I want to protect the inputs of my PC847 opto-isolator, which operates between a voltage of 3V up to 24V, from ESD events and extratension. My idea was to do something similar to the circuit I found on the web, but using an SMBC30CA as a TVS diode.
Now I have doubts. The resistor R3 is supposed to limit the discharge current of the ESD event, in some diagrams I find only the TVS diode without any current limitation, but in doing so the circuit that is inserted at the input would short-circuit if the voltage exceeds the breakdown voltage of the TVS diode. On the other hand, resistor R3, I believe, lowers the effective protection of the TVS diode. In other circuits I find instead of resistor R3 a resettable fuse, which would be a good idea, but I don't know how to size the two components.

Can anyone help me?

 

hi ST,
That TVS diode clamps at 28.2V, so the R3 resistor is required.

E

 

Hello,

I would also place an anti parallel diode accross the led of the optocoupler.
A led can be destroyed when the reversed voltage is higher as 5 Volts.

Bertus

 

"ESD" ?
Doen't that imply a fast very high voltage, like sneakers on carpet and suddenly tip of finger is at 17,000v ?

This is an ok read, but Littelfuse has tons of TVS MOV stuff to look at.
https://www.littelfuse.com/assetdoc...ssetguid=a12f2475-d207-4f54-a7d6-01c4d6eb435c

 

I also agree with Bertus’ suggestion.
But I like to add an anti parallel LED, and that way you have a visual indication that the circuit is “live”.