current limiting resistor for optocoupler (4N35)

Thread Starter

onlyvinod56

Joined Oct 14, 2008
363
hello,
I need to use 4N35 optocoupler in my task.
I confirmed with the emitter side part.
My Vcc is 12V on both sides of the opto coupler.

Emitter side calculation:
From datasheet, Vf = 1.18V If = 10mA
So, R = (12 - 1.18) / 10m = 1.082K


what values i have to consider for the transistor side calculation?
 

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beenthere

Joined Apr 20, 2004
15,819
Optocouplers are not power devices. I usually arrange for more current through the emitter (more like 20 - 30 ma), and use a large resistor on the detector so the signal swings from 0 to Vcc. Something on the order of 47 - 68K in the collector should give a nice transition.
 
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