Teensy 3.2 with USB and external power

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jrdarrah

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I looked at this post an I'm still confused.

https://forum.pjrc.com/threads/32383-Powering-Teensy-3-2-from-external-power-and-still-have-usb-in-with-power-cable-in-it

In one place it said to cut the trace to separate USB power from VIN and put a diode from one pad to the middle pad. Teensy 3.2 doesn't seem to have but two pads. In the backside diagram of the 3.2 which is the VUSB pad and which is the VIN? I don't want to waste a board by getting it backward.

My goal is to run the teensy on USB power while it is connected but have a 4.5V battery source to allow me to save off key data to the EEPROM before having the processor shut itself down via an external mosfet switch inline with the battery V+ feed. In the other post it said to use two diodes one for VUSB to VIN and one for the battery feed to VIN if I understand it correctly. If so which pad gets the anode and which gets the cathode?
 
pXrGO3s.png

097v2xe.png
0eNnxS5.png

green: VUSB
blue: VIN
red: cut
 
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HWguy, your drawing got me thinking about how to build it. I found that the BAT54C in a SOT-23 form factor has two Schottky diodes wired with a common cathode connection as shown in your first picture. I'm designing a shield which will make it easy to wire it.thanks.
 
I'm hoping that will be enough. I only need to power the teensy 3.2 and a 16 x 2 LCD. I'll do some measurements tonight.
 
I sacrificed a USB cable so I could measure the current that my project is drawing. It is only pulling about 46ma with the LCD display. The final version won't have the display so I'm ok on power.
 
HWGuy said:
View attachment 10033

View attachment 10034
View attachment 10037

green: VUSB
blue: VIN
red: cut
Click to expand...

Sorry for bringing up such an old thread, but I am working on a project with a Teensy 3.6 and have implemented the exact schematic as above, where i have a diode on voltage from the battery and also from VUSB, and I have cut the trace between VIN-VUSB. Now I am far from an electronics person and have some questions on what I should expect.

Right now when running of Battery, I see the same voltage on the VUSB pin, and if I have the USB plugged in, I see a voltage on the battery terminal. However if I am running on battery and plug in USB, it switches over to the USB power, which is what I expect. The drop in voltage across the diode is also what I expect, 0.3V. I thought a diode only let electricity flow one way, so why am I seeing a voltage on the opposite side of the diode.

The other confusing thing is I am measuring a voltage of 3.555 at the VIN pin (as expected with ~0.3V drop across diode), however I am only seeing 3.077V at VCC (3.3v pin). I am have measured the current of my sensors etc and not drawing that much, so I assumed I would see 3.3V.

Maybe this is electrical engineering 101, but I can't find a simple answer and have been searching for days. My system seems to work just fine, I just want to make sure I am not doing long term health damage. I have attached the pcb images from this project, in case I have made some dumb error thats easy to spot. Thanks in advance.


08d98399840ebaec70e20a4a5601d3df.png
386ed0853287814b77a2313f6353ede5.png
 
Thanks, funny I remember looking at that couple years ago for another project. That makes more sense, and a small package, so won't take up much room. Thanks a lot.
 
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