Model Forum / General / Railroads / April 2004

A 12 volt automobile battery cannot easily kill you. It CAN under some bizarre
circumstances cause death or serious injury but it cannot "easily" kill you. The 220
Volt 60Hz stuff in your breaker box can kill you a whole lot easier and quicker than
any 12 volt car battery. The battery can only kill you with the greatest of
difficulty.  You must help it along quite a bit to achieve such an end.  The most
probable injury is a minor burn caused from being part of a short circuit.

If you burn your skin off or stab through it then the resistance across the whole
body under the skin is only about 500 ohms; even less, if you stab deep enough. So if
you take a couple of sharp probes, connect them to a 12 volt car battery, and stab
them into your chest, you'll be in a bad way.
However, if you do that it is evident that you were already in a bad way to begin
with.

Now, while it may be possible (perhaps) to show documented proof of death as a direct
result of electrocution from a car battery, it is not something that is a serious
problem. Batteries are no more dangerous than the electrolyte inside and the hydrogen
they outgass as a result of their function.

Read this:
http://engineering-ed.org/computer_repair/documents/ELECTROCUTION.doc

As anybody who's managed to jam his stainless steel watch bracelet between the
positive terminal of his car battery and the negative-ground frame will be able to
tell you, car batteries can deliver quite a lot of current.

Automotive starter motors operate on 12 volts DC, and can draw hundreds of amps for
the (with any luck) brief period when the engine's cranking. Let's assume the battery
delivers exactly 12 volts, and exactly 240 amps is being drawn. With that
information, you can use Ohm's law to figure out the resistance of the circuit
(including the starter motor, its wiring, the internal resistance of the battery, et
cetera). R equals 12/240, or only 0.05 ohms. And the circuit power, mostly accounted
for by the motor, is 12 times 240 - an imposing 2880 watts.

If 240 amps passes through any significant amount of a human body for any significant
period of time, investigators may have to employ DNA analysis to determine who that
stuff they found all over the place used to be. You really don't want to make
yourself part of a 240 amp circuit. Actually, only about 30 milliamps (0.03 amps)
across the human heart has a good chance of stopping it.

Where people go wrong, here, is by thinking that if you disconnect one battery lead
in your car, hold the end of that lead in one hand, touch the battery terminal with
the other hand, shout "Je ne regrette rien!", and then get a partner in scientific
exploration to turn the key, you'll be deader, faster, than someone who's already
started his skydive when he discovers that the thing on his back actually contains
some chocolate and a sleeping bag.

What will actually happen in the above situation - the car situation, not the
skydiving one - is nothing.

The reason for this is that the human body has quite a lot of resistance of its own.
If you've got a good contact - the terminals are big enough that the whole palm of
each hand can touch them - then the resistance across your body is likely to be
between about 2,000 and 50,000 ohms. The thinner and wetter your skin, the lower the
resistance will be.

Even if your resistance is only 2000 ohms - which it probably won't be - that's
enough to drop the circuit current to about 6mA. You may be able to feel that. You
probably won't.

Since your resistance is actually likely to be much higher, the current through your
body is likely to be trivially low. And since your body is part of the circuit, the
current through the whole circuit will also be trivially low. Hence, the car will not
start, and you will not die.

But beware!  You can most certainly fry one of those little flea sized N scale
locomotives if you do not include some kind of over-current protection such as a fuse
or a crowbar curcuit.  Fuses are OK and they are cheap. Crowbars are more complicated
and expensive but work much quicker. You may still ruin a locomotive even if you pop
a fuse.  Not really very likely, but the slim possibility does exist.

Don't open the batteries and expose yourself to the electrolyte.  It's not a crime,
but there's no future in it. You can get nasty acid burns if you're careless or
accident-prone.

Don't allow hydrogen gas to accumulate in closed spaces unless you live in Lakehurst
New Jersey or your name is Hindenburg. and be sure to vent the inevitable acid vapors
that are an integral part of lead-acid batteries.  You can buy sealed batteries these
days that cannot be opened and that will not spill. That is what I would recommend,
but I would still vent them to the outside if possible

NOTE: some of the comments above were C&P from http://www.dansdata.com and some are
truly my own. I have messed about with automobile batteries for fifty years and have
never been killed by one.
Not even injured.

I would recomend about two minor changes that come to mind with a quick
glance at the schematic.  The 3300 uF capacitor should be replaced with
about a 10 uF tantalum and a 0.1uF ceramic in parallel with each other.
This will reduce the chances of oscillation in the regulator.

To clarify how to hook this up the part labled cr-1 is where the battery
hooks up.  See the + and - signs on it?  

PUT A FUSE BETWEEN THE BATTERY AND THE POWER SUPPLY!!!!!!

Ooops, was I shouting there?  Good reason. A car battery can put out several
hundred amps if it is short circuited.

To do reversing on the track use the standard wiring of a switch to make
it work.  A double pole double throw switch will work there.

Thanks for all of the help.
I am going to build the circuit using the LM350 IC and the two diodes on the
output.
Steve