Thanks for the suggestion. I just had my 2nd fault, and also in this case
it was the 0603 10uF 6.3V capacitor that failed short. The MPU is sane.

Is it because it's a cheap chinese capacitor bought on eBay, or such a cap
is intrinsically delicate and requires ESD protection even more than the
MPU itself?

Thanks a lot.
Mario


At 13.54 2013.05.19, Isaac Marino Bavaresco wrote:
>First you must ensure the Zener will not interfere with your signal.
>Zeners have capacitance and current leakage. No problem if your signal
>is slow and of a reasonably low impedance.
>
>I always use some kind of protection to MCU pins that have off-board
>connections.
>
>One circuit I use is comprised of a series resistor from the connector
>pin to the Zener's cathode (Zener anode to ground, obviously) then a
>second resistor from the Zener's cathode to the MCU pin. If I need a
>pull-up I connect it to the Zener's cathode also.
>
>If the resistor's values are correctly chosen you can communicate
>bi-directionally (e.g.  I2C).
>
>The first resistor is to limit current into the Zener in case of an
>accidental connection to +12V.
>The Zener besides protecting against ESD will protect against continued
>connection to battery voltage.
>The second resistor will protect the MCU's internal parasitic diodes to
>VCC in the case where the MCU is not powered and the external connection
>is being supplied a voltage large enough to the point that some of the
>Zener current is diverted to the MCU's pin and damage it.
>
>
>Best regards,
>
>Isaac
>
>
>Em 19/05/2013 08:17, Electron escreveu:
>> Hello!
>>
>> An ECU uses some PIC analog pins to connect to external sensors
>> that return 0 .. 3.3V analog signal, like e.g. a TPS.
>>
>> 0603 10uF 6.3V capacitor in parallel to the PIC input, to smooth
>> things out.
>>
>> Now I wonder if a zener is necessary too, to protect from ESD.
>>
>> OK, they won't harm. But are they really necessary in your opinion?
>>
>> PCB's are already made and space is little, more hand work is non
>> desiderable etc.. so if they are really useful OK, else why add them..
>>
>> Thanks,
>> Mario
>>
>
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