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I am working on a project that requires the use of a PC Power supply with my Arduino Uno. I would like to use a Molex to power my Arduino and I have a few questions. My power supply is rated 18A on the 12V rail, and 28A on the 5V rail. My question is which rail would be best to use, and should I use Vin, 5V, or the DC power plug. Also, would the same advice work for an Arduino nano? All help is appreciated.

Cameron K
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    What other components are going to be drawing power from the PSU, and roughly what currents will they be drawing? A PC PSU typically needs at least some load on some of the rails for correct operation. – Andrew Morton Oct 12 '17 at 20:55
  • @Andrew: I have a PC power supply of the same dimensions and it does NOT need any load. This is not true. Well, it is strange to use such a powerfull supply just for an Arduino. But there is no problem to connect just the Arduino and nothing more. It is also strange to still use Arduino Uno which is the oldest and slowest board with nearly no memory. I would recommend to use a Teensy 3.2 instead which is smaller and offers much more functionality. – Elmue Oct 13 '17 at 00:58
  • To answer the quesion: You can use the Arduino DC adapter and connect it to 12V of the supply OR you can use the 5V pin on Arduino and connect it to the 5V of the supply. But I recommend to use the 12 V because the voltage from a PC power supply is not very clean. The voltage regulator on the Arduino will make a clean 5V from the dirty 12V. See http://playground.arduino.cc/Learning/WhatAdapter – Elmue Oct 13 '17 at 01:22

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Neither, I would use the 5VSB supply to the arduino 5V input, this is the standby supply, not the main 5V supply, the PSU fan won't run, neither will the main part of the powersupply, and your power bill will be lower.

If you need to also operate heavy machinery, you could power the that from the main 12V or 5V and control the power-on input to the power-supply from the arduino, only operating the supply at full power when needed.

If this arduino is an accessory to a PC I would use the 5V molex, unless I wanted it to operate duirng suspend and sleep, in which case I would use 5VSB off the ATX main connector, perhaps making the connection using an idc splice connector, or connecting to the power pins of an on-board USB header.

Jasen Слава Україні
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The 18 A and 28 A means that it is what the PSU (Power Supply Unit) safely can deliver. However, current will only go where it needs to go, the PSU can't force 28 A into your Arduino. Your Arduino will take what it needs which is probably around 10 mA.

And your Arduino will probably use a maximum of 10 mA. This means that you, safely, can connect \$\frac{28 A}{0.01 A}=2800\$ Arduinos in parallel to the 5 V rail.

However regarding the 12 V rail or 5 V rail. There's a linear regulator on the Arduino which will convert 5-12 to 5 V which the Arduino use (if you got a 5 V Arduino, or 3.3-12 V to 3.3 V if you got a 3.3 V Arduino).

So if I were you I'd hook up the Arduino's \$V_{in}\$ to the 5 V rail. I'm fairly certain that the DC power plug terminals are \$GND\$ and \$V_{in}\$. Though I'm not 100% sure, so correct me if I'm wrong in the comments.

I'm also not 100% sure if the Arduino uses an Low Drop Out linear voltage regulator LDO or just a simple regular linear voltage regulator.
The difference between them is that an LDO that is supposed to give 5 V out can take about around 5.1 V - 12 V. (12 V is just an example). And the regular linear voltage regulator need some higher input voltage, say 6.5 - 12 V to step down to 5 V.

The reason why I'm talking about the LDO is because the \$V_{in}\$ goes straight into some kind of regulator to give 5 V to the rest of the board.

TLDR;

Just connect the \$V_{in}\$ to the 5 V rail and GND to GND

Harry Svensson
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    With a little help of Google you would have found the answer: There is no LDO on the old Arduino Uno from the stone age. And the board needs much more than 10mA. http://playground.arduino.cc/Learning/WhatAdapter – Elmue Oct 13 '17 at 01:11
  • @Elmue "by djmatic" – Harry Svensson Oct 13 '17 at 01:31
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You may gang contacts 4x, because if Molex current > 10 A the contact drop exceeds current rating, causing excess heat, so for proper solution, you must splice appropriate connectors for 20A and 30A rating on short wires from PSU or replace with much heavier - 8 AWG

Use separate cable to power Arduino from loads to reduce conducted noise. Add decoupling to load source.

Tony Stewart EE75
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