I'm looking to use an optocoupler+MOSFET to switch "undefined" DC power sources; the opto I'm looking at is FOD817D and the MOSFET is FDPF55N06. The power sources may range from 5V (USB power) to 24V, but I could see this being used to switch as high as 36V at some point in the future, and Digikey lists the MOSFET's Vgs(max) as ±25V... because the power source may be as low as 4.8V I didn't want to use a voltage regulator to limit voltage, so I was thinking I could clamp it with a zener; something hefty like a 1N5355B so it wouldn't clamp until 18V, and can handle up to 5W.
Is this a reasonable idea, and if so, what size (resistance and wattage) resistor should I have for current limiting that would allow for fast enough switching, but still permit enough current to run everything? If not what alternatives are there? Most regulators that can take input voltages ≥36V seem to have a dropout of at least a volt, which would mean that when running on a voltage of 4.8V (USB with some power loss) I'd end up below Vgs(th) and the MOSFET wouldn't be fully on.
Is this a reasonable idea, and if so, what size (resistance and wattage) resistor should I have for current limiting that would allow for fast enough switching, but still permit enough current to run everything? If not what alternatives are there? Most regulators that can take input voltages ≥36V seem to have a dropout of at least a volt, which would mean that when running on a voltage of 4.8V (USB with some power loss) I'd end up below Vgs(th) and the MOSFET wouldn't be fully on.