Mark, thanks so much for this important information. I'm so glad we're getting to the bottom of all my design issues. smile

Originally Posted By: mlord
3.9V on my setup. Not quite enough for reliable operation of the Arduino, which wants 4.75V to 5.25V. Interestingly, this was enough for the Arduino's onboard 3.3V PSU to output a full 3.3V.


About the same for me: 3.86 volts when the empeg is powered off the car's voltage.

Thing is, this works great for me. The Arduino has been very stable in all my tests. But you're right, it would make better sense if I could figure out how to do this properly.


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Then, plug in a laptop over USB (with Vin still connected), and measure it again.
Here, the measured voltage went up to 4.8V.


Mine went up to 5.06v.


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Then, I connected an inline current measuring device on the Vin line. Current draw was about 0.06A, until I connected the "laptop" USB in parallel, at which point current draw doubled to 0.12A. Which means it is very likely back-feeding some current to the laptop (not good).
...
Note: I did not actually risk a laptop, but rather used a USB power dongle instead.


Is it possible that a laptop might be different from a USB charger? For example would a laptop have protection against reverse current?


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The boards we have may or may not match this schematic. smile


I am using this one: https://www.amazon.com/dp/B01H4ZLZLQ/ in case it's relevant.


So here's the question, then. If I need to rethink the power scheme again, how can I do it so that it meets my needs? Specifically:
- Normally powered from the tuner connector, in the trunk, connected to the empeg that is also in the trunk.
- Can leave a USB cable plugged into it in the trunk which I can reach from the front seat and plug into a laptop for debugging from the driver's seat, with no need to unplug/replug anything in the trunk.
- Design needs to be something I can fashion into an easy kit without having to heavily mod the Arduino. So ideally any weird stuff would ideally go into my own PCB sandwich/shield board.

Right now what I've got is working for the above. Though you have shown me evidence that I merely got lucky with that, and I'm running a risk. So what to do?

My first thought is to power the Arduino into its 5v pin from the 5v power supply and then do something like an additional linear regulator to get the 3.3v. But that would just run the reverse-voltage-up-the-USB risk worse (it worked when I tried it before though).

There is an interesting circuit in the original BlueGiga dev board design which allows it to get power from the USB or from VCC correctly, and automatically switch. I don't understand it though, and I'm not sure how to implement such a thing on my board and whether it would even help in this situation.

Any other ideas?

Thanks again for all your help.
_________________________
Tony Fabris