Teensy USB Power Supply Fighting with Linear Voltage Regulator


Well-known member
Hi, I recently made a PCB for a Teensy project that uses a linear voltage regulator to step down 12V used for some analog audio circuitry to 5V to power the teensy. This is working well, except that when I plug in the USB for programming I notice the linear regulator starts getting pretty hot. I'm assuming this is because the 5V provided by the regulator and the 5V for USB are slightly different levels, and are therefore fighting over which supplies the board.

I know the Teensy can be modded to disconnect USB power from the 5V input pin, but I'd like this project to be plug and play with new Teensys and also I occasionally would like to be able to power the digital portion of the circuit through USB only for testing purposes. Currently using a hacked data-only micro usb to program when the main 12V power supply is plugged in as a workaround.

I'm wondering if there is a good way around this in the circuit design. Would doing something like putting a 1N4001 diode in series with the regulator output prevent this voltage conflict without requiring the above workarounds?

Here's what a theoretical schematic would look like:

Screenshot 2023-06-19 at 11.58.42 AM.jpg
Yes, a diode from the output of the 7805 regulator to the Teensy Vin pin will avoid the regulator heating up when you power your Teensy from USB.
Ran into this issue recently: an unpowered L7805CV did not like to be powered with 5V on its output pin - it got hot and shutdown its output.
I inserted a 1N5817 Schottky diode between regulator output and Vin pin. Problem solved. A 1N4001 will probably do as well.

The diode trick will work well as long as the .7 volt drop is OK with your circuit design.

Yes, have a diode from your external power supply.
The 0.7V drop (or 0.3V if using a Schottky diode) - does not matter, it is regulated down anyway to 3V3
(still enough margin for onboard regulator).

If you see schematics:
- the VIN from USB goes directly to VIN pin
- if you power on VIN (with 5V) - it "fights" against the USB! It can damage your computer!
(and the external LDO might not "like" to see a power/voltage driven in backwards)

It looks like, there is a solder bridge (open for external power, but powering from USB will be "broken").
So, a diode in series on VIN should do the job. Just make sure to have enough amperage supported (e.g. a 1A diode, there is a fuse for 500mA).

I assume, you will not use the 5V for other stuff (the MCU is not 5V tolerant). So, 5V from external PSU, with a diode,
onboard 3V3 regulator and 3V3 out on pin.