On Fri, Jan 06, 2012 at 11:00:23AM -0200, Isaac Marino Bavaresco wrote:
> Em 6/1/2012 10:26, Byron Jeff escreveu:
> > On Fri, Jan 06, 2012 at 07:20:08AM -0200, Isaac Marino Bavaresco wrote:
> >> Em 06/01/2012 06:34, William "Chops" Westfield escreveu:
> >>>>> Is there anything I can add to protect the chip from unwanted negat=
ive voltage?
> >>> Well, to start with, there should be a substantial resistor; about 10=
k.
> >>> This will rely on the internal protection diodes to clamp the voltage=
s to levels that won't cause problems.  There is plenty of reason to assume=
 that this is insufficient, but microchip does publish app notes where this=
 is done (apparently successfully.)
> >>> ...
> >>
> >> I prefer to use an external Zener diode plus two resistors to clamp th=
e
> >> voltages, forming a "T" with the Zener as the "leg" of the "T".
> > What's the purpose of the resistor between the zener and the PIC input?
>=20
>=20
> If the RS-232 is connected when the PIC is not powered, the pin will
> have 4.7V and the board will be powered by the PIC's internal parasitic
> diode and large currents may flow through it, perhaps violating some
> parameter.

The series resistor will limit the incoming current. With a 4.7V zener and
a 4.7K series resistor, the maximum current the unpowered PIC will see is 1
mA. All the rest of current from the higher incoming voltage will be
shunted to ground by the zener.

> With the additional resistor (say 1K) the current will be limited to a
> few mA, well inside the allowed range.


>=20
>=20
> >  For
> > quick and dirty testing, I use a 4.7K resistor in series with the RS-23=
2
> > output and a 4.7V zener. For more permanent circuits I use this as the =
base
> > input to a NPN transistor inverter with a pullup resistor to protect th=
e
> > PIC's input from the -0.7V the zener allows when the RS-232 output swin=
gs
> > negative.
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>=20
> When using an NPN transistor I use a small signal diode from ground to
> base to protect the base-emitter junction from reverse biasing. The PIC
> will never see the -0.7V because the base-collector junction will be
> reverse biased, but the transistor's base-emitter junction may be
> damaged as they are usually rated for -5V.

The zener serves the purpose of that small signal diode with the input
voltage is negative as the zener is now forward biased. So the base-emitter
voltage is never more that -0.7V which is nowhere near the reverse bias
breakdown voltage for the transistor.

I haven't had any problems so far, and it's a heck of a lot easier than
trying to track down a MAX232 or equivalent. I haven't seen a USB serial
adapter yet that swings any more than +/- 5V, and every one so far works
fine with 0-5V going into their inputs. With the near universal death of
dedicated serial and parallel ports on PC's, USB serial adapters are my
current bit wiggling interface.

BAJ

>=20
>=20
> Best regards,
>=20
> Isaac
>=20
> =20
>=20
> > The last time this discussion came up, Russel noted to me that allowing=
 the
> > negative voltage to the PIC input, even though it's clamped by the
> > protection diodes, causes the part to operate outside of it's range
> > specified in the datasheet (-0.3V minimum). Hence the transistor in the
> > permanent circuit. I'm unsure what if any damage that 150 uA of current=
 at
> > -0.7V can do to the input.
> >
> > BAJ
> >
>=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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