a few volts.
    One thing you may have problems with is noise from a switching supply though I'm not sure how this compared to noise from a running alternator, if the noise is an issue use a couple ohm power resistor followed by a beefy filter capacitor and a healthy choke to smooth out the power if you need it to be low noise.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, rather than a resistor you could use a couple of large caps with an inductor between them to make a pi filter.

    Using a fixed resistor in series w/cap will get rid of the ripple, but could cause the supply voltage to drop quite a bit if you have a heavy load.

    A 2 Ohm resistor in series with a 12v supply will cause a 2v drop per ampere of load current.
     
    Last edited: Jul 5, 2010
  10. sceadwian

    New Member

    Jun 1, 2009
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    The inductor would be big but it'd waste a lot less energy than the resistor. The one thing I hate about ATX supplies is except on the 3.3v line of some older models there are no external sense lines so you can't remove additional filter sections from the feedbackloop.

    If I'm not mistaken most modern ATX supplies are going the route of supplying nothing more than massive 12V power rails and letting the regulators at the sources deal with the rest. Modern buck converters for specific applications are more efficient than the line loss from trying to supply the fixed voltages from a power supply oriented regulator.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, the inductor could be quite small.

    Two 680uF caps with a 5uH inductor between them would have a -3dB rolloff at around 4.3kHz; 20dB down by 7.7kHz. That's a Chebyshev low-pass 3rd order filter. Should be plenty for a PC supply.

    [eta]
    Here's what I'm talking about:

    [​IMG]

    Plot is attached.

    You really wouldn't want a large inductor in the output filter, because you'd have a LOT of surging going on until it finally stabilized.
     
    Last edited: Jul 5, 2010
  12. sceadwian

    New Member

    Jun 1, 2009
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    The main noise picked up by a mains switcher would still be 50/60hz sgt. Especially audio circuits.
     
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