I'm a bit confused, the LTV826 Optocoupler spec sheet says 20ma for full transistor turn-on with the max being 50ma. Using 24 volts I come up with a 1.2K series resistor.

Sealevel systems sells a DIO card using this Optocoupler and recommends a 3.3 k resistor for 24 volts which is only 7.2ma. The input is basically two back to back LED's.

I plan on using this Optocoupler to pull the data line inputs for a Teensy micro low from a 24 volt source. My initial thoughts were to use 1.2k but wondering why a reputable manufacture would suggest 3.3k.

Any ideas?

 

It's not like there is a cliff there where 1.2K is safe and 3.3K is death.

The spec sheet (that you didn't link to expecting everyone to have one on hand) I found didn't have many detailed specs, but it did have current transfer ratio of .5. That varies with current but is helpful as an approximation... so 3.3K gives 7.2mA of drive which should give you 3.6mA at the transistor.

This should be plenty to drive a logic input.

 

Sorry, didn't link the spec sheet because I thought all that was important was the 20ma spec. Guess I should have also stated I am holding the micro input high (5v) and using the transistor output of the Opto to make it go low.