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Thread: I just fried another Teensy, need an idea to stop

  1. #1
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    I just fried another Teensy, need an idea to stop

    I have fried 2 Teensy's now both in the same fashion. Using an Op Amp to control a high voltage device. The Op Amp fails and allowed the higher voltage to back feed into my Teensy. The last one I was using pin A14 driving the the op amp as a non-inverting amplifier. so 0 to 3.3V was outputting 0 to 23V on the op amp. I need to protect my Teensy's from both 27Vdc and 26Vac that are running around my circuits.
    Rob

  2. #2
    Senior Member PaulStoffregen's Avatar
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    As a first step, place a resistor between the Teensy A14 pin and the rest of the world. If something horrible happens on the other side the resistor, the voltage will mostly go across the resistor. With a little luck, the resistor will limit the current which flows into Teensy to a safe level.

    A value like 4.7K to 10K provides a lot of protection, but even lower values like 1K or 470 ohms will help.

    If you want to do even more, use 2 resistors in series, and places a zener diode between their junction and ground. The cathode (side with the stripe) connects to the resistors and the anode connects to ground. Use a 3.6V or 3.9V zener. Then if something horrible happens, most of the voltage will be between the opamp and the zener, with most of the current that would have flowed into Teensy being diverted to GND through the zener. Depending on the current, the zener might go to 4 or even close to 5 volts, so you still need the resistor between the zener and Teensy.

    Of course, the trouble is this adds resistance between Teensy and your opamp. Maybe that's a non-issue, if you're running the signal directly to the positive opamp input, and if your opamp's input bias current is low, or if you don't care about the tiny extra error due to the opamp's bias current flowing through that resistor. If you do care, often a similar resistor can be added somehow on the negative side of the opamp, which causes a similar voltage error on the other side, so they mostly cancel each other. As will all analog circuit design, small details can range from being insignificant to contributing terrible errors.

  3. #3
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    Thanks Paul, I will give that a shot.

  4. #4
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    Thank you Paul, from my side as well.

    The other question is: WHY did the Op-Amp fail? Maybe it's worth looking for the root cause...

  5. #5
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    The Sim was running on a flight and there was a very brief power flicker in my utility. So quick the clocks never lost time but did start flashing. Well the pc's shut off therefore cutting off power to the teensy and and the op amp. The op amp literally blew a chunk of plastic off its top. No melting at all! Not sure what happened as I was upstairs when it occurred. Power surge? Reverse current? Not sure but it took the op amp and Teensy out together. The op amp was only hooked to the input of the Radio Altimeter.which is a low current 0-30Vdc input to an internal op amp as well.
    Rob

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