Can a 7805 kill a Teensy?

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Veit

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I just killed two Teensy 3.6 in a row and would rather not turn this into a new habit of mine ...

It seems that it has something to do with a faulty switch, please have a look at the attached schematic. This switch did not close reliably but had a faulty connection. When I touched it, it interrupted the connection briefly - and killed the Teensy. When I took the Teensy out of the circuit and connected it to the USB-cable (only) it got hot.

Could it be, that a short opening creates some spikes or a backflow of current that kills the Teensy? The little amp that is also hooked to the 9V power supply draws about 20 mA.

powerSupplyCircuitThatFriesTeensy2.png

If so, can I help the situation by adding some diodes in front of the 7805? Or is there any other recommended way about how to create an absolutely foolproof power supply? I have now ordered a Teensy 3.5 for a replacement. Is this design more robust against spikes?

Any tips or advice is highly appreciated!
 

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If your schematic corresponds to reality, then the switch is not the culprit, but the 7805 missing the ground connection. Without the latter, the output voltage will float up to 8.5V which is definitively too much for whatever Teensy.
 
yes, right after I had posted the diagram I noticed the missing line myself. Of course it is there in reality and I added it five minutes ago to the schematic. The circuit had worked for a while, only when I moved the whole amp to another table I must have created these short disconnects. The amp has a pre-amp and a poweramp-section which drives a speaker, so there are some inductivities around (my project is a guitar amp and I use the Teensy for sound shaping purposes).

On top of the Teensy rides an audio shield and there are links from line-in and line-out to the amp. However, when I blew the second Teensy these connections were unplugged, so I left them out in the drawing.

Would a diode to the left of the capacitors help to prevent current backflow?
 
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Before you discard those destroyed boards, maybe try disconnecting everything and join the VIN-VUSB pads if you cut them apart, to see if they magically work again when used only with USB power? Who knows, you might get lucky. It's happened several times before...
 
Thanks, I will try that. Or at least something similar - I never cut any connection on the board but used a USB-cable with the red wire cut, when I changed the software. If I now connect an intact usb-cable then the one Teensy will get hot, while the other one simply does not react at all. I might try to connect the 3.3 Volt to the boards. I only need to find out where. Pin 3 is an output, so it is probably not the right pin to attach the power supply to.
 
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