Zener before optocoupler

[loup-garou]

hi everyone,

I'd like to control my optocoupler (4N35) with a 22V (filtred but not regulated), its Led has the following carateristics : Vfmax=1.5v,If=10mA,
I was told that a Zener diode is useful in this case to stabilise the tension before the Led (do you agree.... ?), has anyone made such circuit ? which reference for the Zener should I take ?

a schematic will be highly appreciated !


thanks in advance.

 

[duffy]

"stabilise the tension" isn't telling us much, neither is your description of what you are trying to do.

The zener could be in series to subtract a certain voltage to insure the opto doesn't turn on before then, or it could be in a series-parallel configuration to insure it turns on early but doesn't burn out at a peak of 22V.

 

[crutschow]

Depending upon how bad the regulation is you could probably just use a resistor in series with the opto input to give the desired current. If you are just turning the opto on and off, then it should be pretty tolerant of variations on the 22V. You just want to make sure that the maximum voltage doesn't generate an opto current higher than its maximum input rating. The nominal resistor value would be (22-1.5) / 10mA = 2.0k ohm.