# current limiting resistor for optocoupler (4N35)

#### 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.

Last edited:

#### onlyvinod56

Joined Oct 14, 2008
363
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.
How can i use the datasheet to find this resistir (47k - 68K)?

thanku

#### beenthere

Joined Apr 20, 2004
15,819
That's just experience. I probably stuck one in a breadboard and swapped resistors around until I liked the result.