How many volts can be applied to the input side LED?

In principle, a photocoupler can be operated by applying a forward voltage equal to or greater than voltage V_F to the input side LED.
At that time, while there is some degree of flexibility for a power supply voltage, input forward current I_F must be kept below the absolute maximum rating by using the current limiting resistor.
In the reverse direction, a photocoupler must be used within the specified range of input reverse voltage V_R.

Example: Case of transistor-output photocoupler TLP185 (SE

Note 1: Pulse width ≦100㎲,f=100㎐