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  1. #1
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    why does optcoupler behave like this?

    Just bread boarded 4N35 circuit. Input side 5V, 330R on anode. Cathode side grounded. On transistor side, collector connected to +24V. Emitter connected to 10K resistor to ground. Everything works fine. The output is 24-VCEsat. If I now connect base terminal to 100K and ground it. The voltage across emitter is 4V and not 24-VCEsat as expected. It goes into linear region. If I keep increasing RB, the voltage across emitter keeps building up.I guess, RB is a speed up capacitor. Why does it behave this way?

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  2. #2
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    Re: why does optcoupler behave like this?

    The light tries to turn the transistor on. The base resistor tries to turn it off (it "steals" the base drive).
    The circuit will stabilize where these effects cancel each other.



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  3. #3
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    Re: why does optcoupler behave like this?

    Imagine a photo transistor as a high current gain transistor with a photo diode across CB junction. You can easily predict the observed behavior.



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