current limiting resistor for optocoupler (4N35)

Discussion in 'The Projects Forum' started by onlyvinod56, Apr 21, 2009.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    hello,
    I need to use 4N35 optocoupler in my task.
    I confirmed with the emitter side part.
    My Vcc is 12V on both sides of the opto coupler.

    Emitter side calculation:
    From datasheet, Vf = 1.18V If = 10mA
    So, R = (12 - 1.18) / 10m = 1.082K


    what values i have to consider for the transistor side calculation?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Optocouplers are not power devices. I usually arrange for more current through the emitter (more like 20 - 30 ma), and use a large resistor on the detector so the signal swings from 0 to Vcc. Something on the order of 47 - 68K in the collector should give a nice transition.
     
    Last edited: Apr 21, 2009
  3. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    How can i use the datasheet to find this resistir (47k - 68K)?

    thanku
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That's just experience. I probably stuck one in a breadboard and swapped resistors around until I liked the result.
     
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