Dead Teensy 4.1

YaniH

Member
Hello everyone!

I have just been developing a little project and my teensy appears to have died. I am trying to figure out if it is salvageable and what I may have done wrong.

THE MEASUREMENTS ARE PERFORMED WITH THE TEENSY REMOVED FROM MY BOARD AND USING USB POWER AGAIN

Currently, the 3.3v is pulled down (1ohm between 3.3v and ground which then creeps down to .5ohm) I am getting about .175v out of the 3.3v regulator and both the regulator (U4) and MOSFET (Q1) are getting quite hot.

Prior to the failure I was using an external 5v and have cut the USB supply voltage. I was using the 3.3v line to power the 3 74165s as well as the 3.3v for the button switching for those through a 1k resistor to ground. (I don't have a schematic at the moment).

This project has been working fine for the last 5 days the last thing I did before it popped was add one more button via the shift register. I did wire the 3.3v to a button that is connected to the LSB of the 3rd shift register. This singular button was wired to the 3v pin near the Vin and ground for my board. The chips and other buttons were wired to the 3.3v between pin 12 and 24. I can't see why that would matter but thought I would mention it.

My concern is that I am unsure what has popped and why. None of my io changed and nothing connected to the teensy is capable of exceeding 3.3vs.

Apart from too much current draw on the 3.3v line I can't think of anything that might have happened. I also was under the impression that there was overload protection on the 3.3v line.

Any advice would be appreciated. I would like to salvage this board if possible.
 
Attaching Schematic. I am less concerned with repairing the board an more trying to figure out if there is something that would cause this to fail. I am happy to buy another board but I want to avoid this failure occurring again.
 

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Did you reconnect the PCB pads again for testing the Teensy on USB power?

Paul
Hi Paul thanks for the response. Yes, I did. So for clarity, the schematic was the configuration when the board failed. I then removed the board and reconnected the 5v from USB so I could A. confirm that the board failed and that It wasn't being pulled down by my circuit and B. to be able to take measurements that may be more useful without my circuit affecting them.
 
Did you reconnect the PCB pads again for testing the Teensy on USB power?

Paul

Sorry to hijack the thread, but could you elucidate on that for me please? In my high current requirement project, do I cut the track and then feed just the 3v3 input, or as I assume, the 3v3 LO pin as well, for the audio board? If I then try to program the Teensy, does it work as normal without powering the rest of the circuit?
 
Can you show us a photo of the board with switches/shift-registers? There might be something there we can spot.
I assume you've only recently drawn the schematic and the board you mention is a proto-typing board with jumper wires. The schematic would certainly not work as there is no connection from last shift register back to Teensy in order to read the switches.

Can you tell us the full part number of the shift register? I assume it's not 74165, but a 74HC165 (the 74HCT165 would not work properly/reliably with only 3.3V on its power pin).

Only speculation....
Maybe the last switch you added is not connected thru the 1k Ohm resistor. The switch may have been wired (or completely bypassed) such that 3.3V gets connected directly to Ground thus burning out the regulator. Although, you'd expect the 0.5Amp fuse to pop first.

It usually boils down to accidentally touching a 5V signal (or power supply) to one of the Teensy pins.

Can you check the back-side of Teensy to see if any components have been knocked off?
 
Currently, the 3.3v is pulled down (1ohm between 3.3v and ground which then creeps down to .5ohm) I am getting about .175v out of the 3.3v regulator and both the regulator (U4) and MOSFET (Q1) are getting quite hot.
That sounds to me there is a short on the 3V3 line somewhere on the board. Did you visually check for shorts, e.g. solder blobs, metal particles, flux residue, etc?
I also was under the impression that there was overload protection on the 3.3v line.
I think the regulator is still operating - in current-limit mode though.

Paul
 
That sounds to me there is a short on the 3V3 line somewhere on the board. Did you visually check for shorts, e.g. solder blobs, metal particles, flux residue, etc?

I think the regulator is still operating - in current-limit mode though.

Paul
I agree it seems like a short on the board. I did physically check all of my header pins. I also brushed the board with a toothbrush in case there was small solder debris that found its way between pins. I am going to revisit it later. Thank you for your response.
 
Can you show us a photo of the board with switches/shift-registers? There might be something there we can spot.
I assume you've only recently drawn the schematic and the board you mention is a proto-typing board with jumper wires. The schematic would certainly not work as there is no connection from last shift register back to Teensy in order to read the switches.

Can you tell us the full part number of the shift register? I assume it's not 74165, but a 74HC165 (the 74HCT165 would not work properly/reliably with only 3.3V on its power pin).

Only speculation....
Maybe the last switch you added is not connected thru the 1k Ohm resistor. The switch may have been wired (or completely bypassed) such that 3.3V gets connected directly to Ground thus burning out the regulator. Although, you'd expect the 0.5Amp fuse to pop first.

It usually boils down to accidentally touching a 5V signal (or power supply) to one of the Teensy pins.

Can you check the back-side of Teensy to see if any components have been knocked off?
As stated the shift registers had been working fine for 5ish days. Probably about 30 hours of active use in that time. The last shift register works because pin 10 of shift register 3 goes to pin 9 of shift register 2 adding 8 bits to the bus.

I am positive the 1k was present but also irrelevant as I was unable to try using the added switch as the teensy never booted again.

Thanks for the response.
 
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Sorry to hijack the thread, but could you elucidate on that for me please? In my high current requirement project, do I cut the track and then feed just the 3v3 input, or as I assume, the 3v3 LO pin as well, for the audio board? If I then try to program the Teensy, does it work as normal without powering the rest of the circuit?
You cut the track under the board, supply 5v to the vin. You do not supply 3.3v to the teensy though this is possible WITH MODIFICATION. You are able to program via usb so long as the 5v supply is provided. This will provide power to the onboard 3.3v regulator supplying power to the development board.
 
I am going to assume at this point that some small piece of debris caused some sort of connection that popped something on my teensy. Thanks to everyone for their time and responses.
 
As stated the shift registers had been working fine for 5ish days. Probably about 30 hours of active use in that time. The last shift register works because pin 10 of shift register 3 goes to pin 9 of shift register 2 adding 8 bits to the bus.

I am positive the 1k was present but also irrelevant as I was unable to try using the added switch as the teensy never booted again.

Thanks for the response.
I agree with shift register connections from shift register 2 to shift register 3.
However, since there's no connection from shift register 3 pin 9 back to Teensy, Teensy cannot read the switches. Maybe you added a jumper barnacle and that's why it's not shown on schematic.

I suspect you had the 1k resistor in place. It's a question of it being wired as you had intended. I wouldn't be plugging in a new Teensy until you check this carefully. Maybe that 1k resistor is 0 Ohms?
 
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