 For my sherline CNC project I needed a high current power supply capable of powering the logic on the stepper motor drive boards, and deliver high currents to the stepper motors. Suitable power supplies was priced at several hundred dollars. But I had an old ATX computer power supply in my basement and decided to convert this to a suitable power supply for my CNC project.
My ATX power supply is rated at 12V-19A, 5V-25A and 3,3V-18A. In addition it provides -12V and -5V. ATX power supplies comes in two variants, one with 20 pin connector and one (ATX v2.2) with a 24 pin connector. The ATX power supply connector pinout is found in the ATX specification available at www.formfactors.org. An easier description is found at the excellent electronics information page The Hardware Book, from where the below tables can be found. ATX 20 pin connector
| ATX 24 pin connector | | Pin | Name |
| Color | Description |
|---|
| 1 | 3.3V | | Orange | +3.3 VDC | | 2 | 3.3V | | Orange | +3.3 VDC | | 3 | COM | | Black | Ground | | 4 | 5V | | Red | +5 VDC | | 5 | COM | | Black | Ground | | 6 | 5V | | Red | +5 VDC | | 7 | COM | | Black | Ground | | 8 | PWR_OK | | Gray | Power Ok (+5V & +3.3V is ok) | | 9 | 5VSB | | Purple | +5 VDC Standby Voltage (max 10mA) | | 10 | 12V | | Yellow | +12 VDC | | 11 | 3.3V | | Orange | +3.3 VDC | | 12 | -12V | | Blue | -12 VDC | | 13 | COM | | Black | Ground | | 14 | /PS_ON | | Green | Power Supply On (active low) | | 15 | COM | | Black | Ground | | 16 | COM | | Black | Ground | | 17 | COM | | Black | Ground | | 18 | -5V | | White | -5 VDC | | 19 | 5V | | Red | +5 VDC | | 20 | 5V | | Red | +5 VDC |
| | Pin | Name |
| Color | Description |
|---|
| 1 | +3.3V | | Orange | +3.3 VDC | | 2 | +3.3V | | Orange | +3.3 VDC | | 3 | COM | | Black | Ground | | 4 | +5V | | Red | +5 VDC | | 5 | COM | | Black | Ground | | 6 | +5V | | Red | +5 VDC | | 7 | COM | | Black | Ground | | 8 | PWR_OK | | Gray | Power Ok (+5V & +3.3V is ok) | | 9 | +5VSB | | Purple | +5 VDC Standby Voltage (max 10mA) | | 10 | +12V1 | | Yellow | +12 VDC | | 11 | +12V1 | | Yellow | +12 VDC | | 12 | +3.3V | | Orange | +3.3 VDC | | 13 | +3.3V | | Orange | +3.3 VDC | | 14 | -12V | | Blue | -12 VDC | | 15 | COM | | Black | Ground | | 16 | /PS_ON | | Green | Power Supply On (active low) | | 17 | COM | | Black | Ground | | 18 | COM | | Black | Ground | | 19 | COM | | Black | Ground | | 20 | N/C | | | Not connected | | 21 | 5V | | Red | +5 VDC | | 22 | 5V | | Red | +5 VDC | | 23 | 5V | | Red | +5 VDC | | 24 | COM | | Black | Ground |
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My powersupply is an ATX v2.2 type, so all pin numbering in the schematics below refers to the 24 pin table. Cut of all the power connectors so you are left with only the big bundle of cables. Collect the different colors of wires together. Remove the wires that is not needed. For my needs 12V and 5V is sufficient so I just stripped away all the wires for the other voltages, the -5V, -12V and "5V stand by". Some of the wires, like the 5V standby power was cut away on the PCB component side. Some other wires, like the 3,3V cable bundle, was de-soldered from the PCB solder side. Also remove some of the 5V and ground wires so that you are left with 4/5 wires of each color (black, red and yellow). For computer powersupplies to correctly operate they need to have a certain load applied. Therefore a high power resistor must be connected between 5V and ground. In my parts bin I had a set of 10 ohm resistors. Coupling two in series gave 20 ohm load. Using Ohms law (U=R*I) we find the load current to be 5V/20ohm=0.25A. The power dissipated in each resistor (P=U*I) is then 2.5V*0.25A= 0.625W (2,5V as the 5 volt voltage is divided equally over the two identical resistors). To have some margin to save the resistors from being completely toasted the resistors should be able to handle at least 1W dissipated power. The power dissipated in the resistors can make them fairly hot, therefore I placed the resistors directly in the air flow from the power supply fan to help keeping them cool.  Now connect the swith and the power-on indication lamp (if any). The switch connects the green "Powersupply on" to ground to turn on the Power Supply.The switch must be a toggle switch, not a momentary type, as the low level on PS_ON must be held low as long as the power is to operate. For my supply I use an industrial switch with built in 12V lamp. Therefore the lamp was connected directly between one 12V wire and ground. If a lower voltage lamp or LED is to be used you can also connect it to the gray "Power OK" wire. If you use a LED connected to either 5V or "Power OK", you should connect a resistor of approximately 330 ohms in series with the LED to reduce the current to approximately 10mA. Now all that is left is to assemble the encapsulation, and the power supply is ready for use.
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Comments
You will have 24 V between +12V and -12V, but the -12 V cannot deliver as much current as the +12V, so the max current rating of the -12V line would determine how much power you could draw. Typically just around 1 Ampere, compared to the 10-12 amperes typical for the +12V output.
Also the negative supply would be acting as GND for the motors, and would then be at a potential 12 volts below the ground level of the 5V, giving 17V between +5 and the motor ground, which would destroy the drivers.
Connecting two supplies in series to get 24V is in theory possible, but as both power supplys share the same ground potential trough the mains connection you need to do some more heavy modification to separate the grounds of the two power-supplies. I have seen it done somewhere on the web (tried searcing for it again, but couldnt find it), but I do not recommend it from safety point of view (in general, never mess with the ground connection for mains connected equipment, it might kill you).