I'm looking for help and unfortunately stuck for time on a project, so hoping someone wouldn't mind helping me out.
I am trying to calculate the resistor value required for an opto-isolator circuit. I am powering a 24DC magnetic reed switch passing ground through a Siemens SFH615A:
http://docs-europe.electrocomponents.com/webdocs/009c/0900766b8009c194.pdf
Can someone recommend a resistor value, thank you very much.
I have used the calculator available here: http://led.linear1.org/1led.wiz
with a 24V supply, 1.5 forward voltage and 60mA forward current.
I would be grateful if someone could double check
Best Answer
The resistor value is calculated like any common LED
$$ R= \frac {V_{in}-ledV_F}{I_{LED}} $$
The actual value depends on the current you want through the LED
For 60mA and typical VF= 1.25v
$$ R= \frac {24v-1.25v}{{0.060A}}$$ which results to about 380 Ohm
If you want to calculate for VF= 1.5v $$ R= \frac {24v-1.5v}{{0.060A}}$$ which results to about 375 Ohm
Peter Bennett made a good point in the comments below about the resistor power rating, make sure it is appropriately sized.
The resistor power is calculated as
$$ P= {I}^2 * R = {0.06}^2 * 380 ohm = 1.368W $$ so a 2W resistor should be good enough