This text is copyrighted and duplication, re-distribution or publication is prohibited without consent of the author.
Article: Volts, Amps and Waterwheels
Last updated: 10/27/01
The problem with writing these articles is that I always start out with one intention and in the course of writing, it turns into something else. I started with the thought of an article on the solving the "hard start" problem with the 6 volt systems, then realized it would help to explain some electrical basics, then realized that those basics would be best served as their own article.
Then there is the other problem too: I always start out thinking "I'll just hammer out a quick, informative, new article" and it turns into a leather-bound thesis. But in spite, here were are.
First, let me qualify myself (further). I
am an electrical engineer and have been messing around with things electric
since I took my electric gyroscope toy apart when I was 8. I think I was
around 10 before I took any thing apart and put it back together,
and at least 12 before anything that I took apart actually worked again
(a flashlight). And I still remember that moment in history when
I put thin wire across one of those 6 volt lantern batteries, and when
I touched a nine volt battery on some "fine" steel wool (try that sometime).
Fortunately somehow, we never had any structure fires at the Henry household.
The Water Connection
I love a good analogy. And the electricity/water one is one of my favorites, but it does have its shortcomings, so bear with me.
Think of electricity as running through a wire as water runs through a pipe. This can really help it all make sense. Now, lets equate some basic terms:
AMPS: This is how much (volume) of water flowing through the pipe, ("Current")
POWER: This is force that the water in the pipe can exert (read on, it'll make sense)
DC: This means direct current
(in both electricity and water terms)
As long as we are talking about simple DC circuits, POWER is the product of both volts and amps. Think about standing in front of the open end of the pipe, water spewing out. If the pipe were only 1/2" in diameter (low volume, low AMPS) but the water were coming out at, say, 50 mph (high speed, high VOLTS) it would probably knock you down (and hurt too!).
Now if the pipe were, say 18 inches in diameter
(high volume, high AMPS) but only coming out at say 7 mph (low speed, low
VOLTS), I bet you would still get knocked down. Fact is, the force
that the water applies to you could be the same for a whole range of pipe
diameter/water speed combinations. Same for electricity.
The Easy Math
As long as we are talking about basic DC circuits and what are called "resistive loads" (lights, starters, etc.), the math is really simple. POWER is expressed in units of watts, and is simply VOLTS x AMPS. And, as it turns out, water pressure (force) is actually a product of volume and velocity (speed), but it s a little more complicated that simple multiplication.
If we go into inductive and captive loads
like large motors, electronic circuits, things get much more complicated
and the math isn't as simple. And the fact is that there are motors
in VWs, as well as electronics (where? that condenser is just a capacitor
in real life) but we can model them as resistive loads and pretty much
come close to reality that way.
Where Does All That Water Go?
Now, we have established that water and electricity
flow much alike. What about the things that use electricity? Lets think
about every device (like headlights, radio, starter, etc.) as a waterwheel
at the end of this pipe. Things that require a lot of water would be big
water wheels. And, of course they would take a lot of water to turn. If
get a lot of water (lots of AMPS) but get it very slowly (low VOLTage),
they will turn very slowly. And, if you blast too much water at a tiny
waterwheel, you will break it. So for the sake of this discussion, lets
just stay with the notion that a waterwheel (device) is a fixed size and
requires a specific amount of water to turn. Too little, to slow, and it
won't turn like we would like it to. Too much or too fast, and it
may spin faster for a little while, but you are probably going to break
Ok, What About Those Pipes?
By now you might realize that for a given pipe, there is a limit to how much water can go through it at once. You're right (you might be envisioning a dark basement with dripping pipes). There is a limit to how much water can go through a pipe. Try to push too much through and they burst and spray water every where. Well, wires don't actually do that (duh.). Instead they do one of two things.
So far more common is that pipe burst/melted wires and smoke thing.
Lets go back to that waterwheel example and try to paint a picture of smoking wires. This will begin to stretch the old water analogy a bit, but go with me. Let's say that if this waterwheel is stopped by some mechanical force, it won't let water out of the pipe that is feeding it. Pressure will build up in the pipe and it will break. If you engine is seized and you try to start it, the starter motor will not let any "water" into it and you will "burst" (smoke) wires.
This is when things go bad with electricity. Most often we talk about an electrical "short". Electricity actually has to have a path from its source and back to it. (Now the water analogy starts to fall apart more. We all know water is perfectly happy flowing just one way, and creating a big mess at the end) In your car, it may seem as though the wire just goes to the device and stops, but the return path is actually via the metal car body or the "ground". Now when water pressure builds up, it will burst at the weakest point in it's path. Same with electricity, it will create heat and maybe even smoke or fire at it's weakest point. And guess what? It's weakest point is the place which can pass the least current. The thinnest wire, the poorest connection (never the body itself, by the way).
An guess what a fuse is? Its an intentional
weakest link in a path for electricity (called a "circuit") that will break
safely (ie. no smoke or fire) if a certain current limit is passed (the
"value" of the fuse in amps. Now the water analogy really goes to hell.
A water "fuse" would be a valve that shut off flow when the pressure got
to high, and that of course would just create more pressure. Oh well, it
got us this far....
Now lets talk about loss of VOLTage. A "poor"
connection is one which introduces a RESISTANCE. We haven't talked about
RESISTANCE yet, so before we abandon the water thing, lets give it a shot.
Think of a resistance as a constriction in the pipe that slows the water
down, and yes, the volume of water too. Again this is a stretch, but lets
pretend that the water is moving faster before the constriction than after
it in the pipe. The restricted part of the pipe would represent a drop
in the speed of the water. Electrically, a resistance creates a voltage
drop. In fact a light bulb is actually a resistance that drops all of the
voltage across its filament. (but don't go trying to read it with an ohmmeter
because when the tungsten filament is not glowing, it will read zero ohms
or close to it). But some resistance's are bad. A poor connection on a
battery terminal creates a resistance, and hence a voltage drop, for everything
in the car. And worse, a "poor connection" resistance brought about by
corrosion, will create heat within the connection thus encouraging more
corrosion. The "snowball" effect.
6 Volt vs. 12 Volt
Now that you have this understanding, lets abandon water all together and make sure we understand the real differences between 6 and 12 volt systems. If the POWER that a device consumes is the same, and the voltage changes, than the current must change also. If a headlight consumes 48 watts for example, and it is a 12 volt headlight, the the current is 48/12 or 4 amps. If it is a 6 volt headlight, then the current is 48/6 or 8 amps. TWICE the current. And yes, that means the wire needs to be twice as thick. Now think about voltage drops brought on by corrosive or otherwise poor connections. As it turns out, the actual voltage drop is proportional to the current through the connection.
So now you can see the problem with 6 volt systems, they run at twice the current of 12 volts, making them more prone to voltage drops due to bad connections, and they only have 6 volts to "drop" in the first place!!!
With 12 volt systems, the current is halved, and the drops due to bad connections are also reduced.
So why not 48 volts? 100 volts? What is the
trade off? Well, this is a classic power distribution problem. You have
to make concessions between wire size, susceptibility to voltage drops
and safety, and dielectric considerations. It is all but impossible to
feel an electric shock from 12 volts and minimizing arcing and sparking
in a fuel laden vehicle seems like a good idea. I think big trucks use
24 volt systems. High voltage (600v and higher) is used for commercial
and residential power distribution where high dielectric, but thinner wires
can be used and run high over head.
Summary of It All
So you get it now? You are a plumber who
works in your car. You need to make sure you pipes are all fat enough to
carry the amount of water that the things that use it need. You need to
make sure that all of the connections are capable of passing as much current
as the wires that connect to them. Pay particular attention to the battery
terminals and your starter and headlight connections.
Copyright© 1997; John S. Henry