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Beginner's Project: ATX Benchtop Power Supply

Former Member

Hi,

 

As one of my first real projects, I would like to build an benchtop power supply based on an ATX power supply. I'm not planning to modify the power supply itself, but I would like to add a circuit to it to allow a couple of things. I'm a programmer so maybe my instinct of going right away for a microcontroller is bad, but since I would like it to have only one output with a voltage selector (I was thinking of adding an LCD screen and using a clickable rotary switch to make it good looking) to cycle between the ATX's -12V, 3.3V, 5V, 12V and maybe add a voltage regulator for other voltages, it seemed easier to control transistors on each rails to control which one is active or not.

 

While this flow makes sense in my head (4 lines - -12V, 3.3V, 5V and 12V - merging into a single one with transistors acting as "gates" and I guess diodes for safety?), it somehow doesn't sound safe... is it?  I haven't seen a similar build on the internet... people tend to expose each rail invidually... but I think it would be a better learning experience for me to make it work with a single output rail.

 

Any feedback is greatly appreciated before I fry anything image

 

Thanks!

  • 0

    Nice idea.  Remember this.  There will be a voltage drop across each transistor and associated power dissipation.  Depending on how many amperes your circuit will demand; the transistor(s) will have to be rated for the wattage and may need to be attached to a heat sink (large sheet of metal or a thick piece with ribs). 

  • 0

    these psu`s can output several 10`s of amps, transistors aren`t a good idea for this without significant wastage. personally I would go with Relays and use the rotary switch to activate those, also you`ll need a break before make type.

  • 0

    Thanks guys!  This is really appreciated!  I'll work on some schematics and post them here to have feedback!

     

    Thanks again image

  • 0

    image

    Having just finished the first 5 chapters in Programmable Logic Controllers, 4th Ed., by  Frank Petruzella; the Q1 and R3 are looking a lot like the transistor version of the NC switch (normally closed) of a contact relay.  Transistor theory, as taught to me by my instructors (Harold Coombs and George Mershimer), informs me that +0.5 volts will not be enough to forward bias both n-p junctions and also that the lower p-n junction of Q1 is reverse biased and therefore shut down absolutely.  I had thought about using the 12 volt rail to  reverse bias the red led but the RGB LED data sheet indicated that it cannot stand up to the 7 volts of reverse bias.  Clearly, if that was to the only choice, one would need a voltage divider to set the bias on the base of Q1.  The other insight this circuit has given me is that if one wished to make a transistor version of a NO switch (normally open) then the use of a Q2 of type NPN would be the right choice.  Which might be useful just beneath R1 to hold the green LED in an OFF state until the switch was thrown and the rest of the supply rails came online.

  • 0

    So, I spent a few hours learning Eagle and reading some open source schematics as "tutorial" / inspiration...

     

    I want to build an enclosure for this thing, so most things are linked to terminal posts as I don't want to solder the wires.

     

    Build detail:

    - Power OK led

    - Switch to turn the power on

    - Power ON led

    - Control board with "clickable" rotary encoder + "back" button to navigate menus

    - LCD + adafruit LCD backpack to communicate with i2c (not enough pins remaining)

    - Buzzer

    - 5 rails controlled by relays (-12V, 3.3V, 5V and 12V for now, a LM350 should be added to have variable voltage)

    - Added a current mesuring circuit based on ACS712ELCTR-05B-T.ACS712ELCTR-05B-T. to be able to display current used

    - Based on ATMEGA328

     

    Does this schematic makes sense before I start working on the board layout?

     

    Thank you!

     

    image

  • 0 in reply to Former Member

    Nice idea,

    I have one question: where to find the ATX connector on library,
    I would like to do your projec,

    If you have finished the project, would like to share in a zip folder?

    I would have been very grateful

    Thx