Running an Arduino Nano from unregulated power source.

Projects General Guidance
1

I'm sorry if the topic may seem a little unclear, but a little background should make it more clear. I'm currently building a desktop power supply from an old ATX PSU, where I'm planning to use all power outputs (3.3, 5, +-12 and ground), and also implement current/voltage monitoring on outputs using an Arduino Nano reading current levels from three separate ACS712 sensors on the positive outputs and display the readings on a Nokia 5110 LCD display.

To make things easy I thought of just running the arduino from the 5V output connected directly to the 5V pin on the arduino, however when I think of it, wouldn't that make problems with the ballast (a 10R, 10W resistor) also connected to the 5V output if I attempt to connect the arduino to the USB without removing either the resistor or taking the Arduino out first?

The easiest solution is perhaps to take two power resistors in series and connect them to the 12V output?

2

wouldn't that make problems with the ballast (a 10R, 10W resistor) also connected to the 5V output if I attempt to connect the arduino to the USB without removing either the resistor or taking the Arduino out first?

No.

3

Grumpy_Mike:
No.

Isn't the 5V from the USB connected to the 5V pin on the Arduino, so that the resistor will pull almost half an amp from the USB?

4

Isn't the 5V from the USB connected to the 5V pin on the Arduino,

Yes.

so that the resistor will pull almost half an amp from the USB?

Only if the power supply is off.

5

Grumpy_Mike:
Yes.
Only if the power supply is off.

I see. It will usually be off when/if I have to update code.

I think I'll put the ballast on the second 12V rail instead (which is not used), and run the Arduino from the 5V rail. Then I will be able to connect to USB to update code with the PSU turned off?

6

The real solution would be to power the Arduino from the 12V rail in the first place.