power Diecimila with computer power supply

I have retrieved a power supply from an old computer. Would it be sufficient to power the Diecimila with it? I will be needing the regulated 5V delivered by the Diecimila.

Also, If the computer power supply is sufficient, I also have some valves to operate, would it be wise to power these valves from the Diecimila or directly from the computer power supply? The valves require 24 V to operate.


Ahh… More than plenty… The computer power supply likely puts out AMPS worth of 5v not just milliamps. However, I doubt it would have a 24V output. They do output 12V so maybe you could use that. It’s possible that a 24V valve would work on 12V too.

I think they do have 24V - at least mine does - at 2.5A.
There’s a trick to using them when disconnected - some wires to short. Google this, or I’ll find you a link.

Thanks for the quick response! Good that I don’t have to purchase a new power supply for it.

Now the next question is how do I connect the dicimila to the computer power supply (I’m assuming I need the 12V from the power supply so I get a stabilized 5V)?

And if I can only get 12V, I can always have an amplifier circuit to drive my valves, right?

The computer power supply will require a minimum load to be present before it will power on, and you need to determine which of the power supply pins controls the power to the system. Typically the motherboard of the computer does this. (remember that the power switch on a computer is not connected to the power supply, but is attached to the motherboard)

I’m not sure which pins carry the 3.3V or the power-on signal but it should be fairly easy to figure out. You have to do something to those pins to get the power supply to work. The below links may help.

I found these with a Google search: atx power supply 3.3 volt pin


I googled for ATX power supply specs. They do output 5V regulated so that should be able to be used to power the arduino directly with no voltage regulator in between. Also, there are +12 and -12V leads. Taken together that’s a 24V potential.

Yes, you do have to short pins to get the power supply to turn on. Here is a site which discusses this:

A couple things to beware of if you combine the +12v and -12v to get 24V: 1) You will not have a ground lead for the 24V lines. Both wires will be at a potential different than ground and you’ll let out the smoke (and possibly the fire) if either one is tied to ground. 2) The -12V feed usually has no where near the current capacity of the +12V. For example, the +12V may be rated at 7 amps, but the -12v could be as little as 1/2 of an amp. Be sure the least of the two capacities is >= to the load you intend to support at 24V.

One last thing to consider: sometimes computer power supplies rate their current capacities in non-mutually exclusive values. What that means is for example the +5V may be rated at 10 amps and the +3.3V rated at 7 amps, but the total combined current from both can’t be more than 12 amps. Usually this is only on the +5V and +3.3V outputs, but I’ve seen it also on the +12V/+5V and the -12V/-5V.

koyaanisqatsi has given good advice regarding the 24V. (The PSU I used had a dedicated 24V along with the +12 and -12 - most probably don’t.) So if you tie the +12 and -12 together you probably will get only 500ma max - and nothing that can be used for a common ground.

So if you tie the +12 and -12 together

If you tie them together, you will get a short circuit! If you use -12V as Ground and +12V as a power supply, then it will give you +24V, but the “Ground” wire of the PSU will be at +12V and the “5V” supply will be at +17V.

oopps! - totally used the wrong term! thanks for straightening that out. :-X

Thanks for all your replies, i’ll check out those sites about hooking up the supply.

I have also taken out the on/off button so I can turn the power supply on and off. Like when I plug it in and push the button, i hear the fan going. Does this make the things less complicated with the 3.3V thing?

the valve is rated at 24V, 15.4 W. If the -12V only gives me .5 A, than that’s probably not enough, right?

So close, but not enough. You’ll need about 650 mA or more to drive 24V at 15.4 watts.

So… forget about the valves. I’ll get different power supply. So say I’m powering the arduino. I move the selection pin from usb to ext.

  1. Should I put the regulated 5V from the power supply to Vin on the Arduino or the unregulated 12V from the power supply to Vin? And Do I put gnd from the power supply to one of the gnd on the arduino?

  2. After I load the program onto the Arduino, and unplug it from the computer, how to I tell it to run when I use the external supply?


And Do I put gnd from the power supply to one of the gnd on the arduino?

Yes, otherwise it won’t work.

Should I put the regulated 5V from the power supply to Vin on the Arduino


how to I tell it to run when I use the external supply?

Depends on the version of the Arduino you have. There is a link on the Decimila that swaps between USB and external power. Other versions switch automatically.

Thanks! I will try that with the power supply. And further on the last question. I moved the jumper to be on the ext instead of the usb. In this case, can the arduino diecimila still communicate with the computer through the usb, like I would like to see my output on the console. Is it allowed?

Is it allowed?

Yes it’s allowed, the USB serial port still functions no matter which power source is chosen.


I’d like to point out that these days there are all sorts of adaptors that provide 5V on a USB female A connector, aimed at charging mp3 players and cell phones and PDAs and portable game systems; USB is rapidly become a standard power interface, compatible power supplies can be found that are astoundingly cheap ( http://www.meritline.com/apple-iphone-3g-262-237---p-31362.aspx for example)
Something like this should happily power an arduino via a USB cable. (Of course, it won’t provide 9V on the other power pin, nor does it provide any intelligence or communications via USB.)

Similar devices designed for in-car use may be one of the better ways of powering your Arduino in a car, as well. No power dissipation worries (these are almost all “switching” power supplies), and a nice layer of insurance between the nasty automotive power environment and sensitive electronics.

I’ve even been wondering whether it’d be time to get rid of the regulator and external power jack on board like the arduino. Regulated wall warts are a lot more common, and a lot cheaper, than they used to be…