24volt from pc power supply

Discussion in 'The Projects Forum' started by scratch, Oct 26, 2010.

  1. scratch

    Thread Starter Member

    Aug 6, 2010
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    Helo pple.

    Can some one tell me how to get 24 volts at about 10Amp from a PC power supply (across 12v+ & 12- ) . I would like to use 3 of these modified power supply in series to charge 60v batteries pack.

    THANKS
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    +12 and -12 is 24v
     
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Highly unlikely. The -12v line is rarely assigned more than an amp.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I have some bad news for you.

    First, while ATX and ATXplus12 form factor power supplies generally have a pretty robust +12v supply, the -12v supply is quite anemic, usually < 1A, as it is only used for the archaic RS232 COM ports. Expect RS232 ports to disappear over the next few years.

    Secondly, the output returns of the power supplies are connected to earth ground. This means that you cannot operate them in series without some serious problems.

    Thirdly, PC power supplies were designed to support a certain range of load current and to output fixed voltages; if the load is out of the design range, the output will either not regulate properly (if too low), or shut down altogether. This is very different from battery charging requirements.

    Let's say you had just one 12v lead-acid battery that you wanted to charge.

    The normal way to charge it would be to apply a current that is a portion of it's AH rating (usually between 1/5 and 1/10) until the voltage rose to somewhere between 14v and 15v (depending upon battery construction/type and internal temperature) for the "bulk charge" phase. Next, the battery would be charged at a fixed voltage until the current fell below a threshold level for the "absorption" phase. Next, the voltage would be reduced to a "float" voltage level for the "float charge" phase, which can continue indefinitely.

    ATX and ATXplus12 form factor power supplies have to meet all of the standards for size, placement of openings, harness lengths, connector types, voltages, etc - however, their internal circuits can vary widely. It would be very difficult to suggest via the Internet changes that you would need to make to such a set of supplies in order to make them viable battery chargers.
     
  5. scratch

    Thread Starter Member

    Aug 6, 2010
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    Thanks Sgt, yes i can see how that is not going to work.


    And big trransformer (weight) charger is not practical for on board EV charger.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    One possibility, if you can find a supply that has more than one +12V output, both at decent amps, you could unsolder one of the toroid output windings and put it in series with the other one - provided of course you changed the filter caps to handle the higher voltage.

    If the toroid is big enough you might be able to double the turns on one and do it that way too, but this is all wishful thinking as very few would be easily modified.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I've actually seen a 7912 in some ATX supplies, the 24V would only be as beefy as that weakest link.

    The legacy -12V will be gone soon (as the -5V did a ways back). This is seen with the +5V rail has giving way to very high current +12V Rails, stepping it down at point of use supplies for power hungry areas, such as CPU and RAM.

    My newest ATX supply is rated 30A for +3.3 and +5V, 4 +12V Rails at 36A EACH (984 Watts!), and -12V at 500mA. I find that last one funny next to the other ratings on a 1000+ Watt power supply.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    Thats the "just in case" 500ma... ;)
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    With the older components we used a lot more 5V and 3.3V with the 12V mainly hitting the audio, cheap onboard video and mainly for the hard drives. New motherboards use the +12V to step down for the processor and memory and also use it to drive the now higher-end video onboard and especially the plug in cards as well as still being needed for the drives.

    Supply wattage is deceiving as well, if you add up all the rated outputs you'll see there's no way you could load them all down at the same time.

    When buying a new power supply, unless it's being used as a replacement for a really old PC look carefully to be certain most of the power is available from the +12V rail(s) or you're just wasting money. I received one free with a case not long ago that was totally unuseable as many that come with cases are. It's just a way they unload the older supplies that aren't really modern enough to sell.
     
  10. scratch

    Thread Starter Member

    Aug 6, 2010
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    So basically there is no way to get high volts at decent amps from PC power supply ?.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Not really.

    There are "open frame" SMPS modules available that can give amazing amounts of current at about any voltage. Their price is a bit higher than a PC Power supply due to the sheer volume discount of ATX supplies.

    For charging a battery, you would need to also consider a charge controller capable of the current/voltage you are desiring to use. The batteries aren't cheap, and cooking one would be a shame if skimping on the charge monitoring.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Not easily, but as mentioned above places like Jameco sell "open board" power supply modules that should meet your needs. YOu simply supply the case, fuse, line cord and switch.
     
  13. scratch

    Thread Starter Member

    Aug 6, 2010
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    marshallf3 that sound good thank U.
     
  14. scratch

    Thread Starter Member

    Aug 6, 2010
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    No thing at JAMECO.

    need 90v at least 10 amp
     
  15. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    90V @ 10A is 900W.. what in god's name do u want that for?
     
  16. scratch

    Thread Starter Member

    Aug 6, 2010
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    on board EV battery charger. 72volt battery pack.
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    60V @ 17A, But it is medical grade, which is probably overkill for regulation/interference reduction.

    At DigiKey 1000W @ > $0.50/watt

    I know electric forklifts charge from a 220 or higher input, and run 00 Gauge cables to the batteries for charging. Is this similar to what you are looking to accomplish?

    Trying to get more than 1500W from 120AC is a bit too close to the edge of typical circuit limits.
     
  18. scratch

    Thread Starter Member

    Aug 6, 2010
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    can be 90v 5amp
     
  19. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    what about the charging methods.
    U can't just cannot connect a heavy duty supply to a EV battery pack.
     
  20. scratch

    Thread Starter Member

    Aug 6, 2010
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    will build PWM charge controller.
     
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