Power problem with vin 5 and 3.3V

Hi there,

So my arduino was siting for an year in a unfinished project and i decide to complete. First project btw.

The arduino was power by a ATX PSU giving 12.4V to the VIN pin. Everything was as expected. Then when i plug the USB into the computer. Both arduino and computer shutdown instantly.

So far as i tested:
VIN no longer power the arduino with the 12.4 V from the PSU
Powering the arduino via USB gives initially 3.3 V at 3 volts pin and soon drops to 2.5V.
5V pin only gives 2.5 volts. This pin also power the breadboard with a LCD.
By pluging the USB into the computer (wich still working fine), the LCD turns on and soon, 2 to 3 seconds later, it fade away and remain off.

I have no idea what happen or if it is fixable or worth fixing rather than get a new arduino.

Btw im using the PSU becouse i need to control a heater that runs at 12V and drain up to 7 A via PID.

Any ideas on how to proceed and what went wrong so i dont repeat the same mistake are welcome.

Thanks
TeoGr

Which Arduino? Clone or original?

VIN no longer power the arduino with the 12.4 V from the PSU
...
...

If that means that the power led does not switch on, the 5V voltage regulator went to heaven.

It is fixable but you can end up replacing about every component depending on the damage and it might not be worth the effort and money. The voltage regulator needs to be replaced; all components around it need to be checked. You run the risk that another broken component will send the new voltage regulator to haven as well.

Any ideas on how to proceed and what went wrong so i dont repeat the same mistake are welcome.

No idea what went wrong. There are plenty stories like this (relating to 12V and USB) and it's never quite clear as to what happened.

The safe way (for e.g. Uno and Nano):
1)
If you need the USB for upload, disconnect the 12V.
2)
If you need the USB for serial prints, use a separate USB-to-TTL adapter.

With so many Arduino variants around, what I suspect you’ve effectively done is applied two power supplies to it simultaneously.

What I suggest is you get a cheap USB hub like the SABRENT that lets you power or not power the USB channel, whilst still using the USB for data (and replace the Arduino).