Multisim and Ultiboard

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4N35 optocoupler doesn't make sense

Attached is a document showing 3 simulated circuits..
Circuit 1 and Circuit 2 is similar except the LED polarity (direction) is opposite. Circuit 3 is NPN transistor simulation.
For some reason, Circuit 1 amd Circuit 2 simulation doesn't make sense.
Does anybody know why or can anybody please explain..?
I have search the internet to check my connection and it is correct..
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Add a resistor to the Positive terminal of V1 to the opto-coupler and it works fine. I not exactlysure why the origninal circuit is doing this. I thought the Beakdown Voltage of the opto-coupler diode was being exceeded, but the model indicates that the BV=infinite. Maybe Nestor would know at little more in that area than I do. It could be that with that much voltage it is overdriving the opto-coupler LED in the forward direction and exceeding the maximum forward current. Suffice it to say that with the resistor everything works like it should.


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