Over voltage protection from piezo

Discussion in 'General Electronics Chat' started by seayaker, Jan 21, 2018.

  1. seayaker

    Thread Starter Active Member

    Jan 27, 2009
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    I want to protect a micro-controller board from voltage spikes generated piezos. The board can only handle 3.3 volts, would just a 3.3v zener diode in parallel work? I'd like to keep it as simple as possible and I read that a resistor in series has an effect on the signal, Also, If there are more than 1 triggered at the same time even though they're on different pins does that add to the over all voltage in the board?
     
  2. Sensacell

    Moderator

    Jun 19, 2012
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    Schematic of your plan?
     
  3. dendad

    Well-Known Member

    Feb 20, 2016
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    545
    A series resistor from the piezo to the ADCpin, with a 3.3V Zener from the pin to Gnd. Cathode (stripe) to the pin.
     
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Put the zener on the piezo side of the resistor.
     
  5. dendad

    Well-Known Member

    Feb 20, 2016
    2,101
    545
    That would most probably work ok too in this case as the piezo is high impeadance.
    I would do it the other way though.
     
  6. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    The other way round, on the negative excursion of the piezo signal, The forward conducting zener would be directly parallel with the PIC protection diode and it would be a toss up which way the current goes. Having the resistor between the zener and the chip will keep the current through the diode in the chip to an absolute minimum.
     
  7. m.majid

    Well-Known Member

    May 28, 2008
    53
    3
    AVR MCUs have internal protection diodes
    Check datasheet
    if your mcu doesnt have some, you may add yourself
    Here is from ATmega328 datasheet :
    _20180121_163122.JPG
     
  8. ebeowulf17

    Distinguished Member

    Aug 12, 2014
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    Right, but it's quite easy to overload them. They're tiny and can't dissipate much power before failing. It's better to add external protection in situations where you expect to encounter out of range voltages.
     
  9. seayaker

    Thread Starter Active Member

    Jan 27, 2009
    36
    0
    what about a varistor?
     
  10. ian field

    AAC Fanatic!

    Oct 27, 2012
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    The piezo has capacitance, so a sensible precaution is to diode clamp that input to Vdd so switching off cant collapse Vdd faster than that input.

    The input probably already has Vdd and vss clamp diodes, they can probably only handle about 10mA so external ones are worthwhile. A hefty piezo could bump Vdd up, so you need either Vdd voltage protection or an input clamp.
     
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