Convert 12V power supply output to 9V

Other Hardware General Electronics
21

On the UNO:

If there is no Vin voltage, the regulator has no 5v out.

If we connect external 5.3v to the 5v pin and the UNO is plugged into USB, the T1 MOSFET (gate is LOW from U5A-1) allows the external 5.3v go to the PC thru the USB cable.

And if the PC is turned off . . . ?

22

IMO
We cannot expect new users to remember which Arduino can and which Arduino cannot be plugged into external 5v at the same time USB is being used.
New users, get into the habit of disconnecting the external 5v power supply when uploading sketches to the Arduino.

For new users, my words of wisdom is to :nerd_face:, never connect external 5v to the Arduino 5v pin, when the Arduino is plugged into USB.

23

Yes, I agree with you there. I suppose there is an assumption made (these days, though now years ago) that any 5V external source would be well regulated switching dc to dc converter compliant with TTL standard of the allowable voltage
range of 4.75V tp 5.25 V. In the interest of caution I suppose both supplies would need to be vetted with a DMM to assess their compatibility. I would say that if they are both in the range of 4.95V to 5.05V that there is no need for concern. When they wrote the TTL spec they didn't have the technology we have today. If they were to update I suspect they would change it to 4.95V to 5.05V. I would never connect external 3.3V to the 3.3V pin of the board regulator because it has input power from the 5V pin and would have voltage on the output. The T1 mosfet is controlled by the comparator and with no external Vin the mosfet would be ON. (AFAIK).

24

Yes.

  • 3.3v on -
    0v on +
    =
    LOW on Comparator output
    =
    T1 turned ON
25

And remember, if the PC is then turned off but USB still plugged in . . . ?

26

In that case the PC could be subject to 'phantom
power' (back-feeding), the same as having voltage
on arduino pins when it is powered off.
I have no information on what the consequences
could be. If you blow the USB regulator in the PC
it would not be easily fixed and could require replacing the mother board or worse if it was a laptop. Best practice is disconnect the USB cable
BEFORE powering off the PC.

27

Hence the statement:

28

EDIT

LarryD 29

:+1: