FW: Schematic and technical info on MSD6A ignition

[Tom Cloud]

>Hi all,
>       I've gotten zapped by motorcycle ignitions, 110V ac etc. but I
>believe that 12V dc is more dangerous due to the high currents 
>involved. The first thing I learnt in basic electricity is that 500mA
>of current can kill you and car circuits certainly draw lots!

well Gregory, glad you brought that up ... technically,
at least if you ignore parts of Ohm's law, you're
correct.  However, I=E/R and the "R" is your body.
Body resistance of a reasonably slender (i.e. not
obese), dry person is several hundred k, so the
current that 12 volts can produce is relatively
small -- maybe 50 micro-amps.  Now, if you want
to stand in salt water, you're asking for more
of a problem, and being upset or fat will lower
your body resistance -- but not enough for 12 V
to be of any consequence.

Also, the 500 mA figure is probably not quite
right either.  Fibrillation of the heart is
said to be started with currents (from one hand
to another -- a heart path) starting at about
100 mA or so.  Below that current is the "tingle".
Above about 500 mA or so is contraction of the
heart, preventing fibrillation.  If you survive
the higher currents -- after you're removed from
them -- (i.e. not charcoaled), you stand a better
chance of living than if your heart has gone into
fibrillation.

Of course, all of this is talking about current
through the chest cavity -- through the hands
and arms, basically.  Now, if you want to put on
your metal beanie and stick your head into your
MSD ignition, you might live, but I guarantee
your launch response times will be a tad off   ;-)

So, for practical purposes, if you're dry, not
standing in water, not sticking your head into
the wall socket, etc, voltages up to and including
120 VAC generally cause unpleasant sensations.
The joltage from your ignition system, while not
posing a threat from the current, will make you
hurt yourself with muscle contractions.

Two of the worst shocks I've ever gotten:
 - when I was 16 (1959), I built a 2 kW ham rig
(legal limit -- 1 kW  ;-)  out of 872 mercury
rectifiers and a converted utility pole 3 kVA
transformer.  Had almost 1 uF of filtering,
swinging choke -- put out 3200 VDC at an easy
800 mA (guarantee short circuit was many times
that !!!)   I was trying to neutralize a pair
of 400X (seems that what they were called) in
a push-pull final.  Plates getting red hot.
Turn off HV and adjust neutralization cap
going to grid.  Turn HV back on and check it.
Forgot to turn HV off -- don't know why I didn't
glom onto the fact that the plates were still
bright red hot.  had left forearm resting
on the bare aluminum 19" relay rack panel
(i.e. good _SOLID_ ground) and reached in
and took a good grab of the metal heat sink
plate caps (IOW, the 3200 volts).  Knocked
me about 10 feet -- probably would've been
further, except there was a wall there!!!
My mother heard the noise.  I couldn't get
up, couldn't raise my arms, don't know how
I kept my head up.  Couldn't speak -- could
barely think, but I knew that she better know
what had happened, so when she asked if anything
was wrong and if I was alright, I could only
try to smile (man, was that hard to do) and
try to wag my head.  Don't know why she left.
Glad she did.  Asked her later and she said she
was fixing supper, though she wondered why I
was sitting back against the wall, wouldn't get
up or speak to her and looked so funny.  I shook
for over an hour.

Second experience was working on one of my cars.
Was bumping the starter over with a switch connected
to the starter relay with clip leads.  Touched the
wire going to the starter when I bumped it.  Got
_major_ lesson in inductive reactance or cemf.  The
inductive kick off the starter is probably at least
300 volts when the relay opens -- and you can be
sure that's at some real _LOW_ impedance (i.e. high
current -- well, consider that it's trying to keep
the power/current through itself unchanging, so
200A at 12 V is 2400 W therefore 300V kickback would
love to dump 8 A into a load, even though briefly).


Tom Cloud <cloud at peaches.ph.utexas.edu>