AVR-Chat

Clamping to the Vcc rail in this app might not be a good idea. If the
current draw of the entire cct is low, and the Ain voltage goes too high,
it'll just pull the rail voltage up too. So then you need a Zener on the Vcc
rail. So just use one on the a/d in, instead.

Regards
Roland


2009/8/12 Don Kinzer <dkinzer@easystreet.net>

>
>
> --- In AVR-Chat@yahoogroups.com <AVR-Chat%40yahoogroups.com>, "Dave
> McLaughlin" <dave_mclaughlin@...> wrote:
> > I would put a 5.1V zener diode across the input to the AVR ADC
>
> If you do so, you must be careful to select a size for the upper resistor
> in the divider so that the current is limited to a safe level for the zener.
>
> Most modern AVRs do have clamping diodes to Vcc and ground on all I/O pins
> but you can add an external clamping diode (instead of the zener). The
> external diode should have a forward voltage lower than 0.5V since that
> appears to be the forward voltage of the internal clamping diodes. Here,
> again, the upper resistor in the divider must be large enough to limit the
> current to a safe level for the clamping diodes.
>
> Don Kinzer
> ZBasic Microcontrollers
> http://www.zbasic.net
>
>  
>


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