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Thread: PIC Input protection.
www.piclist.com/techref/microchip/ios.htm?key=input
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face BY : Russell McMahon email (remove spam text)



>> Let's see.  12V one side of the diode, VCC the other.  Either VCC
>> will
>> get pulled up towards 11.4V or VCC holds its own, the diode goes
>> boom
>> and goodbye protection.

http://www.pacificsun.ca/~john/ELS/photos/IO_Circuit.jpg

Assuming that _I means input and _O means output, as is clearly the
intention then:

As shown it is definitely bad IF L_I is low impednace and/or high
energy..
Even if the regulator can absorb the energy you are indeed attempting
to use a diode to pass an indefinitely large current.
It may be that L_I is impedance / resistance limited outside the
circuitry shown.

A possibly important other issue is that L_O will rise to about 5.6v
for values of L_I much above 5.6V. Depending on what L_I is driving
this may cause problems with current flowing in body protection diodes
in connected ICs. As D4 is (almost) obviously attempting to protect a
circuit from voltages above 5V then odds are L_I is driving an IC with
VDD or Vcc = 5V.

Better solutions, given the limited information here are:

1.    Connect the anode of D4 to the other side of R46 which will
protect it against higher energy inputs and limit the amount by which
Vcc can be pumped up. This protects D4 and Vcc but still exposes L_I
to excessive overvoltage.

2. As per 1. but make it a Schottky diode.  This will work well enough
in almost every case where L_I drives a 5V IC.

Here's the Yahoo Groups Electronic Lead Screw page showing what this
is part of

       http://tech.groups.yahoo.com/group/E-LeadScrew/

Notably

       Member John Dammeyer has led the way in these discussions. He
has designed a system, which is described in a comprehensive article
"Electronics Gear Control" in the magazine "Circuit Celler" -
Nov/2006, Issue 196, pp 36-43.




       Russell McMahon


_______________


     "ELS" - Electronic Lead Screw - is an alternative to full CNC
for simple bench lathes - especially those lathes which do not have a
"Quick Change Gear Box". For such lathes, it would be very useful to
have a cheap method of sychronizing spindle speed to lead screw speed.
If this can be realized electronically, then all threading can be done
without the necessity of changing manual gears. Also, all saddle
traverse speed ratios become possible.

     The aim of this group is to develop a simple, cost-effective
electronic and mechanical system for implementing ELS - which in
theory would be cheaper than CNC, and which can be ported to
hobby-class bench lathes.

     Member John Dammeyer has led the way in these discussions. He
has designed a system, which is described in a comprehensive article
"Electronics Gear Control" in the magazine "Circuit Celler" -
Nov/2006, Issue 196, pp 36-43.

     Suggestions and fresh perspectives are welcomed. Please follow
posting guidelines to maximize readability and archiveability of your
ideas.

     The photo at right is of a Taiwanese lathe with Electronic Lead
Screw.

<015601c76c78$b920ac30$d201a8c0@y2k> 7bit

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