I manufacture a device that uses a 0-5V TTL data input to a 4N35 optocoupler. I have a 47 ohm resistor and T 1-3/4 LED in series with the optoupler to give me a visual input status and current limit. All works fine in normal use, but it blows the resistor, LED, and/or resistor when customers accidently connect the 24V power supply input to the data input connector. I need to come up with reliable overvoltage/overcurrent protection on the data input. A fuse is not an option since the device will be potted in epoxy. I was thinking that replacing the 47 ohm resistor with a PTC would be a good option. Is there a better way to do this?