# Using an old computer atx power supply as a stepper power supply

I have a thee amp stepper motor along with a pololu driver.the driver says it can handle two amps per coil and had a current limiter on it. The power supply says it can provide 12 volts at 15 amps. If I use this power supply.am I going to burn every thing out?

The Polulu driver can handle 12V? If so it can handle that supply... You might be wise to measure the output voltage on no-load to check its behaving.

Yes, the product page says it can handle between 8 and 35 volts. http://www.pololu.com/catalog/product/1182

Computer Power supplies regulate the +5V output so in order to get the other outputs to track you must load the +5V output with at least 100ma or 50 Ohms.

greengiant83: I have a thee amp stepper motor along with a pololu driver.the driver says it can handle two amps per coil and had a current limiter on it. The power supply says it can provide 12 volts at 15 amps. If I use this power supply.am I going to burn every thing out?

You can answer this question yourself using Ohm's law.

Just because a PS can deliver up to 15A doesn't mean that 15A will be flowing, unless you have little enough resistance (high enough load) to actually draw that much. As long as your motor is rated less than 2A draw, you should be fine.

Just because a PS can deliver up to 15A doesn’t mean that 15A will be flowing, unless you have little enough resistance (high enough load) to actually draw that much.

That is right. To the OP, just remember that amperage is drawn (by a load) not pushed (by a power supply). Your PS can withstand up to 15 amps of power draw, anything less cannot be a problem.

I used an old pooter PSU what's handy is the "standby" voltage is on all the time you can then command 5 and 12 on when you're ready useful if you want to sequence the power on

MarkT: The Polulu driver can handle 12V? If so it can handle that supply... You might be wise to measure the output voltage on no-load to check its behaving.

If you run a switching power supply with no load it will blow up. A switching power supply will keep trying to increase it's out put until it hit it rated voltage. If there is no load there is no voltage so it just keeps trying until it smokes. So don't run it with no load.

If it was me, and it soon will be, I wouldn't do it the way described in hobbycnc. I would leave the At or ATX power supply alone and connect an old floppy drive to give a load to the switching supply. Mount the supply inside another box. Then construct linear DC to DC supplies to outputs on the outer box. This would give you a 12 volt, 9 volt, 5 volt and a 3.3 volt outputs, with the proper dc to dc supplies. And you can made an adjustable output for anything in between. Adafruit even has a kit to make an low dropout dc to dc supply. So then all you would need is a couple of panel meters and you are good to go. It looks like an hour and a half project to me.

When I do mine I will make an instructable and show what I am talking about. But there is no need to open the comp power supply. If you did it that way it would look like crap anyhow.

joseph_m: If you run a switching power supply with no load it will blow up. A switching power supply will keep trying to increase it's out put until it hit it rated voltage. If there is no load there is no voltage so it just keeps trying until it smokes. So don't run it with no load.

I have never heard that. Perhaps you can point me to the source of that information. I have my switchmode power supply (which is my mobile phone charger) plugged in and have often forgotten to switch it off after charging my phone. Mind you I have done the same with 3 others as well and all any of them have done is cool down when the phone is unplugged!

I also stated that CPU Powersupplies regulate to the 5V output, it must be loaded for the auxillary outputs to deliver to rated capacity.

Late response to this thread but I thought I might add a few helpful notes and some helpful tips if not mentioned above.

I find it very useful to use an old or new tower desktop computer power supply. Not only it can delivery high ampere but it has both 6v and 12v output for cases such as popolu motor drive that run on 6v yet attached with a 12v stepper motor (such as Nema 17HS2408 stepper motor).

My preference is a power supply with an on/off switch right on the box.
If it has a green line within the main stack of wires then you can use that as an external on/off switch.
If you have both a switch on the box and a green wire then to bypass the green wire switch is to loop a wire from the green socket to any of the black (GRD) wires.

In most computer power supplies the red and black are the 6v wires. And the yellow and black are the 12v wires.

A jumper cable with a pin can easily fit and fastened and inserted into the old floppy disk or CD connector wire plug which has those mini inserts and just the right size for a jumper cable.

I hope this helps future generations.