Taking power from an ATX molex

Hello folks,

I'm planning a project that will be closely tied into a desktop PC, and I'm going to need some power both for the spaghetti electronics and a whole bunch of fans.

Can I simply use the the +5V line from a PSU Molex to power the Arduino?

I don't need to worry about all the trickery of getting the PSU switched on, keeping it on, and so forth, because when the PC is off, my project is off. If I do end up needing always-on, I should be able to work out some shenanigans such as this project from Contesti.

I was going to connect the +5V line to the Uno's Vin pin. That is that regulated, correct? Or do I have to go through the edge barrel-connector to get through to the VRM on the board?

No you need to connect the 5v output of the PSU directly to the 5v line of the arduino.

If you are using analogRead() and want good results it might be wiser to use the 12V output of the ATX supply to the Vin pin so the noise on the 5V supply doesn't affect the ADC so much - it might make a difference. Computer PSUs are usually fairly noisy since they are designed for power logic rather than analog circuitry, and because they are connected to a noisy load (the computer!)

If you have a manual with the computer mainboard it might say how much current it takes and you can see how much spare the PSU has available for extra stuff - usually there's a reasonable amount of spare capacity unless you've filled all the drive bays and installed a mental graphics card.

While you are at it check the PSU fan is running freely - sometimes they start to stick and run slower and slower and then the PSU gets hotter and hotter - nip this in the bud!

MarkT: spare capacity unless you've filled all the drive bays and installed a mental graphics card.

Um... yes and yes :D It's a brand-new PC with dual GPU's and all sorts of bells on, but it's also a 1000W PSU with lots of spare modular connections.