External Power and USB on 4.1

[BlueCoder]

I did a search and it's still not quite clear. On ALL the most recent Teensy boards (I have 3.6 and 4.1) if you have external power and want to connect USB for programming you have to cut the power either in the cable or on the board between pads?

If YES then the power must be hooked up in serial no? If so connecting both would result in 10v power? But looking at the schematics it looks to me that the power is connected in parallel. If that is so then why does the power need to be seperated?

I'm using my Teensy for controlling many 5v LED strips with logic converters. Lots of long strips so I have a 40 watt external power supply.

 

[mborgerson]

But looking at the schematics it looks to me that the power is connected in parallel. If that is so then why does the power need to be seperated?

You are correct that the external and USB5V are in parallel in that they both connect to the input of the Teensy regulator that provides 3.3V. You need to disconnect the USB5V by cutting the jumper between the pads on the bottom if you are going to use external 5V power. The separation is necessary because the USB5V and your power supply are very unlikely to be exactly the same voltage. When they are different, the higher voltage supply will try to force current into the lower-voltage supply. Your external supply may be OK with that, but USB ports don't like accepting current from a higher voltage source.

Once you cut the trace on the bottom, you can use a few diodes to prevent the current sourcing problem:

[FONT=Courier New]
               Schottky Diodes

EXT 5V ------ -->|-------------*
                               |
                               *--------------- Teensy VIN pin;
                               |
USB 5V ------ -->|-------------*
[/FONT]

The diodes protect against reverse current flow, but have the disadvantage that they will drop the nominal 5V by about 0.2V. Without the diodes, if either the PC to which USB is connected or the external supply is powered off while the other is on, bad things can happen without the diodes.