Path: utzoo!mnetor!uunet!amdahl!ames!ncar!gatech!mcnc!decvax!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: Automobile shock hazard
Message-ID: <1088@neoucom.UUCP>
Date: 9 Apr 88 03:44:18 GMT
References: <560001@hpcljws.HP.COM> <334@eos.UUCP> <5773@watdragon.waterloo.edu> <3779@medusa.cs.purdue.edu>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 37
Summary: Inductive ringing when circuit opens


The inductive kick delivered by the starter solenoid is likely to
be worst when the solenoid disengages.  This is due to the effect
of V=L dI/dt.  The dI/dt is a pretty big number as I tends to go
from something like 10 amps for a starter solenoid to zero in a
couple of microseconds.  The result is that V = some big negative
number (like 250 volts, for instance).  If one's hand is on the
keyswitch, which just happens to have the lock barrel be hot (some
do), you just might get a friendly jolt!

Maybe that is why VW keys are platic-coated around the top, huh??

As I said previously, the metal horn button on my Chevy G10 van
used to shock the bajeezus out of me due to the kick from the horn
realy, and the realy coil was less than an amp!  (The horn itself
was about 15 amps).

Modern fancy radios and ECM (thats Electronic Control Module, not
Electronic Counter Measures!!) electronics in cars are very heavily
bypassed with protective diodes and multisection pi filters on the
vcc inputs.

I currently have a 1983 AMC Alliance that has a Bendix TBI fuel
injection system (the CA models have Bosch multiport EFI).  Just
about everything in the car is equipped with supressor diodes.  I
was pertty impressed that they were pretty careful about EMI/EMC
design.

$5 question: why is the Alliance turnsignal indicator so
ridiculous?  It is a one lamp design like the VW bug, but unlike
the Bug, AMC used a USA 4-wire turnsignal design -- which means
they had to use two diodes to combine L and R into a single lamp!
Seems like it would have been easier for them to just use two bulbs
and be done with it.  A lamp must be cheaper than a diode, even
these days.

--Bill