Input protection

[Nic van der Walt]

>Well, seeing that he drew it for my benefit... (I only have a "glass
>tty") and I'm used to squinting when ASCII art goes drunken...
>
>>> 
>>>                                   --+--- +5v
>>>                                     l
>>>                                     l
>>>                                  -------
>>>                                    / \
>>>                                   /___\      1N4148
>>>                                     l
>>>                                     l
>>>                       +---------+   l
>>>   12v source    >-----l   13k   l---+------------>  input
>>>        input          +---------+   l               to uP
>>>                                     l
>>>                                     l
>>>                                  +-----+
>>>                                  l     l
>>>                                  l 10k l
>>>                                  l     l
>>>                                  +-----+
>>>                                     l
>>>                                     l
>>>                                     l
>>>                                   --+----- GND
>
>Is that better?

I have about 30 thousand automotive alarms running around and experience
shows the
following:

Never ever ever dump energy to VCC.
Never rely on simple voltage division. The 12V line goes from 7V to 16V
worst case. And those
200V spikes someone mentioned does get into everything... ;-(

The best solotion is:

IN---10K---------------10K---OUT
           |   |   |
          --- 10K 10nF
          / \  |   |
          ---  |   |
           |   |   |
          GND GND GND    

The diode is a 4.7V zener. You could get away without the cap up to a
point. This circuit works
as well as the best ESD protection circuit money can buy (Harris SP720)
with the advantage that
is needs no connection to VCC at all.

Make sure your supply is well protected. Minimum is a load dump tolerant
regulator like the
LM2931 with a tranzorb and a reverse polarity diode protecting it.

Regards
N.
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org