power supply help

Discussion in 'The Projects Forum' started by jimit, Nov 11, 2010.

  1. jimit

    Thread Starter Member

    Oct 28, 2010
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    i am willing to make a muiti-voltage power supply in which i can get an out put of 12v,9v,3v.
    but i cannot get the right circuit diagram for it can some one suggest me a good circuit :D
    thx in advance
     
  2. magnethead

    Member

    Nov 9, 2010
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    OR....you could do it the easy way, and steal an old power supply from a computer. I have mine set up for 1.8 volts, 3.3 volts, 5 volts, 7 volts, 8.7 volts, 12 volts, and 24 volts.
     
  3. jimit

    Thread Starter Member

    Oct 28, 2010
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    i know that and i also bought one but it didn't start.
    rather i would like to make it than spending money on an old power supply from a computer.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
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    Take a look at the LM317 chip. But be aware of the fact that you will need some sort of cooling fin, for the LM317 in your application.
     
  5. jimit

    Thread Starter Member

    Oct 28, 2010
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    i will take care of the heating factor of LM317 by using heat sink and a fan
    but how will i get all three different voltage with it 3v,6v,9v:D
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    We need to know you requirements for current.

    The three voltages can be supplied by either three LM317s, or by changing resistors that adjust a single LM317's output.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I hope you have some sort of current regulation... or your projects could go POOF!
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Are you really in need of specific output voltages or could a variable supply work?
     
  9. DigitalReaper

    Member

    Aug 7, 2010
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    When you say you tried a PC power supply and it didn't start, did you just plug it in or did you short the power on wire to ground? Without that short the supply remains in standby mode.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,145
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    My PSU also auto-protects itself if it sees a brief over-current (short) on its outputs. It won't restart until you cycle the power off and back on. It'll just appear dead.

    To the OP - power supplies pulled from old used computers are cheaper than free, as you can often be paid to take them (the computers) away. They make excellent workbench power supplies, even if you need to use a LM317 to provide a voltage they don't offer.
     
  11. jimit

    Thread Starter Member

    Oct 28, 2010
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    i tried this but it didn't start
     
  12. jimit

    Thread Starter Member

    Oct 28, 2010
    57
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    i would like to use three LM317s cause its hard to work with variable resistors but can you show me how it is dun
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    You need to do a couple of things in order to get a computer power supply to work.

    First, it needs to be an ATX or ATXplus12 supply. These have a 20-pin (ATX) or 24-pin (ATXplus12) motherboard connector.

    You need to use a power resistor or two from the +5v supply to ground; unless there is a couple of amps' load on the 5v supply, it won't regulate properly.

    Then there is a green wire which is the enable; you short it to ground to turn the supply on.

    Google "ATX Bench Supply" for lots of ideas. I have an old Compaq 250W ATX supply that I converted into a bench supply years ago; I outfitted it with banana jacks/binding posts and it's worked just great.

    However, if you really want to go the LM317 route, that can be done too. I'll post a schematic in a bit.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    See the attached schematic. The output voltages shouldl be a bit closer with the real components than they are with the simulation.

    Note that C11 thru C13 are optional, but help a great deal to prevent "overshoot" on start-up, and keep the output voltage more stable.

    C2 through C7 are mandatory.
    C8 through C10 are optional; they help a good deal with transient response.

    C1 can be larger; it will help with ripple reduction.
     
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  15. apqo1

    Well-Known Member

    Oct 5, 2008
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    Last edited: Nov 25, 2013
  16. jimit

    Thread Starter Member

    Oct 28, 2010
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    i tried it what SgtWookie said me to do but the problem is that when i connect it to the main power and switch it on the fan runs for a second and then runs off it does not start at all what now
    i forgot to say that i connected a bulb instead of the power resistor some 6v bulb
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    What do you mean by "the main power" - I hope you didn't try plugging it in to a wall outlet? :eek: As shown, it's to be connected to a 12v DC supply. It needs something like a "wall wart" plug-in supply that has a transformer and rectifier.

    What voltage and current is the fan rated for?
    What voltage and current is the bulb rated for?

    Look, this is a low-power supply. Linear regulators dissipate a lot of power as heat. You need a way to get rid of the heat, normally a large heat sink for each regulator.

    Note that the tab on the regulator is "live"; it's connected to the OUT pin. If you short it to ground or the input, you're going to have big problems.
     
  18. themindflayer

    Member

    Oct 29, 2010
    44
    1
    You will need 1 IC per voltage level ..... if you want accurate o/p voltages .... otherwise a series zener could be a good bet assuming not much power being drawn by the PSU ....

    you can find the circuit(s) in the datasheet of LM317 .... you also have fixed voltage ics like 7809 in your arsenal ....
     
  19. jimit

    Thread Starter Member

    Oct 28, 2010
    57
    0
    man i am not talking about that one i am talking about the pc power supply hahahahaha:D
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    dOH! (in my best Homer Simpson voice) ;)

    Was the supply working when it was removed from a computer?

    Did it have a 20-pin or a 24-pin motherboard connector?

    Some ATXplus12 supplies might require a load on the +12v supply along with a load on the +5v supply; with no load it may go higher than expected, causing the supply to shut down due to an over-voltage condition.
     
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