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DC 17V from PC power supply?

Discussion in 'Electronic Basics' started by Mr. Man-wai Chang, Jun 25, 2013.

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  1. 1. If I shorted the +5V and +12V in the molex connector,
    would I get +17V? What's the max. current?

    2. Could I use 17V to charge a notebook battery rated 16V and 3.8A?
    Would it hurt the battery?

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  2. Thanks! Expected answer! :)

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  3. OTOH, should I attempt -5V and +12V pins at the power connector? Or
    should I try +5V and -12V?

    Which one is safer and could supplies 4A?

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  4. Tom Biasi

    Tom Biasi Guest

    If you manage to find 17 volts the current rating will that of the lower
    rated source.Look at the supply label. It will say what the current
    rating of each supply is. The 5 volt supply can supply much more than
    the 12 volts but you will need to use the lesser rating.

    Tom
     
  5. P E Schoen

    P E Schoen Guest

    "Tom Biasi" wrote in message
    Usually the +5V and +12V supplies are fairly high current. And in many cases
    only the +5V supply is directly regulated. It might be possible to reverse
    the diodes and output capacitor on the +12V supply to make it -12V, and then
    you should get 17V between them. The outputs are referenced to ground and
    may be tied to chassis, so you may have to float it. You can probably find a
    nominal 16-20 VDC laptop supply capable of 4 amps for about $10 and it would
    be much safer, smaller, and more convenient. But if you want to play with an
    ATX or similar power supply, there are schematics available that give a
    general idea of how they are made.
    http://www.pavouk.org/hw/en_atxps.html

    Another possibility is to change the feedback resistors so that you get 17V
    from the 12V supply (while the 5V output will go up to about 7 volts). Just
    make sure the PSU doesn't have a crowbar on the 5V supply.

    There are other web pages showing ways to hack these PSUs to make battery
    chargers and get various voltages and even make DC-DC converters out of
    them.

    Paul
     
  6. Daniel Pitts

    Daniel Pitts Guest

    Potentially. Also, battery charging circuits aren't just applying the
    rated voltage. There is a lot more to it than that.

    Based on your recent posts here, I suggest finding a good book about DC
    electricity. Maybe taking a class or two. Or three. I just finished the
    third (DC, then AC, then Linear Circuits). There are a lot of things
    that are just intuitive with electronics, but a lot that isn't. Just
    guessing and trying will end up in a lot of magic smoke, and potentially
    loud bangs and fires.

    I'm a huge fan of HyperPhysics, though that isn't where I learned
    electronics from. It might be a good starting place though...

    <http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/cktetrcon.html#c1>
     
  7. P E Schoen

    P E Schoen Guest

    "Mr. Man-wai Chang" wrote in message
    You can probably use a surplus or used notebook supply of similar specs.
    Here's one with 19V and 2.65A for just $4:
    http://www.mpja.com/19-Volt-Desktop-Power-Supply-265A-EIAJ-Plug/productinfo/19078 PS/

    And if you need 4.74A and can spare $15:
    http://www.mpja.com/19V-474A-Desktop-Supply-Delta/productinfo/30336 PS/

    You can always add one or more diodes in series with the output to lower the
    voltage. And these units may also have built-in current limiters and battery
    charging profile circuits (although those would more likely be built into
    the laptop computer - at least that's how I would design it - I wouldn't
    trust an external device that could be swapped with something else).

    Paul
     
  8. No, because probably one side of each is already grounded inside the pwoer
    supply.
    NO, because these likely are fancy batteries and you don't just push
    current through them, it has to be controlled based on the state of the
    battery.

    Leave the batteries in the notebook, and charge it that way. The charger
    already came with the notebook, it also powers the notebook. If the
    charger/power supply is missing, then find one that will do the job. I
    see them endlessly no longer big because they are switching supplies. I
    pulled an XBOX 360 supply out of a pile of junk when the students moved
    out at the end of April. Then a few weeks ago, I found another one,
    except the actual XBOX 360 was with it.

    Michael
     
  9. The minus supplies, if they are even still there, have minimal current,
    were way in the beginning because RAM might have needed some negative
    voltage (I can't remember if those RAM were still in existence in 1981
    whtn the IBM PC came out, but they were lower density RAM), and of course,
    RS232 interfaces need -12. FOr much of the time, those neagative supplies
    have eitehr been underused or not used at all.

    Plus, the negative supplies will also have one side ground, just like the
    positive supplies. And even worse, -5 plus +12 is 7volts, the minus
    subtracts from the positive.


    Michael
     
  10. I'd forgotten about the dip switches on those.

    But how many were actually shipped with low density RAM? I thought it was
    the 4K RAM that needed negative voltage, and I thought by the time the
    IBM came out, it would use 64K RAM, which I thought did away with the need
    for negative voltage.

    Michael
     
  11. Jamie

    Jamie Guest

    No
    Jamie
     
  12. Guess this project is too dangerous for a beginner! Thank you all! :)

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  13. JW

    JW Guest

    IF it's possible, and I have a feeling that it's not, you'd better flip
    the polarity on the electrolytic caps as well!
     
  14. Jasen Betts

    Jasen Betts Guest

    neither, both -5 and -12 are typically capable of less than 1A (0.2A
    on a device at hand)

    -5 has been discontinued in new PCs

    If you need 17V and 4A you could try an aftermarket adjustable laptop
    power brick, It might be easier to find one at 18V or 16V though, but
    there are 17V ones out there.
     
  15. Which I find all the time in the garbage. Or that time I needed a power
    supply for a Powerbook 1400, I opened up an inkjet printer I'd dragged
    home from the garbage, and it had the right voltage (those tend to have
    higher voltages than +12, but still decent current). The external power
    supplies have the advantage that they are already cased. The inkjet
    supplied a standalone power supply board, making it easy to extract, but
    not casing.

    But, the problem is that applying voltage to a battery isn't a good thing,
    if the battery is some more recent kind, it needs a proper charger.

    Michael
     
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