Back-to-back Diodes?

Discussion in 'General Electronics Chat' started by blah2222, Jul 27, 2012.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi, I am starting to look into EMG/ECG circuits and was always curious about patient protection from an improbable surge of current through the electrodes. Though highly unlikely, I was wondering if that's what the use of these back-to-back diodes in the following schematic are for:

    [​IMG]

    IN+/IN-/BODY are the three electrodes.

    From my understanding, the ac electrode signals will be too weak to turn on the diodes so they will pass unaffected, but any significant DC voltages will be clamped at 0.7 V to protect both the person and the circuitry?

    Does this sound about right?

    Thanks again,
    JP
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Close enough. You really don't need all four, just figure if they start to conduct if the DC voltage exceeds the power supply voltage, and you should be safe. Static can be thought of a form of DC for this purpose.
     
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  3. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    33
    Can these just be the standard 4000 series? Also, do you mean six not four?

    Thanks
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Actually, two.

    Even BJT chips can be blown, you know.
     
  5. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    33
    Sorry maybe, I am not following but I see three pairs of diodes on that schematic, where is this number two coming from?
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    I don't know what the requirements are for circuits connected to humans, but if I only needed signals that were a fraction of a diode drop in magnitude, I would put back-to-back diodes between every pair of lines touching the patient or that could connect one of those lines to the supply or ground, if possible. If there even a chance that those electrodes could accidentally contact someone where their skin resistance had been compromised (such an a wound or during surgery), I don't want to let even 9V get to them. Performance issues arising from that policy could be examined and mitigated on a case-by-case basis.
     
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