Adding a Schottky across solder jumper on Teensy4.1

paulfer

paulfer

Member
Hi All,

Why can't/shouldn't I:
  • Cut the said solder jumper,
  • Then add a diode across as per my crude drawing,
Besides the Vf surely this would protect the USB port but still provide power to the board from the USB, when I am in the bushes and don't have a 5V supply handy.

dimensions_teensy41_photo2.jpg
 
Yes, adding this diode won't hurt anything and will help protect the USB host should you connect USB while you also have VIN power applied. The Teensy is good down to 3.6V, so the Vf drop won't matter. In fact it will take a bit of the load off the regulator

For complete protection, you would also include a diode from VIN to the external power source with the cathode also pointed towards VIN to prevent USB from possibly trying to back power the VIN external power source. That diode would need to be external to the Teensy, so may or may not be applicable in your case.
 
KenHahn said:
For complete protection, you would also include a diode from VIN to the external power source with the cathode also pointed towards VIN to prevent USB from possibly trying to back power the VIN external power source. That diode would need to be external to the Teensy, so may or may not be applicable in your case.
Click to expand...
Yes good point. Diode added.
 
Anyone have any photos of how they've soldered a diode onto those two pads? None of the SMD Schottkey diodes I have on hand are small enough.
 
I used to scratch that out, but then I realized it doesn't matter and just creates more work and inconvenience. My USB cables never get warm from the voltage difference.
 
I use a SOD-523 / SC-79 package style which is pretty small, but fits nicely on the pads. I normally still mount it at an angle to ensure good clearance for the pins.

1775659284834.jpeg


I used to scratch that out, but then I realized it doesn't matter and just creates more work and inconvenience. My USB cables never get warm from the voltage difference.
Click to expand...

If you are connecting both VIN and USB power without the mod, you are directly connecting the 5V USB power on your computer to whatever power supply and voltage that is driving the VIN pin. It probably won't hurt the Teensy, but it could potentially damage something like your computer USB port. How great that risk is, I don't know.
 
I've only connected nominal 5V regulators, but that's a good point because somebody else could theoretically hook up 3.6V or 5.5V. USB power wires vary from 20 to 24 AWG and lengths usually vary from 3 to 15 feet.

6 feet (black + red wire) of 20 AWG is only 0.061 ohms.
30 feet (black + red wire) of 24 AWG is 0.77 ohms.

Voltages of 5.5V and 5.0V have a difference of 0.5V.
A 3 foot cable with 20 AWG might cause problems.
0.5V/0.061ohms = 8.2 Amps!

If voltages are 5.05V and 5.10V, that's only a difference of 0.05V, and the voltages usually change a little with current flowing.

With a 15ft USB cord with 24AWG power wires the current would only be 0.05V/0.77ohms = 65mA.
 
And there's contact resistance to consider, should be milliohms but can be > 0.1 ohm in bad cases, and there are 4 contacts in the power/ground cable wiring all in series too...
 
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