Need Help!- Hacked ATX PSU Tripping off with motor load

[RotarySMP]

Hi Guys,

I am making a Proxxon Collet spindle, and bought a Johnson 12V motor to power it.
http://www.cnczone.com/forums/showthread.php?t=103610

I hacked a 300W ATX PSU to power it, and it runs fine on 3.3V and 5V, but when I switch on the PSU with the motor connected to 12V, the motor twitches (hard, with a spark at the brushes) and then the PSU trips off. If I turn of the PSU, it resets after about 2sec (motor twitches slightly) and then I can start again.

My guess is the motor windings are a short to this PSU (rated to 16A on +12V), so the short circuit protection trips.

How do I run this motor on this PSU? I am guessing some sort of inrush limiting resister until the motor is running an the back EMF limits peak current (at least that is how I soft started my big toroidal transformer for the mill).

[Al_The_Man]

Sure, with no controller and the motor connected directly to the supply the current is whatever the resistance of the armature is at the time of switch on.
This high current is probably exceeding the 12v rating and the PS shuts down, the 5v supply is usually has the max. power rating for a PC supply.
If this is a spindle do you not intend to vary the speed? or just run it full tilt.
If you soft start it, what about when you put it under load?
Al.

[RotarySMP]

At this stage I was going to vary the speed between 3.3V, 5V and 12V

I guess a motor speed controller is the solution.

The soft start I did on the toroid uses a relay to short the resister back out of the circuit.

[Jesse B]

I would really implement a speed control of some sort, personally.