250W Lab Power Supply
If you need a power supply that can deliver high currents, consider converting one from an old PC. In my case I needed to power an electronics project I'm working on, and i didn't like the idea of shelling out the kind of money a PSU with similar capabilities costs in the shops. To give a couple of example of possible uses; power source for an 12V car amp, cheap alternative for replacing your broken WLAN access point, flatbed scanner, printer, LCD monitor, or whatever -transformer. Or as in my case: a lab power supply.
Disclaimer
The conversion process is really simple as there's only a matter of wiring and re-wiring. However, here's a disclaimer for all you US-citizens reading this: use caution if you decide to try this out yourself, there's dangerous voltages inside and the current delivered from the PSU can - and will - start a fire if you're not careful. There's a potential risk of loss of life, should you try this yourself - if you don't know what you're doing; don't ! Also, I haven't checked this text for errors, so the risk is entirely your own if you follow these instructions. Now then...
Parts needed
I got hold of an old ATX PSU - remember to make sure it's working first. I managed to get everything else I needed from Clas Ohlsson (scandinavian hobbyist store chain) . It included a on/off switch, LED:s, binding post, heat shrink tubing, crimps, resistors.
Disassemby
I ripped the PSU open, disassembled everything. I carefully drained the electrolytic capacitors with a resistor to avoid some shocking experiences later on. I cut off the wires to the fan and the mains. I then unscrewed the circuit board and removed it from the box. The reason for this was to avoid damaging the PCB when drilling the holes later on.
PCB has been removed from the case
On the subject ATX
Here's the pinout of the ATX-connector that is connected to the mother board:
My particular PSU has the following specs:
Panel
I measured the free space inside the box to see where i could put the connectors, switch and LED. I already decided that I would make a custom panel for it later on so I started out by making a drill stencil to get accurate placings of the drill holes:
Trying out the different possibilities
The jagged line beneath the terminals shows which area is free to use and which is accupied by electronics on the circuit board.
Drill stencil (click to enlarge)
I may aswell say it up front: I'm a sucka' for designing panels for electronics casing. I may not be good at it, but I do enjoy it a lot :) below is the panel, the white area will be cut out to allow for air circulation through the grid of the case. It was printed with an inkjet printer on A4-paper that had an adhesive backside (basically a big sticker). Then I glued on a a protective layer of the kind of plastic you can put on books to protect their covers. The excess material was cut away (I left about 10mm extra above,below and on the sides so that I could fold that over on the case's back side).
Panel artwork background (click to enlarge)
Panel artwork complete with background and labels (click to enlarge)
Drilling stencil superimposed on panel artwork (click to enlarge)
You can download the Corel Draw 12 file containing the panel artwork here.
Drilling
The stencil was taped on, and the holes drilled. Stencil removed and disposed of.
Paint & Panel Job
I spay painted the part of the front panel that would be visible (the square in middle with the grid). I left the rest of the unit unpainted since it has a tendency of scratching off over time, and I also wanted to keep the right side unpainted as it contained the spec:s for the PSU.
Full s pecs of the PSU
When the paint had dried I taped on the panel and cut out the drill holes in it.
Rewiring
I cut all the connectors off. Good riddance.
Connectors has been cut off
Then I bundled all wires of same color and crimped them together. I left one black and one red uncrimped for future use. There's a lot of black red and yellow wires there, so I crimped them 4 and 4 together, or 3 and 3. Also remember to crip the brown "power good" -wire together with the orange ones. The purple +5VSB was not used.
Wires has been crimped
LED
I connected a LED in series with a resistor between +5V and ground so that the LED would light up when the PSU is active. Approximate resistance to use is 300 Ohms. Calculate it using Ohm's law : R=U/I, ehere R is resistance sought, U is the voltage drop over the resistor and I is the desired current. I used a green LED which has a voltage drop of about 2.1 V and needs a current in the neighborhood of 10mA, so R=(5V- 2.1V) /10mA, R=290 Ohms. Depending on your brightness preferences, you can use another current in your calculation. With another type of LED you need to adjust the voltage drop in the formula also. So there. I shorthened the connectors of the LED to about 10mm, soldered the resistor directly to on of the LED:s connectors, soldered the wires to it. Finally I put 20mm heat shrink tubing over the exposed areas of the LED connectors and resistor..
On/off switch
My PSU had an on/off switch on the back (connecting/disconnecting mains power). However I decided to put a switch on the panel too - this is easy as it was an ATX PSU. They have a signal line which you can use to activate/deactivate the unit. Caution: The panel switch doesn't disconnect the mains power from the PSU , it only activates/deactivates the PSU. I connected the switch between the green wire and ground.
Binding posts
I screwed the binding posts in place and bolted down the connectors of the crimps to the binding posts.
Bolted down
Reassembly
I put the circuit board back into the box. I also soldered back the mains and fan wires.
Don't force it, use a bigger hammer!
It was as challenging as ever trying to fit everything inside snugly, but eventually I got everything inside and slapped the lid back on!
Wires has been tucked into place
Testing it
I checked for any short circuits both with my eyes an my multimeter, it looked good. I decided to go for it and power the PSU up. It worked. What a monumental waste of time it would have been otherwise.
Running on mains power for the first time
Ta-daa!
Thanks for reading!
Peter Sandström
27.2.2008
