Optocoupler Turn On: Varying Emitter Voltage

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SUMMARY

The discussion centers on the operational characteristics of an optocoupler when varying the emitter voltage. With an input voltage of 5V and a voltage drop across the optocoupler of 1V, the total current flowing is 4mA. The Vcc on the transistor side is set at 12V. The consensus is that the transistor will turn on when the collector-emitter voltage (VCE) exceeds 0.2V.

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When exactly does a opto coupler turn on, if the emitter voltage is varied.
See attached.
Input voltage for the opto is 5V. Drop across the opto is 1v. Total current flowing is 4mA
The Vcc on transistor side is 12v.
What should the emitter voltage be, so that the transistor turns on.
 

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I'd say it can turn on when VCE>0.2V
 

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