5V on Vin leads to 4.8V on 3.3V pins?

[Eumldeuml]

Hello all!

Today I noticed a very strange issue with my relatively new Teensy 4.1: When I switched from USB power to external power I noticed that there is 4.8V on the 3.3V pins. Before attaching external power I disconnected the USB cable of course.
When applying less than 5V to Vin, the 3.3V pin also reads lower voltage until Vin is approx. 4V. Then I read 3.3V on the 3.3V pin next to Vin, while the one on the other side of the board is about 0.2V lower. The weirdest thing about all this: My Teensy still works, as far as I can tell. Everything else I read in this forum indicated that a voltage on these pins only a little higher than 3.3V instantly fries the whole board.
I tested it without anything attached to the board with a multimeter and an adjustable bench power supply. When on USB power, everything is as it should be, just not when applying external power.
How can this be? Should I be worried or just carry on since the Teensy apparently doesn't really care?

Thanks in advance!

 

[tonton81]

This usually happens when your DMM battery is low (offscale readings), also make sure you have a good ground path when testing voltages

 

[Eumldeuml]

I don't think it's the DMM since every other measurement I took was the expected value (e.g. the voltage output measured at the power supply matched the displayed voltage pretty close).
There is indeed a possibility that it has something to do with the ground path (maybe the dreaded ground loop?) but I don't know how I could mitigate that especially since there was nothing else connected to the Teensy except USB or Vin when tested.

 

[mjs513]

When applying less than 5V to Vin, the 3.3V pin also reads lower voltage until Vin is approx. 4V. Then I read 3.3V on the 3.3V pin next to Vin, while the one on the other side of the board is about 0.2V lower. The weirdest thing about all this: My Teensy still works, as far as I can tell. Everything else I read in this forum indicated that a voltage on these pins only a little higher than 3.3V instantly fries the whole board.

Confused the pin next to Vin is a GND pin or am I reading this wrong. It goes Vin, GND, 3.3v

 

[Eumldeuml]

Confused the pin next to Vin is a GND pin or am I reading this wrong. It goes Vin, GND, 3.3v

You are right, I could have put that a little more clearly. What I meant is the 3.3V pin that is directly next to the section where also Vin is located. This was only to differentiate from the 3.3V pin in the middle of the opposite side of the board. Sorry for the confusion!

 

[mjs513]

In that case you really are getting a strange result. Right now I am powering a T4.1 from a 7.1v LiPo battery through a 5v regulator to the Vin pin without any issue and am reading about 3.3v on the 3.v pin. So not sure what's going on. If you power the T4.1 through the USB cable instead of the cable do you still read 4.8v? Just a double check.

 

[Eumldeuml]

In that case you really are getting a strange result. Right now I am powering a T4.1 from a 7.1v LiPo battery through a 5v regulator to the Vin pin without any issue and am reading about 3.3v on the 3.v pin. So not sure what's going on. If you power the T4.1 through the USB cable instead of the cable do you still read 4.8v? Just a double check.

Nope, when powered through USB everything measures as expected.
I'm really not sure if and what I should do about it, especially since it doesn't seem to be a problem (for now?). If I didn't measure the pins I never would have noticed anyway. And since the Teensy was connected to Vin for at least several minutes and it didn't kill it, I can only imagine that somehow I'm measuring something wrong. On the other hand I don't really know how you could screw up a simple voltage measurement between Ground and V+...

 

[KurtE]

Maybe something else connected up? Maybe only the external power only, that is feeding in power to on of the IO pins or the like? Needless to say not good.

Like how are you measuring the voltage going in? Do you have it connected to an analog PIN?

In all cases like this, I disconnect everything (or try to) except the VIN and GND and then measure...

 

[Eumldeuml]

Maybe something else connected up? Maybe only the external power only, that is feeding in power to on of the IO pins or the like? Needless to say not good.

Like how are you measuring the voltage going in? Do you have it connected to an analog PIN?

In all cases like this, I disconnect everything (or try to) except the VIN and GND and then measure...

When I first discovered it, everything else (a level shifter and a stepper motor driver and an encoder) was still connected. To rule out any interferences with those components I unplugged the Teensy from the breadboard but the problem still persisted.