Killed my first Teensy! Now, why...

Hi all,

Well, I finally killed my first Teensy. Sadly it was a 4.0, and while I have a bunch of 3.2 on hand, I am now waiting for more 4.0 to continue my project.

While I'm waiting, I'd like to try to figure out what exactly I did so that I don't do it again.

What I was working on was trying to convert a project using a T3.2 to 4.0. It communicates with a 5V I2C display driver, so I added a level shifter. All I had on hand was a clone of this MOSFET shifter from Sparkfun:

https://www.sparkfun.com/products/12009

I connected the GND line to ground, the HV line to Vin, and the LV line to the 3.3V line. Then I connected two of the LV In lines to two Teensy pins. I connected the other side to my display driver, plugged in the USB and got...nothing. Scoping the 3.3V pin I see a low voltage oscillating signal, like the 3.3V regulator is trying to start up and failing because of a short.

So I'm guessing HV got into the LV side and fried the chip. I'm also guessing it's because of the questionable quality of the level shifter I used. I've ordered more Teensy 4.0s and some original Sparkfun level shifters.

Is it safe to assume that this circuit should be safe? These clones use MOSFETs with no markings so it's impossible to tell what they are. They ones on the board I tried do not meter out correctly, but ones on a fresh board do. It's possible that the MOSFETs were bad to start, or whatever happened took them out as well.

Finally, I had an audio board connected at the same time. I can't test it until I get another T4.0, but if I killed the SGTL5000 as well, is it likely that connecting it to a new Teensy then kill the Teensy?

Thank you!

I have used the Sparkfun level shifters and have never had issues. They use BSS138 transistors. If you look at the documents tab of the product page you mention, you will see they have a link to the BSS138 datasheet as well as a schematic of their board.

Hi Kurt,

Awesome, thank you for the info, I'm glad to know they are safe and that they work. One of my concerns was that perhaps if the voltage regulator on the Teensy was slow to start up there would be a time where there was HV present, but no LV and that might cause issues. Indeed, that might be the case with the knockoff MOSFETs on these boards. Hard to know. Perhaps at some point I'll wire one up to a couple of power supplies and see if I can catch it sending a spike on the low voltage side.

Were you using it for I2C?

Thanks!

I2C - Maybe... I have used them for several different things, like driving LEDS, or Uarts for driving servos.

Example one of the last two boards I had fabricated were setup to use 4 of these transistors (the other one two). The psuedo picture below was one to talk to Lynxmotion Servos and their adapter board. Two of them were to drive servos at 5v. The other was to talk through their board to XBees. The XBee part is sort of screwy as XBees talk at 3.3v, but their board upshifts it to 5v to talk to Arduino UNO boards, so I then have these two to go back to 3.3v... So again in this case all of them are for Serial communications


You will notice the 4 transistors and all of the resistors near the bottom left of the board