Let's Make Robots!

Check the purple wire first (a.k.a. "so that's how my computer died")

So the power on my CNC machine was going to come from a computer power supply, and I had a recently decommisioned computer lying around.  Instructables and other sites have some good instructions on "ATX power supply to DC"  and "ATX power supply to lab power supply" (search strings).  After wiring the thin orange to the thick orange, and the thick green to black, and a red wire to a "sandbar" load resistor, I thought it should be working.  After all the changes, I still did not even have 5V coming out of my purple "standby power" wire. 
Eventually I remembered something about that computer and how it had come to be in pieces and used for parts:  one day about a year ago it just didn't turn on, and I thought it was because the power supply died.  I think I confirmed my speculation.

Lesson learned: On an ATX supply, test the purple wire first.


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Ha ha. Thanks for sharing. I love swaping stories of projects or ideas gone wrong, implementation problems, and just plain goofs, mistakes and disasters.

We can all learn from them.

My own (growing) collection of goofs is posted at Ignoble Idiocy, and I invite others to post their mistakes as well.

Good luck with the CNC machine!

With the problem diagnosed, I went to the TRW montly electronics swap meet and picked up a 300W and 400W supply for $15.  I tested the purple wire on each, and got 5V on each.  Now I can start modifying them.


Sounds like you are on your way. So will you wind up with a bench power supply with 3.3, 5, and 12 volts?

I've got an old ATX supply lying around, I think. I might need to dig it out. I've been using a variable PS I built from a kit, but it might be nice to have the common voltages available in a fixed voltage supply.

I plan to keep the 3.3V and 5V leads even though I'm not using them for the CNC.  The different voltages will be mounted to a terminal block like this one: http://www.radioshack.com/product/index.jsp?productId=2103229.  I'd like to keep all the drivers, the PSU, and the terminal block mounted in a metal computer case, but the CAT5 pairs that go to the stepper motors might be long enough to cause voltage drop or EMI (the stepper motors will have 12V current alternating on and off, and I don't want to make an accidental 2 watt transmitter).  A bench-top supply and drivers mounted on the machine would let me keep the unshielded stepper motor wires short and quiet compared to the box-mounted plan.

Whatever the configuration, a PSU lab supply won't look as nice as your coffee grinder supply :)