On Thu, Jan 26, 2012 at 01:02:18PM -0500, Dave Tweed wrote:
> Byron Jeff wrote:
> > Short of using Schottky diodes for the external diode, in order to
> > guarantee they conduct first, what else can be done?
>=20
> Use a transistor already!
>=20
> Take a jellybean NPN transistor, and connect a diode antiparallel across =
its
> B-E junction (A to E, K to B). Connect the E to ground. Now B can't go mo=
re
> than +/- 0.7V away from ground.
>=20
> Connect as large a resistor as needed between the source voltage and B. F=
or
> example, suppose you want to allow a current of 1 mA at an input voltage =
of
> +/- 300V, which would make the resistor 300k (make sure it can handle the
> voltage and power). This still leaves plenty of current to drive the
> transistor near the zero-crossing, i.e., 10 uA @ 3V.
>=20
> Connect the C to the MCU input, using an external pullup if necessary.

Actually I did that and it changed the timing on the pin. When I changed
out the signal diode with a zener, the timing reverted back to its normal
behavior.

BAJ

>=20
> -- Dave Tweed
> --=20
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--=20
Byron A. Jeff
Department Chair: IT/CS/CNET
College of Information and Mathematical Sciences
Clayton State University
http://cims.clayton.edu/bjeff
--=20
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