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I'm building a small device using arduino that should be connected to my pc via a usb header to exchange data with the PC.

What I'd like to achieve is that arduino turns on when the pc is on, whereas it shuts down when the pc is turned off. If I rely on the usb port to provide power to the arduino that might not happen, since some motherboards still provide power to the usb connections even when the pc is off.

Therefore I'd like to power the arduino using the 5v rail directly from a Molex. I know that theoretically I can connect that rail straight to the 5v pin, but what about power spikes? How can I be sure that the 5v input voltage is properly filtered and stabilized? Would it be better to use a LM317 and transform the 12v rail into 7.5v to feed arduino from the vin?

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    The Arduino will be fine with the 5V rail as long as you don't have a garbage-of-the-garbage PSU. After all, the rest of the computer is much more sensitive than the Arduino.
    – Ignacio Vazquez-Abrams
    Commented Jun 29, 2016 at 8:08
  • Yes, I think that as well, but just to be sure what kind of filter could I build to prevent possible (even if unlikely to happen) spikes?
    – xanz
    Commented Jun 29, 2016 at 8:28
  • The 5V on the USB port, is coming from the same power supply as the 5V from the molex. You could also switch to e.g. an Arduino Leonardo, which would be able to detect whether the PC is actually on.
    – Gerben
    Commented Jun 29, 2016 at 8:49
  • So you're saying that if the USB is powered when the pc is off then so is the molex 5v rail? What about the 12v then? I might be able to regulate it down to like 7.5v with a lm317 and then feed it to the vin pin... Changing the board is not a viable option for me in this case.
    – xanz
    Commented Jun 29, 2016 at 8:53
  • FYI only the DC Power port (12v) and USB (5v) are regulated, vin is not. 3v and 5v pins are only for output.
    – Jacksonkr
    Commented Jun 30, 2016 at 1:49

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The 5v of an ATX power supply will be perfectly adequate for powering an Arduino.

It will already be very heavily filtered and stabilised.

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  • Now @Gerben made me think that maybe the 5v rail is still powered on when the pc is off. If so, then will a regulated 12v rail work? What do you think? ps. I need 15 rep here to upvote you, so if you upvote the question that might help :)
    – xanz
    Commented Jun 29, 2016 at 9:01
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    An ATX supply has a 5v standby rail that is always on which is used to control the computer's power circuitry. The main 5v rail will be off. The only time I have seen a computer's USB providing power when turned off has been when an external powered hub is back feeding power into the port.
    – Majenko
    Commented Jun 29, 2016 at 9:04
  • Some motherboards do have the option of providing standby power to USB for wake-up purposes, but that usually has to be explicitly enabled by the user.
    – Ignacio Vazquez-Abrams
    Commented Jun 30, 2016 at 1:15

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