.-----------------.
              |\                 \
              | \                 \
              || \  /\ /\ /\ /\ /\ \              The
              ||| \_________________\
              ' |||                 |         SCAVENGER'S
               \ ||                 |
                \ |       ezod '06  |     Desktop Power Supply
                 \|_________________|


            .----------------------------------------------------.
            |               Windsor 2600 Meeting                 |
            | First Friday of the Month, CAW Student Centre, 7PM |
            '----------------------------------------------------'


[ DESCRIPTION ]

The scavenger's desktop power supply may well be the world's most cost
effective way to generate commonly used voltages at relatively high currents.
This file details the easy construction of an excellent switch-mode power
supply from an old ATX PC power supply and a few bucks worth of common parts.


[ MATERIALS ]

First, and most importantly, you need an ATX power supply from a PC. I leave it
to you to secure one. Any ATX supply will do. The example used for this file
is a Power Tronic PK-6145DT 145W unit stripped from a Pentium-era Gateway box.

Also, you WILL need the following:

    - SPST switch
    - 5 binding posts

And you MAY need some or all of the following:

    - 10 ohm 10 watt sandbar (coil) resistor
    - 1 (additional) binding post
    - standard 20mA LED
    - 150 ohm 1/4 watt resistor
    - heatsink compound
    - small heatsink or heatsink/fan combo


[ CONSTRUCTION ]

An ATX power supply's motherboard connector has the following pinout (older
supplies may not have the orange +3.3VDC line):

                  +12VDC  YEL  ---  (+][+]  ---  RED  +5VDC
                   +5VSB  PUR  ---  [+](+]  ---  RED  +5VDC
                  PWR_OK  GRY  ---  [+](+]  ---  WHT  -5VDC
                     COM  BLK  ---  (+][+]  ---  BLK  COM
                   +5VDC  RED  ---  (+][+]| ---  BLK  COM
                     COM  BLK  ---  [+](+]| ---  BLK  COM
                   +5VDC  RED  ---  [+](+]  ---  GRN  PS_ON
                     COM  BLK  ---  (+][+]  ---  BLK  COM
                 +3.3VDC  ORG  ---  (+][+]  ---  BLU  -12VDC
                 +3.3VDC  ORG  ---  [+](+]  ---  ORG  +3.3VDC

As you can see by opening the supply, all the lines of the same color from the
motherboard connector and the rest of the connectors run back to the same
large pad on the circuit board. Cut all of the connectors off, and desolder
all the extra external wires leaving one of each color, except red and black,
of which you'll need 2 and 5 respectively.


                              - Binding Posts -

Drill some holes for 5 or 6 binding posts (depending on whether you have/want
the +3.3VDC line) in the top of the power supply box. Solder each to COM,
+5VDC, -5VDC, +12VDC, -12VDC and (optionally) +3.3VDC. It may be a good idea
to label them on top of the box somehow.


                               - Power Switch -

Attach an SPST switch across PS_ON and COM. The power supply will run as long
as it is closed. See the Standby Power section for a tip on how to make this a
momentary contact button.


                            - Power OK Indicator -

For a neat power indicator that also tells you the power is clean, mount a
standard 20mA LED and power it, in series with a 150 ohm resistor, across
PWR_OK and COM. This is optional; if you don't need it, desolder the PWR_OK
line and one COM line from the board.


                              - Internal Load -

Since this is a switching power supply, it requires a load to operate
properly. Some older supplies will require you to add the 10 ohm, 10 watt
sandbar resistor across the +5VDC and COM lines. If you do need this, it is a
good idea to heatsink the sandbar somehow; any way that keeps the heat down
will do. If you find your supply doesn't require you to add the internal load,
desolder the second +5VDC line and one COM line from the board.


                              - Standby Power -

The +5VSB line is available as long as the power supply is plugged in, but it
can only source very limited current. It could be used, for example, to gate a
transistor for the PS_ON line, so that the power switch can be a momentary
contact button like on a PC case. If you don't need this, desolder the +5VSB
line and one COM line from the board.


[ USAGE ]

The scavenger's desktop power supply can be used for most circuits that don't
mind being on a switching power supply. The +5VDC and +12VDC lines will
generally have plenty of current; the example supply discussed above can do up
to 18A on +5VDC and 6A on +12VDC (so long as the total doesn't exceed 142W).
The -5VDC and -12VDC, however, have much lower currents available; on the
example supply they have only 0.3A each. Test the outputs with a multimeter
before using them on anything important. Keep an eye on the heat for the first
few uses, especially if your supply needed the internal load.

Picture of Example Supply:
  http://www.mavrinac.com/files/textfiles/pcpowersupply.jpg

If all goes well, happy hacking!


::: mavrinac.com