Path: utzoo!mnetor!uunet!husc6!ncar!gatech!mcnc!decvax!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (20 Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: Automobile shock hazard (was: shock box)
Message-ID: <1057@neoucom.UUCP>
Date: 25 Mar 88 03:59:43 GMT
References: <560001@hpcljws.HP.COM> <334@eos.UUCP> <5773@watdragon.waterloo.edu>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 47
Summary: Zapped by the horn relay!


Back when I was a hippie communist radical (are you listening NSA?)
in undergrad school, I owed a very disreputable purple 1969 Chevy
G-10 van.  The van had a very large metal button in the center of
the steering wheel for sounding the horn.

The metal button was attached to the horn relay's coil, and
pressing the button completed the circuit to ground.  The fun was
that if you were touching anything metal (or had on leather soled
shoes), you got your brains shocked out when you let go of the
button.

The trick was that the metal button delivered a nasty inductive
kick from the relay coil when the circuit was broken by releasing
the button.  It was a very graphic illustration of why supressor
diodes are placed across relay coils.  It was also a good
illusration of poor design.  It would have been relatively easy to
design the button so that it was not electrically hot.

I'd imagine that something similar was happening with the
Volkswagen key switch.  Probably an inductive jolt from the starter
solenoid as the key was released.  Anybody that has worked on a car
knows that a starter solenoid is capable of sizable v=L di/dt.

Also, up to 200-300 volts can appear across the primary side of the
ignition coil, as the coil functions as a ringing choke resonant
circuit when the points open up.  That is why the "condenser" is
across the points.  The condenser's primary responsibility in not
to limit RF emission, as you'd first guess.  Of course, CDI
electronic ignition changes the picture a bit.  There is a good
description of igniton systems in a 1939 book on IC engines by
Walter Lichty.

I also used to own a 1956 Caddie that had a Wonder Bar delco radio.
The output used a pair of 12V6 beam power pentodes.  They had a
plate voltage of about 350 volts that was derived by a mechanical
vibrator driven power supply.  In a sense, it was a component audio
system.  The AM tuner was separate from the power amp / power
supply.  The amp also had an extra <unused> audio input.  It was a
very good AM receiver, which I still have stashed somewhere in my
basement.  As a matter of fact the delco radio saw some duty in the
purple van.


(Sufficiently old-farted?)

--Bill