Thanks for the link. Also found this one on the wiki:

http://en.wikipedia.org/wiki/Linear_regulator#Simple_zener_regulator

However..... just thinking what if the Vdd regulated to 5V and the input pin
does not? The doc says should not exceed by more than Vdd+0.3V... That leads
to other problems as I suppose if the Vdd is much more than 5V (7 for
example) then the data line also pretty much the same or might be just a bit
less.

So two options come into my mind:

1. regulate somehow the input as well which may or may not alter the signal
a bit

2. leave it as it is and pray :-)

Opinions?

(3. use optocoupler or whatever? But then the additional components will
cost more than the chip itself...)

Thanks,
Tamas



On 11/08/06, John Chung <kravnus@yahoo.com> wrote:
>
> Took me some time to understand your idea.
>
> http://www.tnt-audio.com/clinica/regulators_noise3_e.html
>
> A picture of what it is suppose to be(based on your
> description.)
>
> John
>
> --- Olin Lathrop <olin_piclist@embedinc.com> wrote:
>
> > Jinx wrote:
> > > For example, enter 10 (volts in), 4.7 (zener
> > voltage), 10 (mA load)
> > > and click Solve -> 265ohms 0.1W resistor, 0.1W
> > zener
> >
> > Yes, but the resistor will drop 2.65V all the time
> > when the load is 10mA,
> > even when the input voltage is the 5.0V it's
> > supposed to be.  That means the
> > PIC Vdd will be 2.35V with 5V in.  The 10F will
> > operate down to 2V, so maybe
> > that's OK, but the OP should at least be aware of
> > this limitation.
> >
> > If this is not acceptable, you could try a 5.6V
> > zener with emitter follower
> > as a crude regulator.  That requires one additional
> > NPN transistor from the
> > plain shunt regulator described above, but will only
> > drop about 700mV when
> > the input voltage is below about 5.7V.  Above that
> > it will regulate
> > reasonably well to 5V up to the C-E voltage limit of
> > the transistor.  The
> > extra $.03 for the MMBT4401 or whatever will be
> > worth it in many
> > applications.
> >
> > By the way, 10mA is quite high for a 10F itself,
> > although the whole circuit
> > could easily draw that.  The OP needs to provide
> > some real specs.
> >
> >
> >
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