Turn a PSU into variable power supply

Discussion in 'The Projects Forum' started by XOIIO, May 1, 2013.

  1. XOIIO

    Thread Starter Member

    Mar 1, 2013
    I want to turn a PSU into a variable power supply, I need the schematics/parts list to do so.

    Ideally I want to drop the 450v or so from the capacitors to 100v (the max my digital voltmeters can read) and have that variable from 0-100 volts.

    If that is not possible, I'd like to have a 24v variable source (would taking two 12v lines in series work in a PSU?) from 0-24v.

    And, if that does not work, I want to make 12v variable from 0-12v.

    I am looking for simple schematics I can make on a breadboard (can't print circuit boards), and need to be fairly small, and a full parts list.

    I have an xl file here with all the parts I have salvaged from other PSUs, would any of those voltage regulators work?
  2. Meixner


    Sep 26, 2011
    What kind if "PSU" do you have ?
  3. XOIIO

    Thread Starter Member

    Mar 1, 2013
    just an ordinary desktop one. I've been making them into bench PSUs for quite some time but never variable ones.
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Gee Whizz.. where do you get 450 Volts across capacitors from?
  5. paulktreg

    Distinguished Member

    Jun 2, 2008
    Your XL file is empty!

    You're better off leaving the ATX power supply well alone and building from scratch.
  6. XOIIO

    Thread Starter Member

    Mar 1, 2013
    I've seen some with that much for the caps, but I opened the one I have and it only has 200v for the caps (probably just older ones have higher voltage) so that makes it easier.

    Eh, supposed to be xml, anywys dropbox messed it up I guess.

    And I'm not talking about schematics for the power supply itself, I'm talking about a variable voltage regulator circuit I can hook the 12v or whatever from the power supply into.

    Also I said digital voltmeter, not multimeter. I am using a digital voltmeter as a display for the voltage,search them on ebay and you'll see.

    Also in the many power supplies I have taken apart I have not seen one that uses the capacitors to recity the AC (120 in my area fyi), and even if they do, I don't think this one does because there are a couple diode bridges before the capacitors.
  7. Wendy


    Mar 24, 2008
  8. XOIIO

    Thread Starter Member

    Mar 1, 2013
    Ahh, that schematic is great, are you aware of a voltage regulator that can take up to 100 volts? (and one that is not variable that would limit 200v to 100).
  9. atferrari

    AAC Fanatic!

    Jan 6, 2004
    Are you trying to regulate 450V down to 12V?

    Sorry, it is not clear to me.
  10. XOIIO

    Thread Starter Member

    Mar 1, 2013
    No, a power supply has 12v, 5v and 3.3v outputs, but I'd like to have a variable one as well. The power supply which I have has 200v on the caps, I need to halve that and then use a variable voltage regulator so that I can have a variable output from 0-100v.
  11. ScottWang


    Aug 23, 2012
    If your current doesn't need that much, you can using +12V and -12V → LM317 → 21V (Adjustable), if you want to get some more voltages, then you need to adjust inside of PSU.

    If you need more current then you can using two atx power as +12V-0V in series with +12V-0V → LM338 → 21V (Adjustable), if you want to get some more voltages, then you need to adjust inside of PSU.

    other IC:
    LM1085 -- 3A
    LM1084 -- 5A

    ATX Based Bench Power Supply to LM317.

    Here is a variable power 0~330V.