Hi Forrest,

Beware that the "knee" of such low voltage zeners is often very soft.
In other words, a 3.3V zener may start to break down at 2.5V and not
really conduct well until 4.5V.

Sean


On Sun, Oct 26, 2008 at 8:59 PM, Forrest W Christian <forrestc@imach.com> wrote:
> I'm using a Zener diode as an input protection diode for a 16f886
> running at 3.3V.   Currently I have the circuit designed with a 3.0V
> zener (1n4619) between the input pin and ground (reverse biased of
> course).   At 3.3V, Vih (min) on the '886 looks like either 1.9V or
> 2.64V depending on whether I am talking a TTL or Schmitt trigger input,
> so I'm expecting a 3.0V zener to be perfect for this purpose - high
> enough so that the voltage is over Vih, but low enough that the input
> will never exceed Vcc.
>
> That said, a lot of the sample circuits I've seen seem to prefer a 3.3V
> zener for this purpose.   Other than not thinking through the design
> completely (and the intuitive idea that we want to clamp at 3.3V, so we
> should use a 3.3V zener), is there a reason why the examples show a 3.3V
> zener and not something lower, like the 3.0V zener I currently have
> specified?
>
> For those of you who will want more information such as what the input
> circuit looks like....  This is for a switch-closure input.   From VCC
> to the PIC Input: I have a 1n5819 Schottky diode connected through a 10K
> pullup resistor to the external switch input.  From the external switch
> input, it goes through another 10K pullup resisitor to the PIC input and
> the cathode of the diode..    Of course, the anode of the diode is
> connected to Vss, (as is the other "side" of the switch input).  The
> theory being that a voltage accidentally injected at the input (in
> relation to the ground), will not be able to raise the VCC rail due to
> the 1n5819, and the 10K current limiting resistor will limit current
> through the zener to ground.
>
> So, am I correct in thinking that the 3.0V zener is the correct diode
> for the job, or is the 3.3V more appropriate?  (It's too bad that 3.15
> or 3.2V isn't an option).
>
> -forrest
>
>
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