Computer power supply

Discussion in 'General Electronics Chat' started by boatsman, May 11, 2015.

  1. boatsman

    Thread Starter Senior Member

    Jan 17, 2008
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    Is it possible to modify an old computer power supply to operate a 12 v impact drill?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    you can make it variable,what voltage and current do you need?
     
  3. Reloadron

    Active Member

    Jan 15, 2015
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    Depends on what the PSU can supply for current and how much current the impact drill needs for operation. Overall I don't see why it would not work. See Dodgydave's post, I assume 12 volt drill but still need to know the current. Even most older ATX form factor PSUs will deliver about 18 - 20 Amps.

    Ron
     
  4. upand_at_them

    Active Member

    May 15, 2010
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    It's very likely that the drill requires more current than the power supply can provide.
     
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I doubt it. It is likely that the electronic overcurrent protection will not allow the drill to start and if it does, it may cut out on high loads.

    A simple linear supply would be ok as it could deliver the high peak currents necessary for driving a motor.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    older computer psu's for pc's did not have any current limitting on the 12 volt output, and even the regulation, if any was done through the 5 volt stages. up till the atx, the 12 volt output wasnt very much current.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Elektor did a project for that a few years ago, but that was to supply 12V halogen lighting.

    The project wasn't trivial - it involved winding a custom replacement for the existing transformer.

    By far, the biggest power draw is shared between the +5V and +3.3V rails - and their windings take up a lot of the space on the transformer.

    You can probably get a couple or few amps from the +12V output, but you have to load both the 3.3 and 5 outputs to ramp up the pulse width in the SMPSU controller.

    Since what you have in mind involves a trailing lead anyway, you might find it easier to rotate a couple of motorcycle batteries between charger and drill.

    Flooded cell motorcycle batteries aren't that expensive, but they spill if you knock them over - the latest thing is SLA or AGM motorcycle batteries that can't spill, but those types aren't cheap.

    The ideal set up would be at least one "intelligent" charger/conditioner, like the Optimate for one example, they can be left float charging for long periods. If you work on site - a very rough & ready fast charger (the "intelligent" ones take too long) may be needed to finish a big job.
     
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I just bought new 300 watts AT (not ATX) power supply, label says 10 A maximum on +12 volt rail.
     
  9. Reloadron

    Active Member

    Jan 15, 2015
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    Yeah, and I am willing to bet the Maximum on the 5 V rail is up around 30 A. Those power supplies are designed for greater 5 V power just as their early brethren were. They have their uses. :)

    Ron
     
  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    You nailed it. Label says +5 V rail is 30 A maximum.
     
  11. ian field

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    Oct 27, 2012
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    When I had access to the skip outside an end user disposals agent, I won a couple of power boards - all the 12V rails came from a 20VA transformer, each unit had a compact 5V switcher module rated for 180A output.

    One PSU I rescued is 12V only and about 1/4 the dize of a typical PC tower - can't remember the current rating, but it isn't small.
     
  12. ian field

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    I remember having seen +5V rails on PC boxes somewhat higher than 30A on relatively low rated units.
     
  13. Reloadron

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    Jan 15, 2015
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    Oh yeah, much higher. My understanding here is years ago as the home computer spawned, with its business counterparts and machine counterparts it was still pretty much a TTL world. The big power demand was on the 5 volt supply. As things evolved all of that changed to where the demand is now on the 12 volt supply. I remember getting into VXI panels with huge 5 volt bus rails, I mean like 50 or 70 amp copper bus bars in there. Today it's a different ball game.

    Ron
     
  14. ian field

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    Oct 27, 2012
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    When a local trader kept bringing PC PSUs for repair that had nothing wrong with them, it became a total pain checking them over for safety etc, and then finding there was no job in it for me. So I went to see him and find out what was going on. Turns out he'd just grab any HDD that happened to be laying about and use it as a test load. Very occasionally an old HDD drew enough at 5V to get some PWM going and there was (just) enough on the 12V to get the motor spinning. Sometimes the PSU would fire up without complaint, but there was still a steady stream of falsly condemned units.

    Eventually I managed to persuade him to get a proper test load unit, it also had LCD readout for the voltage rails.

    With all the fancy new logic families and chip sets, I get to wondering how they still get the current draw on 5V to get the PWM chip moving. Increasingly, people put two or more HDDs in, then there's DVD drives and various fans etc.
     
  15. Reloadron

    Active Member

    Jan 15, 2015
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    So his test methods left a little to be desired? :) That was funny Ian.

    This is getting off topic but.... years ago I was involved with a website that tested computer hardware. The focus for me was PSUs. Myself and another built a test load bank. The only merit to testing all this junk (and seriously most of it was junk) was after testing and evaluating we got to keep the hardware. There was so much hype and advertising surrounding the stuff it was unbelievable.

    Ron
     
  16. ian field

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    Oct 27, 2012
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    An article about tech support calls that was published in Silicon Chip a number of years back is pretty hilarious:-
     
  17. ian field

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    My PC PSU test load was basically cobbled together with a strip of copper clad and an assortment of car bulbs.

    Everything is open, so I can get at it with the DMM test prods, and scope it for ripple if needs be.
     
  18. boatsman

    Thread Starter Senior Member

    Jan 17, 2008
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    Thanks to all you who replied. I don't know how many amps the drill requires but there are ten slightly smaller than 'C' cells in the power unit. I would expect the power drawn by the drill would be at least 10 amps.
     
  19. ian field

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    Oct 27, 2012
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    It could be that much if the cells are Ni-Cd - Ni-Mh cells have a higher internal resistance and just can't equal Ni-Cd.

    Not too sure about lithium though, they seem able to shift a fair bit of current. If they match a standard size, you can probably find typical spec on the website of a manufacturer.

    Funny story about the difference between Ni-Cd and Ni-Mh:- When the battery gave up in my rechargeable shaver, I set about installing some brand new Ni-Mh cells. The original; 2x AA Ni-Cd rated 500mAh, were replaced with 2x AA Ni-Mh rated 2400mAh - the running time turned out to be only half as long as the originals did when new - the mystery was solved when I noticed how hot the cells got, all that extra energy had to be going somewhere!
     
    Last edited: May 13, 2015
  20. Reloadron

    Active Member

    Jan 15, 2015
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    Well if you have a PSU I guess nothing to lose by trying. :) Would be nice if you had a large cap to put across the PSU. Hopefully a PSU capable of 20 amps 12 volt. There are a few considerations. Have you ever used an ATX form factor PSU outside of a computer?

    Ron
     
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