Perfect Power Supply

Discussion in 'General Electronics Chat' started by KrakenFan, Dec 1, 2008.

  1. KrakenFan

    Thread Starter New Member

    Dec 1, 2008
    5
    0
    I'm looking for a power supply circuit to build that has a dedicated 3,5,9 & 12 volt power supply each with their own binding posts out front and a 0-24v variable on another set. I would also tap the 12v to add a car lighter socket to the front for working on car powered items. I would like a volt meter (easy enough ) and Amps too. I suppose it could have a variable current limiter as well but I don't exactly get that as yet as I'm new to electronics. Is this too much to ask of a design? Does anyone know of such a beast? Preferably that doesn't include a ton of transistors? I came close here
    http://members.shaw.ca/roma/supply-4.html but I got lost when it gets to the switching to select the voltage. I guess I understand the 5 position switch but worry about finding one around here. Is there somthing eaiser for a first timer?

    Thanks for any help herding me in the right direction,

    KrakenFan
     
  2. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Well this is a very good first major project to work on for something starting out in electronics. After of course buying a good digital multimeter (they are cheap these days).

    Now on to your project:

    It would be better to break down the project into subsystems. For a bench DC power supply you really don't want to have to switch between the different fixed and on variable voltage. What if you required two voltages at the same time say +12 and +5? Just have different power output connectors for each voltage source. So you really only have to construct 4 fixed voltage power supplies and one variable supply. One important specification that you haven't provided is what is the maximum current you want from each fixed and the variable voltages to be able to supply? Current costs money, voltage is cheap, so this is the most important spec you will have to settle on.

    I would suggest as your just starting out that you consider buying power supply module kits. Then you can concentrate on the building and assembling it into your mega-supply. The kits have schematic so you will still learn how they work but you won't have to worry about calculating and selecting each and every component. Then you can just work on how you want to mount everything, power switches, AC fuse, etc. to complete it. You will have to figure out where to obtain one or more transformers to feed the voltage regulators, but each kit will tell their requirement.

    Here is one firm that has several kit modules, both fixed and variable:
    http://www.alltronics.com/cgi-bin/category/KP

    For display voltages, small digital displays aren't too expensive if you shop around. You really don't need one attached to each of the fixed voltages, just for the one variable supply. You always have your Digital multi-meter to check if you think one of the fixed supplies is OK or not.

    http://www.alltronics.com/cgi-bin/item/PMLCD/S/3-1-2-Digit-LCD-Panel-Meter


    So start simple and learn as you go.

    good luck
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A good 1st project is converting an ATX form factor computer power supply to a bench supply. There are LOTS of how-to's out there for the conversion, and it's not very hard to do.
    You wind up with: +3.3v, +5V, +12V, -5V, -12V; the positive voltages you'll have 8A or more available.
    You can scrounge an old ATX supply from a dead computer, and the whole project will cost you less than $20; far less if you shop around for binding posts.
    This Google search:
    http://www.google.com/search?hl=en&q=ATX+bench+supply&aq=f&oq=
    will get you LOTS of instructional pages on how to do the conversion.

    Radio Shack carries 6- and 12-position rotary switches if you really want to go that route.

    But, try the ATX conversion first. It's easier and cheaper.

    Oh, in many of the ATX bench conversions, they'll tell you to cut off most of the black wires near the PCB. Don't do that. Have a separate ground binding post for each voltage; the negative voltages only need 1 ground wire each. The +3.3, +5 and +12 should have at least 2 wires each for the ground binding posts.
     
  4. Metalfan1185

    Active Member

    Sep 12, 2008
    146
    0

    I have done this, this conversion make an excellet power supply, if you find an atx with a switch on it, it's perfect.


    Due to precise Voltages on RAM and Processor chips, ATX Power supplies offer very stiff regulation, Short circuit protection, and decent current outputs. They never really get too hot either. I stongly suggest you build one of these.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Don't worry about finding one with a switch. The ATX standard supplies have a green wire which will turn on the supply when grounded.

    There is a purple wire that's 5v standby, useful to use with an LED to indicate that the power is connected to the supply.
     
  6. Metalfan1185

    Active Member

    Sep 12, 2008
    146
    0
    That's true, mine happened to have a switch on the back, so i connected the green wire to ground and used the switch from the mains to turn it on and off. Nice to have, not required though. Im thinking of converting another one, but i need the resistor (you need to put a small load on the PSU so it has a current reference for proper regulation) you'll learn more about that if you convert one.

    Good Luck!
     
  7. KrakenFan

    Thread Starter New Member

    Dec 1, 2008
    5
    0
    Actually, as with most things forum related. After posting I spent more ( a lot more ) time reading over the instructions on the link I gave and it's starting to make much more sense to me. The only part I'm a bit dazed aout is on the enlarged layout picture at the top right is a pad labeled "To Regulator". Would that go to the regualtor at the bottom of the diagram for the one push button digital switching diagram? Thanks for the help and the ideas on the ATX design. This is a great forum. I can see learnign a lot reading through these posts here!

    KrakenFan
     
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