Hacking an ATX PSU

Discussion in 'The Projects Forum' started by André Ferrato, Jan 18, 2016.

  1. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Hello, i decided to make a cheap bench supply to test a wide of range of breadboard circuits as i envolve myself far into the depths of electronics. So i went and dismantled the ATX PSU from an old computer that i had lying around in my house, and as it is 200W, it can provide sufficient power for any breadboard circuit and even some other stuff that i plan to do.

    It's not a very old PSU, but it has that -5v rail, so it may be a bit old.

    I came here to ask a few questions, because i know that a lot of guys here already did this kind of thing( Did an ATX bench PSU).

    1) I want to make a cheap variable power supply, so i decided to for the LM317 voltage regulator, as it is linear, it has some inherent voltage drop( datasheet says Vin must be at least 3v bigger than Vout). So if i connect to the 12v rail and try to vary it's voltage, i'll not get a lot.

    So i am thinking, connecting between 12v and -12v it's possible ? Because i'll give a lot more headroom for varying.

    Also LM350 it's a possiblity to extend current capability.

    2) Also to know what voltage am in, i could use voltmeter, with those panels, but i don't have one lying around and to buy one from ebay it takes 2 months to reach my house. I thought of making one with comparators, the only problem is that the resolution is going to be small, isnt it ? Extending the number of comparators and leds could make this really expensive. Any alternative ?

    3) I was taking a look at this thread: variable ATX power supply and i saw the guy trying to mod one of the transistors, really smart move, but i don't know if this is possible with mine. If i upload some pictures like the guy did, can someone locate that amazing schematic ? I don't know here to find!

    Thanks in advance.

    Also there is a thread of @Dodgydave : variable ATX psu that explains how to do this modding that i asked about, but the videos arent available. He stated 4.9v to 28v on the 12v rail and that's really awesome. And 2v to 10v on the 5v rail, if i could mod that, it would be perfect.
     
  2. ISB123

    Well-Known Member

    May 21, 2014
    1,239
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    Linear regulators have terrible power dissipation and are not very efficient. LM2576 is better choice.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    You need to find out which chip the atx uses, then i can show you how to modify it to variable by altering the feedback loop of the chip, the videos on youtube have been deleted by me.
     
  4. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    The LM2576 is kinda out of my reach for now, thats why i would like to use the lm317. I intend to do an much better lab bench power supply, using microcontroller, with built in scope, but i'll need to wait dor my components to arrive, so this one is just to compensate the time.

    Yes dodgy, i'll find the ic's and try to find what i'm dealing with.

    Also, any answers to the other questions?
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
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    The easiest one to modify is the Tl494, or Ka7500 chip, the more modern ones today have deadicated chips with built-in over-under voltage shut down, which can be overridden.

    Plenty of atx circuits here...

    http://danyk.cz/s_atx_en.html
     
  6. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Okay i got some pictures of the PSU:

    Optimized-IMG_20160119_105044198.jpg
    This is a top view picture.

    Optimized-IMG_20160119_105207893_HDR.jpg
    Bottom view

    Optimized-IMG_20160119_105559.jpg
    An four-legged IC

    Optimized-IMG_20160119_105256758 (1).jpg
    Some dust, any idea how to remove from hard places ?

    Optimized-IMG_20160119_105235572.jpg
    Should i remove these things ?


    Optimized-IMG_20160119_105125721_HDR.jpg
    The IC, it seems to be an AT2005B, but there some other writing on him.

    IMG_20160119_105505516.jpg IMG_20160119_105521870.jpg
    These two images are pretty big i know, it's just for you to see the name of the IC's, i intend to delete them soon.

    Also there are two 3 legged TO-220 ic's named "701 D304"(two of them) and "G705" or "C705"(The name is almost erased).

    Any chance this is the main ic ? AT 2005B
     
  7. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Need what?
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Yes its an AT2005,
    You need to fool the sense pins 3v,5v, 12v by putting preset resistors on them and setting the voltage, also the current sense PT needs to be Approx 0.7v.
    I used 10k presets to set the sense pins to 3v, 4v, the5v sense pin can be connected to the chips own 5v supply.

    If all is working the 12v rail will swing from 4v to 28v, if so all the output capacitors need to be uprated to 35v or higher.
     
    Last edited: Jan 19, 2016
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    DSC_0003.JPG

    Better picture..
     
  10. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Hmm, where do you get all this ? I'm wondering.

    It will swing from 4v to 28v with the load capability of the 12 volt rail ? (Mine says 16A)
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Yes mine is variable from 3.9v to 29.6v on the 12v rail, the 5v goes from 1.9 to 11.6v, and you maintain the current of the psu.

    You only need to replace the capacitors on the 12v 5v outputs, there are only four.
     
    Last edited: Jan 19, 2016
  12. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    I feel it will take a very good skill of reverse engineering to do this... As it has many components. I think for now, i'll stick swinging a part of the 12v rail to 1.2v-9v using the LM317 chip. When i have more time to invest, i'll definitely try this. And it i'll be a little costly for me if i need to exchange every output cap.

    Do you know a solution for that second question that i asked ? About the LED panel voltmeter?
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
  14. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    I know i know! But like i said, it takes 2 months for this item to arrive. As i want to complete this so i can keep doing my tests really fast, i'll for now stick to the LM317. BUT when this arrives i'll surely install.
     
  15. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    LM317 swinging from 1.25 to 11.5v, pretty good for a lame ass psu.
     
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