Sunday, September 9, 2012

ATX-Based Benchtop Power Supply (2010)

In 2010 I got tired of continuously having to wire up breadboard power supplies.  I found I often wanted to just prototype something quickly with a few parts I had lying around, but I frequently gave up before starting because I was too lazy to fish out and wire up yet another LM317 or something.  Thus, as is the fashion when nothing too serious is required for a benchtop power supply, I put together a small ATX power supply breakout box with an adjustable voltage regulator.

I began by planning out a simple circuit.  I put a voltmeter on the adjustable voltage branch as well as an ammeter on the common ground:

Circuit for the ATX power supply breakout box.
I laid out the faceplate elements on cardboard to make sure things fit as intended:

Mocking up stuff on leftover cardboard.
Cutting the cardboard to the correct dimensions.
Final mockup.
Then, I carefully transferred the markings to some acrylic sheets I had scored and snapped to size:

Transferring the layout markings from graph paper to acrylic faceplate.
Making sure things line up.
Then, I drilled all the required holes and openings on a drill press borrowed from my dad.  I believe I used a sheet metal nibbler to square out the edges of the doorway-shaped holes, for the meters.

The cutouts.  (This picture was actually taken after I removed the protective film, later on.)
Cutouts.
Then, I test-fit everything to make sure things would line up properly, that there was enough space, etc.:

Testing for fit.
Will it close properly?
Faceplate will look OK it seems.
Since the test fit looked promising, I went ahead, peeled the protective film from the acrylic and assembled everything:

Wiring up the faceplate.
More faceplate action.
Adding the actual guts of the breakout box.
And it turned out great:

Looks good.
From up high.
From the side
Underside.
Off.
ATX supply on in standby, not delivering power yet.  (That's the red LED.)
Final result, delivering power as indicated by the green LED.  In case of panic just slam the safety cover shut.
In the end, everything works great except the ammeter.  That was a dumb mistake; I didn't test it in-circuit before putting everything in place, and I didn't realise that it required special wiring.  (I never looked into it but I suspect it actually acts as a voltmeter, and you have to wire it up across a specified value of resistor for it to work properly.)

After making use of this thing for a couple of years, I might say a couple of things about the design:
  • Though I didn't think the zipties around the back would be sufficient to hold the meters in place, they are still there in 2012 and the meters haven't moved.
  • On the other hand, it was hard to find an inspired way to hold the molex breakout in place; it sucked on day one and it still does today.  (As you might be able to tell from the pictures I built a sort of acrylic clamp.  Won't do that again, the connector keeps slipping out, the acrylic bends, etc.)
  • Instead of using three nuts per screw, I now use standoffs I cut myself from 1/4" plastic water piping.
  • The safety cover on the switch came in handy many times.  Because it's spring-loaded, it's much easier to slam the thing shut than to flick a pretty rigid toggle switch.
  • Acrylic is too fragile a material for something that will be manhandled like this; though it hasn't happened yet, I'm always afraid I'm going to break the thing.


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