PNP with Optocouplers

Discussion in 'General Electronics Chat' started by markdem, Jun 24, 2014.

  1. markdem

    Thread Starter Member

    Jul 31, 2013
    73
    38
    Hi All,

    Just got a quick question regarding the attached circuit.
    (Source:http://picmania.garcia-cuervo.net/r..._high-level_loads_with_optocouplers_83704.pdf)

    It says;
    "
    The voltage
    rating of the phototransistor is irrelevant since its
    maximum collector-emitter voltage is the base emitter​
    voltage of Q
    1 (about 0.7 V).
    "
    I don't understand how. If Q1 is conducting, would the voltage between the collector and emitter of the optocoupler not be whatever the source voltage is? I bet it is just my lack of understanding of something, but I just can't get my head around it.

    Thanks
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi mark,
    Are you mixing up fig #7 and fig #8

    Your drawing is fig 8 and your text clip for fig 7.???

    E

    EDIT:
    The voltage across the opto collector/emitter is never higher than the Vbe of the driven transistor. [0.7v]
     
    Last edited: Jun 24, 2014
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,773
    1,103
    If the transistor is conducting its emitter is at +V and its base is at +V -0.7. Therefore the voltage between the collector and emitter of the photo-transistor is +V - (+V -0.7) = 0.7
     
  4. markdem

    Thread Starter Member

    Jul 31, 2013
    73
    38
    Thanks for the feedback guys,

    Eric, No, I am not mixing them up. I just need the PNP version. I think the text is valid for both.

    I still, however, don't understand why the Vce is not at +V. What drops the voltage?

    Thanks again.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    Vce indeed sees V+. But the Vbe drop is only about 0.7V so that is the voltage the opto sees. Remember the Vbe is relative, not absolute to ground.
     
    Last edited: Jun 25, 2014
  6. markdem

    Thread Starter Member

    Jul 31, 2013
    73
    38
    That made me click. Now I get what is going on :)

    Thanks all.
     
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