I wish to isolate an incoming signal from my 3.3v tolerant microprocessor. The parameters for the circuit are:
*Signal voltage 5V to 48V DC. (I don't know where in this range, as it's up to the manufacturer of the signaling device) *Microprocessor is 3.3V tolerant CMOS. *I'd like circuit to be as simple as possible. *I'd like circuit to be as small as possible.I've been trying to do this and have had some success using a zener diode to limit the incoming signal to the optoisolator to 3.3 Volts. Unfortunately, as the voltage is unknown ahead of time, the zener resistor value must be small to provide a full 10 mA to the optoisolator (NTE 3040, same as 4N27) at the low end of the voltage scale. This, of course, can make the Pz pretty high at higher voltages.
For example: A 3.3 V Zener Diode needs a resistor of 170 ohms for a signal of 5 volts to provide 10 mA to the optoisolator, having a Pz of
33 mW. The same circuit, when given a 48 volt signal will have a Pz of about 12 watts. This seems high.I'd really like some guidance on this topic. If there's another optoisolator, technology, or approach I should use, please let me know.
-Ken
PS: Excuse me for the overly explained problem. I'm very new to this and am figuring it out as I go.