PC ATX PSU - Generating -5v for use on older computers

Discussion in 'Power Electronics' started by sirlemonhead, Oct 13, 2016.

  1. sirlemonhead

    Thread Starter New Member

    Oct 13, 2016
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    Hi everyone,

    I'm looking to do something to get -5v from a modern ATX PC power supply. Older computers used this -5v for some things (certain expansion card) but the -5v line was eventually phased out of the ATX spec. This is a problem for people adapting modern power supplies to use in older PCs.

    What could provide this, using the +12, +5 or even +3.3v rails? There is a -12v rail but I dunno if sharing that would be a good idea (it's generally only around 800ma) ?

    Ideally to get 300-500 amps output for the -5v.

    I don't really know what to look for in terms of a pre-built circuit or what I'd need to get myself to make something to do this. Any suggestions would be great!

    Thanks!
     
  2. ci139

    Member

    Jul 11, 2016
    341
    38
    3x (up to 9x to give them some time to live) 850W 5V PSU-s "syncronized" with extra circuitry (all can't rebuilt) to source to the same load line
    usually there are 6A (1440W) limit or 10A(2.4kW) fuse on mains-phase
    you need minimum 3 x 850W / 80% = about 3.2kW 16A line to hook anything there
    i guess the 3kW DC PSU is a certified industrial apparatus that costs 10x more than that when it was designed for home use
     
  3. sirlemonhead

    Thread Starter New Member

    Oct 13, 2016
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    woops I meant to say 300-500 milliamps up there!
     
  4. JWHassler

    Member

    Sep 25, 2013
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    A quick and direct way would be an isolated DC/DC converter, like this one.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Do they still have a -12V output? - could be as simple as a 7905. Many AT PSUs actually did that inside the PSU box.

    Try googling "inverter topology" - something resembling a flyback converter can be rigged to give negative output from positive input. Its only worth that much effort if you need more current than a 7905 can give.
     
  6. sirlemonhead

    Thread Starter New Member

    Oct 13, 2016
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    Thanks for the suggestions guys :)

    Ian, I did see the 7905 before.. I'm quite new to electronics. The -12v is still present on new PSUs but it's generally only a few hundred milliamps. I don't know would sharing that rail not possibly pull too much current?

    Someone else from the retro PC is looking into this and is going to try this circuit: http://www.ebay.com/itm/DC-DC-Buck-...hash=item1c6ac0d7dd:m:mTkMdgP4g18r-iGIGSD69UA

    Would that work?
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Its no good for a -5v because its not an isolated output.
     
    Last edited: Oct 15, 2016
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    Some of the older lower powered AT PSUs were flyback - almost al modern types are half bridge push-pull converters.

    With a push-pull converter; its possible to add a negative rectifier to the same winding that feeds the +12V output.

    Some older one's had 78xx/79xx regulators, most now use tight coupling on the secondaries. It may be as simple as a negative rectifier on the 5V secondaries.

    Some ATX PSUs I've reverse engineered, had a single regulation error amplifier with the +5V and 3.3V rails summed in with a resistor network - so its pretty much "near enough" situation. The 5V secondary is usually CT with 2 rectifier diodes. You need to use Shottky-barrier types, I'd go for 40V types for safety. The electrolytics need to be low ESR types - you may need to scare up a choke coil to make a Pi filter on the output.

    Stay well clear of the primary side - you don't need to touch that.
     
  9. sirlemonhead

    Thread Starter New Member

    Oct 13, 2016
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    0
    A 7905 does seem like the easiest way.. I think the idea my friend has is something that can be added externally to an existing ATX to add the -5v capability.. He's concerned about the heat generated from the 7905 though.

    This is what he found in an ebay bought ATX to AT converter that boasts the -5v addition... [​IMG]

    I believe it pulls from the 3.3v rail.

    I did just open a Compaq OEM psu I have lying around that's got the typical ATX connector and has -5v "built in" rated at 0.3A - It has a SEC KA7905 inside :)
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If you can get inside the atx psu, its only a matter of putting in two diodes on the 12v winding in the opposite direction with a couple of smoothing caps, and it will give you a Negative output, then you can put a 7905 on it this will give you a 1 amp supply.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    If its a half bridge push-pull - you can put the diodes straight on the CT 5V secondary.

    To all intent and purpose; the current is what the diodes you add can handle. The regulation will be the same as the +5V output.

    The diodes would have to be SB type.
     
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