AVR-Chat

Well, I sent this message well over two weeks ago, so I'm not sure why  
it's being posted now, but, any way... :-)

Why not? The idea is to protect the microcontroller from a non-common  
mode surge on its I/O pins.

That is what an optocoupler is designed to do: as well as isolating,  
in practice, the microcontroller 'sees' any voltage spike only on the  
ground (common) connection, which is easily protected by proper  
earthing practices.

So even if the distant signal supply and the microcontroller power  
supply are on the same power outlet, any surge that comes back through  
the cable would not go into the I/O pin of the micro controller.

David Reaves


On Wed Jul 7, 2010 12:59 am ((PDT)) "Tim Mitchell" wrote:
> -----Original Message-----
> From: AVR-Chat@yahoogroups.com on behalf of David Reaves
> Sent: Sat 19/06/2010 17:07
> To: AVR-Chat@yahoogroups.com
> Cc: David Reaves
> Subject: [AVR-Chat] Re: ATMega16 I/O port protection diodes
>
>> This is exactly the purpose logic opto-couplers ("opto-isolators")
>> were invented for. Complete isolation, of several kV.
>>
>> You can buy them in SMD and DIP packages, in singles, dual and quad
>> configurations. A couple dollars and you're safe.
>> Very cheap insurance.
>
>
>
>
> No, the original poster was wishing to power an LED (for a railway  
> signal) down a long length of cable, without needing power supply at  
> the far end. An optoisolator is no use in this situation.
>
> -- 
> Tim Mitchell