Quote:
Originally Posted by Martin G7MRV
These are bipolar of course, but would the junction voltage drop make any real difference in this case?
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As long as you drive the optocouplers fully on, there should be next to zero voltage drop across the collector-emitter terminals. You're maybe thinking of the base to emitter forward voltage which doesn't apply here. Obviously, they are polarity sensitive devices so the collector of the output transistor has to go to the 'most positive' side of the gap which the transistor will be bridging.
The only voltage drop to worry about here is the forward voltage drop of the photodiode, where you will need more than a specific minimum voltage to turn the diode on. I generally get away with using 330R series resistors when driving optocoupler inputs from 3V microprocessor pins.