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Connection drawing

A pc psu provides power for:

  • arduino uno (via de standby 5v power + ground so it always has power)

  • raspberry pi (via a 5v + ground connection)

  • usb hub (via a second 5v + ground connection)

Arduino is connected from an analog port to the psu POWER-ON connector to shutdown / start the psu after a button press has been detected. (how to detect pi powered off state is for another day).

  1. Can there be any problems here?

  2. I would eventually like to use the psu 12v to power 3 to 9 rgb leds. Can i do that?

  3. i know all electronic things connected together should have a common ground. Does a psu have a common ground for all black wires? or should i use power from the same pair of wires for all things?

    • ethernet/hdmi cables to the pi not shown but they will be connected.

    • i've read that it would be better with 1ohm resistance on the green wire (psu power on)

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Seems reasonable at a glance, couple of thoughts:

  • All the pins marked "GND" on the connectors will be connected together, so can be treated as a common ground. An easy way to test this kind of thing is by using a multimeter set to continuity mode (or ohms mode) and probe between the wires.

  • I don't use Arduino, but I thought the DC in needed higher than 5V. According to this page, it needs between 7-12V. This makes sense if it's using a 5V linear regulator as it needs some headroom to regulate correctly. You may need to use the 12V rail, although lower is better (e.g. between 7V and 9V) if you plan on using a lot of current with the Arduino, as the on board regulator dissipates more power and gets hotter the higher the input voltage.
    EDIT - to clarify about input voltage:
    Looking at the schematic, the Arduino UNO uses a 5V NCP1117 linear regulator at it's DC jack input. From the datasheet we can see the dropout voltage is around 1V (hence the absolute minimum 6V input given in the link above). Supplying the input with 5V will produce an output of ~3.3V (1V + D1 diode drop) - the ATMega will probably still work at this voltage, but the 5V rail will no longer be a regulated 5V rail, so it's obviously not ideal.

  • Yes, you can use the 12V rail to power the LEDs (assuming current limiting will be used correctly) The number will be limited by the current sourcing capabilities of the rail which should be in the supply specs. Up to 9 small RGB LEDS should be no problem though.

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  • \$\begingroup\$ From what i've found online, arduino can work on 5v, but will not have alot of power to drive things. So, if i use another 5v to power other devices it should not be a problem. But i will keep reasearching on that. But if it's using a 5v regulator to get clean 5v, the switching power supply does provide that, from what i can understand about pc psus (as opposed to linear power supply that drop voltage when the load increases). \$\endgroup\$ – vlad b. Sep 26 '12 at 13:13
  • \$\begingroup\$ The Arduino UNO schematics I have looked at don't have a switching regulator, they have a standard linear regulator which needs at least a volt headroom, plus it has a diode in front of it which will drop some voltage (how much is not certain as the part is not specified, at least 0.3V is a safe bet though) See edits, it will likely work for just the ATMega, but it's far from ideal. If you don't want to use another rail, then I would maybe consider bypassing the regulator as the 5V from the ATX should be reasonably regulated (maybe add a zener a basic protection) \$\endgroup\$ – Oli Glaser Sep 26 '12 at 21:50
  • \$\begingroup\$ what if i connect a 5v rail from the psu to the 5v pin of the arduino and do the same wiht a ground. what i'm not sure is if there are going to be any issues if i connect the arduino to 12v then connect them over usb or other pins. im thinking now of lighting up leds with the pi and reading them with the arduino to get a status message (linux booted, shut down - can safely close the psu, etc) \$\endgroup\$ – vlad b. Sep 26 '12 at 22:39
  • \$\begingroup\$ If you input 12V, all the pins will still operate at 5V due to the regulator, so it will be just the same and compatible with other 5V logic, USB etc. If instead you connect to the 5V rail directly you need to make sure the regulator input side is not connected to ground (i.e. leave it floating) - many linear regulators do not like the output pin voltage higher than the input pin voltage. Safest would be to bypass it, snip the output trace (can easily be reconnected with a bit of solder/wire) or remove it completely. \$\endgroup\$ – Oli Glaser Sep 27 '12 at 0:12
  • \$\begingroup\$ Now the hdd does not get enough power (it power-cycles every 2-3 seconds). Can i split the usb wire and connect the +5v to the psu. Not really sure about the ground - hub, psu or both ? Can you recommend me a good book or site for learning these things? right now i'm piecing things together from the web \$\endgroup\$ – vlad b. Sep 27 '12 at 6:06
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  • The Arduino won't work if you connect 5V supply to that connector, since a 5V voltage regulator of the Arduino is connected to that connector. The input voltage should be higher than 6V if you want to use that power supply connector (dropout voltage, meaning Input-Output, of the voltage regulator should be higher than 1V). But the voltage regulator on the Arduino board cannot source much current when the input voltage is around 6V. The Arduino would operate itself on 6V supply but it could be problematic if you use many GPIO pin because the GPIO pins drain current from the Arduino. So you may want to use 9V~12V.
  • Otherwise, you can directly connect 5V pin and GND pin to the 5V power supply, but the above one might be better.
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Why don't you just power the Arduino off the 12v rail? It works, I've done it. But don't forget that a PSU needs a minimum drain to run properly. Connecting a 10W 10Ω resistor between 5V and Gnd does the trick. Google "DIY bench ATX supply".

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