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I have to power an Arduino UNO from the 12v line of an ATX power supply.

Can I just wire the 12v (yellow wire) and GND (black wire) from the PSU to Vin and GND on Arduino?

Arduino specs says it's recommended input voltage is 7-12v (being the actual limits 6-20v), so it's ok?
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I'd measure the voltage first as many ATX supplies need a minimum amount of current to regulate properly, so with no current load (or the relatively small current load of an Arduino) you may see 12V turn into 13V, 14V, etc.

But if it's steady at 12V, you should be OK.

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I think this won't be a problem, as the PSU I would use already powers a lot of devices. Anyway, I'll measure it to confirm. If it does provide 12v, can I wire to the Arduino as I said before?

And supose I have to use only this Arduino board on the PSU, and supose it doesn't function properly because of the low load of the Arduino, how could I workarround this?
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If it does provide 12V, I don't see why you couldn't wire it as you said.

If it's >12V you can work around it by providing a "dummy load". Find some big fat power resistors and hook them up to 12V, ensuring you do not exceed their power ratings. I'd start with a small load, say 0.1A (120 ohm resistor, 2W or higher power rating) and see if that fixes it. If not, you may have a supply that looks for a minimum load on another one of the outputs (e.g., 5V or 3.3V) and regulates everything else in relation, so you will have to put a dummy load on 5V (maybe) to get 12V down to normal.

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Hi, The Good Old Load for a PC power supply is a 12 volt automobile tail light or side marker light bulb. Easy to get.

Also acts as your "Power On" pilot light!

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If you have an ATX supply, you have 10s of amps of well controlled 5V DC.  Why not just use that directly?

I currently have a '328 running off the stand-by 5V of an ATX.  The Ardu' grounds the control pin of the ATX when it wants to fire up a massive LED light (well, only about 20W worth but still more than stand-by power), draws the power it needs and then turns the ATX back to stand-by again after.  The 5V supply is entirely stable enough in spite of PWM control of 20W of LEDs.  Arduino's on-board regulation not required.
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Quote
The Ardu' grounds the control pin of the ATX

Good idea for many applications!

Is the usual ATX power-on pin a 5V control signal? Did you drive it directly from an Arduino output or through a transistor / driver etc??
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If your using a PC ATX power supply, why not just use the +5v?
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The Ardu' grounds the control pin of the ATX

Good idea for many applications!

Is the usual ATX power-on pin a 5V control signal? Did you drive it directly from an Arduino output or through a transistor / driver etc??

Just pull it low through a 1K and the whole thing comes to life.  It reads +5V when on standby.

So yes, it's a normal 5V control and I just connect it directly to a digital pin throu' a 1K (I'm sure direct 0R connection would be fine but I'm nervous like that).

PS the ATX provides a "power OK" signal on the grey wire that goes to +5V when the supply considers the power to be stable.  My next trick will be to wait and read that until it goes high so that I know I have good power before switching on my load.  Not actually done that yet thou'.
« Last Edit: March 09, 2012, 10:29:47 am by Dr_Ugi » Logged

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"If your using a PC ATX power supply, why not just use the +5v?"
As I said before, the PSU I'm gonna use already powers a lot of devices, which drain power from two of the 12v lines of the psu. The Arduino would be located near such devices, which are far away from the psu. In order to reuse the infrastructure I'd just use the 12v lines.

But it's not that dificult to use the 5v directly from the psu... If I decide to do this, how should I wire Arduino up? Could I use the same schema descbribed before (Arduino gnd <-> psu gnd, Arduino Vin <-> psu +5v)?

Dr_Ugi said on the 5v "you have 10s of amps of well controlled 5V DC". That means on the 5v I wouldn't have the instability issues of a low load psu as described before by RuggedCircuits?
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If your ATX is already powering other things successfully on the 12V supply then there should be no issue using the 5V supply too.  In fact you can typically take about 0.5-2A from the 5V "standby" power even when the main power from the ATX is turned off (check the specs of your particular supply).

You would just connect +5V (red wire) from the ATX to the 5V pin of your Arduino and Gnd of your Ardu' to Gnd of your ATX.  Don't use the "Vin" because you need 6.5V or so on that pin to power the regulator.  If you are using standby power then take it from the purple wire to 5V on your Ardu' instead.

Edit - But in any case, if you have a stable 12V that should be OK direct to Vin.  If it's a little high, you can just run it through a few ordinary rectifier diodes first - 1N4001 etc can handle 1A of current and drop the voltage by about 0.8V each, so if you are reading 14V just put it through 3 diodes in series to burn off a little of that excess first.

« Last Edit: March 09, 2012, 07:16:02 am by Dr_Ugi » Logged

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I have straight wired both PS-ON and P.G. to the I/O on an UNO. Both are TTL logic per ATX standards and do not need limiting resistors or pullups. I would caution against sending 5v out to PS-ON. Don't set output high, just LOW. Code as digitalWrite(pin, LOW) and toggle between pinMode(pin, INPUT) for off and pinMode(pin, OUTPUT) for on (open and ground).

Also, watch out when using standby power to 5V input rather than 12 volts to Vin. using the USB at the same time can cause problems.  If you use Standby power directly to the 5v input you bypass all the power protection and switching logic, make sure you turn off power supply or unplug before connecting USB and disconnect USB before turning on ATX master switch(and standby power), otherwise your ATX could send power to your computer. This means that serial communication via USB will not be possible when running using standby power.
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