# Any diode to protect an optocoupler with low voltage drop?

I have this design. By a possible mistake from user, the circuit would receive 12 V AC, and may destroy the optotransistor on reverse voltage cicle.

I can put a diode D1 before the resistor, but the diodes I already know, may have voltage drop or resist just small currents (300 mA), but the AC source can have 2000 mA.

I need a Diode with Low Voltage drop.

The low voltage drop is required to can handle the logical input at right side.

The 5.0 volts is not a source, it comes from a switch connected to a device sealed. When optocoupler gets on, the logical inputs receive a signal and the device acts.

Which diode can I use?

• is the load suppose to have a dc or ac voltage? – drtechno May 3 '18 at 16:21
• It might help us to know your intended circuit function...for any switch state, "logical input" gets 5.0 v. The opto-isolator does nothing. – glen_geek May 3 '18 at 17:09
• the 5.0 volts is not a source, it comes from a switch connected to a device sealed. When optocoupler gets on, the logical inputs receive a signal and the device acts. – Ando May 4 '18 at 3:40
• the load doesn't matter for optocoupler, its works for other circuit. – Ando May 4 '18 at 3:41

That's for the reverse direction. The maximum possible current for the forward direction will - approximately - be your peak voltage (about 17V: $12\cdot\sqrt{2}$ V) divided by R1's resistance. You haven't specified the resistance but you should be able to calculate that.