=20
100 ohm resistors on the clamp diodes are in case the numpty installing t=
he
unit connects it up with the polarity reversed. The 1N4004 diodes are the=
re
as they are rated for 400 volts (this is easily achieved in the ignition
circuit).

The two 18k resistors are necessary because they each have a breakdown
voltage of 200 volts. One 40k resistor may not provide enough insulation =
for
the 300V+ that appears on modern ignition systems (back EMF).

I don=92t know about the 4.7k across the diode but to be honest with you =
I've
used this circuit a number of times and it works without any problems at
all. I personally use an interrupt pin rather than a comparitor input (th=
is
circuit has been around since the days of the 16F84).

Just my personal experiences with the circuit.

Dom

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf =
Of
Philip Stortz
Sent: 06 September 2004 01:29
To: Microcontroller discussion list - Public.
Subject: Re: [PIC:] Car Tacho to PIC input pin protection advise

ok, why the 100 ohm resistors on the clamp diodes?  i'd think you'd want
them without the series diodes (the two 18K series resistors should be mo=
re
than adequate for current limiting).  why the 4.7k resistor across the
diode?  it seems like you'd want to leave that out unless you are worried
about capacitance?  and why the 10k resistor to ground?  is that also for
the capacitance (which should be mostly from the capacitor).=20
also, how are you protecting the 5V supply?  the transistor would also
appear to be somewhat unnecessary if you were using a comparator input,
though i see it could be used for level shifting.  a lot of questions i
know, i'm just trying to understand the logic behind most of that design,
which appears to use extra components (like 2 18K resistors instead of on=
e
around 40K, i can't see power dissipation being a problem).  i'd also use
1n4148 or 1n914 switching diodes instead of the slow 1n4004 rectifiers
(which could also be anything from 1n4001 to
1n4004 diodes i'd think?) as you want fast diodes for limiting transients=
,
and the current rating should be more than adequate.  Please explain, i'm
sure you have reasons.


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=20


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