What is the purpose of this diode ?

Discussion in 'General Electronics Chat' started by yehdev_cc, May 7, 2014.

  1. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Looks like it's a reverse polarity protection diode.
     
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  3. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you very much for the reply,

    I see now, if the polarity is reversed, the circuit will function the same, I guess.

    I have one more question, does it provide any "protection" ? if the input polarity is reversed (input = GND) and the diode is not present there, what might go wrong in this circuit ?
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No, if the input polarity is reversed the circuit will not function, because there will be no current through the opto and discrete LEDs. That is, unless the reversed input voltage is high enough for the two LEDs to go into reverse conduction, acting like zeners. LEDs do not like this, and the diode you are asking about provides a lower voltage bypass path to GND to protect them.

    ak
     
  5. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Why won't the diode conduct ?
    If I understand the term "reversed polarity" right, instead of GND at the diode anode, there will be positive voltage, much higher - typically - than the forward ON voltage needed. And we'll get this voltage (nearly) at the same node on the zener cathode, just like the case of the normal polarity.

    did I get something wrong there ?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    The diode WILL conduct, but as noted, the circuit will not function in its normal way.
     
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  7. BobTPH

    Active Member

    Jun 5, 2013
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    The diode will conduct if the input goes negative. It will limit the voltage across the optocoupler to a diode drop, which will protect it from reverse breakdown.

    Bob
     
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  8. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you all, I much appreciate your answers.

     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    You can get AC input opto-couplers that have an inverse-parallel pair of IRLEDs, with one of those the reverse protection would be redundant - but you'd have to supplement the existing volt-drop zener by making that an inverse-series pair.

    The AC input optos are/were fairly common on dial up modems. Although some manufacturers decided it was cheaper to use a regular opto and add a bridge rectifier.
     
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