Breakdown voltage in optocoupler

Discussion in 'General Electronics Chat' started by clintonb, Apr 19, 2016.

  1. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I had a problem for a while with optocoupler and triac interfaces. I was told that eventhough connecting the collector and emitter of the optocoupler to a 120V AC source, the voltage will never ever be 120V because of the breakdown voltage of the diac.

    What is the breakdown voltage exactly and what happens if the voltage supplied actually exceeds this value?
     
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,781
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    The coupler's isolation breakdown EMF and the Vce of the integral BJT are distinct specifications...

    The breakdown 'voltage' is that EMF at which the isolator/coupler's 'input device/circuit' to 'output device/circuit' insulation may be expected to fail...

    If the 'input to output' potential exceeds the isolation breakdown EMF the isolator/coupler will likely be destroyed with possible damage to connected equipment...

    But then, perhaps, I misunderstood the following statement?
    FWIW a 'DIAC' is a bilateral switching device (essentially a triac sans a gate) -- It exhibits no 'breakdown EMF' per se -- Perhaps you mean 'avalanche' or 'breakover' EMF (i.e. Vbo)? --- Such is merely that potential at which the device (abruptly) 'goes' from 'blocking state' to 'conducting state'...

    Provision of a schematic would greatly assist us to assist you:)


    Hope this helps!
    Best regards
    HP:)
     
    Last edited: Apr 19, 2016
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,781
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    @clintonb

    I see that I may have misunderstood your post -- please re-read post #2 (for the edits near the close of same):)

    Best regards
    HP
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
    1,950
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    I think it would help getting specific if we knew which optoisolator you were using? A circuit would help, too.
     
  5. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    0
    The circuit I was using is simply out of the datasheet. When I began going through the theory behind the circuit I ran into the gate resistor. Every tutorial and video seems to skip over this simply stating that it just is with different users coming right and others burning resistors. On one example they simply used a 1/4 ohm resistor on what I thought was a 220V potential difference.

    The best explanation I have so far is that the voltage isn't 220V but rather the breakdown voltage(as per explaination) of the optocoupler emitter-collector and therefore power rating is calculated as V=breakdown voltage and I=required current.

    The problem with this is that the resistor value changes depeding on the AC voltage which one would think would stay the same given the explaination.
     
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