My understanding is that the Current Transfer Ratio of an opto-coupler defines the relationship between the current flowing through the LED, and the maximum supply current of the output. e.g. "5ma through the LED means a maximum of 15ma can flow between the emitter and collector"
Some datasheets show illustrate this relationship using Normalised Current Transfer Ratio which I'm finding quite a bit harder to grasp.
The opto in question I'm using is the MCT62 (http://www.mouser.com/ds/2/149/MCT62-186891.pdf). I need around 50ma to supply the load on the opto's output.
What calculation do I need to use to determine the required resistor on the LED?
Also, how can I determine what resistor on the LED can deliver the most current?