Dear all,
I am designing an project for automobile application , I which I require to protect my circuit from all possible type of practical failure.
For this project I have decided to build my own arduino board .
I will be taking 4 field digital input into my board , Fields input voltage will be in 20-28VDC(For logic 1) and 0 for logic 0.
To protect the circuit from high input voltage I have used optocoupler PC847 , and to protect the Inputs from ESD i have used TVS diode array SP0505BAJT. ( 5 diode array ,4+1 spare i have kept for future)
As I am designing this type of circuit for the first time , I require your view whether the selected circuit is okay or not, The value of resistance that I have used is okay or not and the most important TVS diode that I have selected is okay or not .
Kindly also provide if any additional type of protection will be needed for input.
Opto isolation is a good step. Where in the diagram have You fitted the magic TVS diodes?
Please find the area marked in image
You probably don't need the diode array with the opto-isolators. An LED with a series resistor isn't particularly sensitive to static discharge. (There are always exceptions like lightning strikes. 
)
The value of resistance that I have used is okay or not and the most important
The values look a little high for the current into the opto-isolator's LED, and if you're trying to make a [u]voltage divider[/u] the "bottom" resistor should go to ground.
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Go ahead and try it in reality.
https://www.mouser.in/datasheet/2/240/Littelfuse_TVS_Diode_Array_SP05_Datasheet.pdf-777043.pdf
Datasheet link for the diode
DVDdoug .
As my application is in automobile , we require ESD protection. And online also i could not find a single source stating the optocoupler or led with resistance in series will not be sensitive to ESD. Kindly provide some reference or explanation please.
Voltage Divider :- Definitely i was trying to create a voltage divider but as the 1K ohm resistor through optocoupler is connected to ground I thing no additional grounding will be required.
I have create a voltage divider as the reverse standoff voltage of selected tvs diode is 5.5 and my input is 24.
with selected value of resistance (7.5+1 K ohm) with 24V dc input 2.68mAmps of current will get generated. It is right opamps can take uptill 20mAmps but what is the point of giving high amps to it. high amps will only increase the power loss across resistor.
Is there any down side of using opamps under low current.
Also is there any lower limit for current is there for the opamps , as in the datasheet it shows the characteristic curve for 0.1 mAmps also.
