Teensy 4.1 powered from both the USB cable and VIN.

J

jimmie

Well-known member
I have a board with a Teensy 4.1 that is powered by a 5VDC power supply to VIN. However, I have been upgrading the firmware through the Teensy's USB (while also powered from VIN) numerous times without a problem ....

I understand that it is not recommended. But what may happen to the Teensy 4.1 if it is powered from both the USB cable and VIN input at the same time?

Will this potentially damage the Teensy, or the USB port or both?

Is the likelihood of damage high?

Thanks.
 
The Teensy 4.1 isn't affected if it is powered from both the USB cable and VIN input at the same time.

T41schematic.PNG

The problem occurs at the USB port of your PC or at your external power supply. Whichever supply has the highest voltage will feed the other supply.
I would be most worried about the USB port of your PC being damaged.
Best practice here is to insert 2 Schottky diodes as indicated on the image above.

Paul
 
As noted p#2 it should be avoided and assumed harmful.

A couple of solutions - beyond diode placement:
> cut the bottom side VIN<>VUSB corner trace, but that requires external power to program or operate, or adding a jumper setup when not externally powered to selectively restore from VUSB inboard hole to the VIN pin
> carefully open a known good cable and cut the 5V line (typically red?)
> use an external Dongle that allows removing the 5V power while the other D+/D-/GND signals pass.
 
Thank you very much PaulS, defragster and Bricomp or the detailed answers.

If I use 2 Schottky diodes, which diode?
 
Is the likelihood of damage high?
Click to expand...

To answer this part of your question, no, the risk of damage is low. USB ports on laptops are traditionally protected from overvoltage, I think up to something like 21V. I've been working this way for close to 10 years with my laptop and benchtop supply connected simultaneously and never had an issue.

Regardless, I have recently started using a USB isolator like this one, just in case, because i started working with PCBs with a higher voltage on the same board.

https://www.amazon.com/GeeekPi-Isol...1682602311&sprefix=usb+isolato,aps,177&sr=8-3

I prefer using this isolator because I don't have to change any physical configurations on the board or Teensy. However, be sure to disconnect the laptop when you turn off your bench supply, or you'll burn out the isolator's 1W transformer, which powers the optocoupling (if your 3.3V components + teensy exceed that demand).
 
Hi, actually just the schottky diode close to the Vusb input is needed. This diode has a forward voltage drop of about 380-410 mV @ 0,35A
The USB voltage needs to be higher than Vin from more than 400 mV to conduct. The reverse voltage on this diode will never be more than about 5V (the Vin voltage)
It would be a good idea to implement this diode at board manufacturing, so no cure about powering Teensy 4.1 from USB or from External Vin....
I have added this single diode and, up to now, no problem at all.
Cheers,
Zoltan
 
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