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Getting 24 volts out of an ATX power supply. — Parallax Forums

Getting 24 volts out of an ATX power supply.

Martin_HMartin_H Posts: 4,051
edited 2012-12-06 08:43 in General Discussion
I have motors that require 24 volts. Until I get batteries that output 24 volts, I'm thinking of using the motors in a tethered mode to prove out some ideas. I have an idea about how to supply them with 24 volts, but I'd like to ask if it would work before I risk smoking something. I have an old ATX power supply which have +12 and -12 volt rails. Can I use the -12 volt rail as a virtual ground and use the 24 volt potential between them to power the motors?

Comments

  • stamptrolstamptrol Posts: 1,731
    edited 2012-12-05 11:04
    Technically, you're Ok. But, the limiting factor will be the current capability of the power supply. Many times the -12 supply can only supply a small fraction of the current that the +12v side can supply.

    Don't forget you could also put two identical 12 volt supplys in series to get the 24 volts.

    Cheers,
  • Martin_HMartin_H Posts: 4,051
    edited 2012-12-05 11:40
    Thanks, the current issues is definitely one of the potential smoke points for the power supply. I hadn't through of two 12 volt power supplies. It turns out I do have two spares and could do that.
  • SRLMSRLM Posts: 5,045
    edited 2012-12-05 13:51
    stamptrol wrote: »
    Don't forget you could also put two identical 12 volt supplys in series to get the 24 volts.

    Just out of curiosity: do the ATX power supplies share a common ground? That would invalidate the simple series hooking of the two supplies, since it would create a short. This post seems to indicate that is the case: http://electronics.stackexchange.com/questions/7657/24v-3a-from-atx-power-supply
  • Martin_HMartin_H Posts: 4,051
    edited 2012-12-05 14:31
    SRLM wrote: »
    Just out of curiosity: do the ATX power supplies share a common ground? That would invalidate the simple series hooking of the two supplies, since it would create a short. This post seems to indicate that is the case: http://electronics.stackexchange.com/questions/7657/24v-3a-from-atx-power-supply

    I just read that thread and I would tend to think that the AC sides share a common ground, but the DC sides would not. So they could be put in series. One person responded that they tried it and it worked.

    I also found out that the -12 volt rail is generally rated at a quarter amp, so it wouldn't work for motors.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-12-05 14:35
    It depends upon whether logic ground is connected internally in the PS to earth ground. If it is, I suppose you could cut the ground prongs off of the AC cords and, for safety, plug both into a GFI outlet.

    -Phil
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-12-05 14:57
    I thought I read once that you can not necessarily pull current from a positive voltage to send to a negative one on an ATX supply? IIRC (and I don't think I am) it was in a SparkFun comment. I thought the negative voltages were supposed to use ground as the higher voltage source. Again, this is very much third hand knowledge.
  • Martin_HMartin_H Posts: 4,051
    edited 2012-12-05 15:14
    Phil Pilgrim (PhiPi) wrote: »
    It depends upon whether logic ground is connected internally in the PS to earth ground. If it is, I suppose you could cut the ground prongs off of the AC cords and, for safety, plug both into a GFI outlet.

    Good point. I think I could test this with a continuity tester? If the DC ground wasn't connected then this couldn't be the case?
  • idbruceidbruce Posts: 6,197
    edited 2012-12-05 21:26
    Martin_H
    It turns out I do have two spares and could do that.

    What do you mean spares? Spare 12V wires do not mean seperate rails. Most supplies now come with two or more 12V rails, but you would have to ensure that these are on seperate rails to get 24V.

    Bruce
  • idbruceidbruce Posts: 6,197
    edited 2012-12-05 21:28
    Oh spare PSs... I see :)
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-12-05 21:37
    What about a couple of 12V wall warts? I would think that would be a much easier setup.
  • idbruceidbruce Posts: 6,197
    edited 2012-12-05 21:42
    With a couple wall warts, you are probably looking at 4A max. I would suggest making a cheap linear power supply.

    Transformers - http://www.mpja.com/Transformers/products/45/

    Bridges - http://www.mpja.com/Bridges/products/111/

    And either get your capacitors at the same place or RS

    EDIT: And 2A wall warts ain't cheap
  • idbruceidbruce Posts: 6,197
    edited 2012-12-05 22:05
    @Martin_H

    I don't know how much amperage you need, but if you might consider making your own linear power supply, and don't need more than 2A per motor, then these two items in addition to a capacitor should get you some power close to 24V

    http://www.mpja.com/18V-2A-Center-Tapped-9-0-9-Transformer/productinfo/7843+TR/

    http://www.mpja.com/6A-50PIV-Square-Bridge-Rectifier-Wire-Leads/productinfo/5241+BR/

    V (Peak) D.C. = 1.41 X Sec. V A.C.
    V (Avg) D.C. = 0.90 X Sec. V A.C.
    I D.C. = 0.62 X Sec. I A.C.

    I have attached some information for your consideration.
    pdf
    pdf
    Design Guide for Rectifier Use.pdf
    259K
  • idbruceidbruce Posts: 6,197
    edited 2012-12-05 22:10
    You could also just look at this page of 24V supplies

    http://www.mpja.com/24-Volt-Power-Supply/products/541/
  • CuriousOneCuriousOne Posts: 931
    edited 2012-12-06 00:42
    In most cases, 12V output coil on transformer in ATX power supply is wound with two(four, six) etc paralel wires. You can unparalel these wires, and use the freed coil with added diode and capacitor to make stronger -12V output, which, with default +12v output will give 24V output.
  • ercoerco Posts: 20,259
    edited 2012-12-06 06:43
    1A stall current for Martin's motors. Ask Santa for two 12V 7AH SLAs. Great for traction. :)
  • idbruceidbruce Posts: 6,197
    edited 2012-12-06 07:28
    @Erco
    1A stall current for Martin's motors. Ask Santa for two 12V 7AH SLAs. Great for traction.

    Now you know as well as I do that Christmas is a long way off for those that need to experiment and tinker NOW :)

    In the meantime, he could also just ask a couple of friends to borrow one of thier extra automotive batteries. Two of them and he would be good to go until Santa delivers the goods.
  • Martin_HMartin_H Posts: 4,051
    edited 2012-12-06 08:43
    erco wrote: »
    1A stall current for Martin's motors. Ask Santa for two 12V 7AH SLAs. Great for traction. :)

    Those will probably come, but I want to build a proof of concept on the cheap before I invest the money for the SLA's
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