Well,
To answer your questions, it depends on how much you really want to
understand about it. In real layman's terms, the voltmeter can be summed
up as follows: (assuming the car is running)
< 12 volts -- something is broken somewhere
12-13 volts -- a marginal electrical system
13+ volts -- basic electrical system is ok
So, in your case, you can assume that the alternator is doing a decent
job of supplying adequate electricity for the car, since you're getting 13
volts when the car is running.
Assuming that you want to understand a little bit more of what the
voltmeter is measuring, I'll try to answer your question. Please note that
this all gets quite complicated, since many different factors can affect
the voltage measured by the voltmeter. What follows is my verbose attempt
to clarify things using the metaphor presented earlier (which probably
actually means that things will be muddied...) The explanation below
probably won't help a non-scientific person much, but could be of help to
anybody that understands dams and reservoirs, but not electrics.
Lawrence Schilling wrote:
> OK, but is the 13 on the Voltmeter the amount that COULD flow or the amount
> that IS flowing.
> Jeremy DuBois wrote:
> > Take as a metaphor a large dam blocking a river. Imagine the
> >water is the electricity. Voltage can be seen as the amount of force with
> >which the water is pushing against the dam. If you've got a high dam,
> >you're going to have a lot of voltage from the backed up water. When the
> >dam is opened, and the water starts flowing, that flow of water is like the
> >amperage. The more you open the dam, the more amperage you're going to
> >get.
> > So, in other words, the voltage tells you how much electricty the
> >curcuit could possibly flow, and the amperage tells you how much it's
> >actually flowing at the moment.
Well, to continue the metaphor, (as all metaphors, it's not perfect, but
serves pretty well in this case), the 13 volts reflects the "present state"
of how high the water level is behind the dam. It turns out that with a
dam, the amount of force being exerted against the dam has little to do
with how much water is behind the dam, but only with how *high* the water
level is. (The force against a one-foot high, by one-foot wide dam, which
is holding back water up to, but not past, its full height, has the same
amount of force pushing against it regardless of whether that small dam is
holding back a reservoir containing a gallon of water, a swimming pool full
of water, Lake Winnepesaukee, or the whole Pacific Ocean.) This amount of
force is analagous to voltage.
When a given sized hole is punched in the dam, the amount of water (per
second, or any other time unit) that comes out of the hole corresponds to
the amperage. (Amperage is often referred to as "current" by EEs.) The
size of the hole is analagous (but inverse) to the resistance. (That is, a
small hole corresponds to big resistance, and a big hole corresponds to
small resistance.) Also, the higher the water behind the dam, the faster
the water will pour out.
So, if you punch a big hole (small resistance) in the dam, this results in
lots of water (amperage); a small hole (big resistance) lets out only a
little water (amperage). (To give some perspective, the oil pressure
light punches a tiny hole, the parking lights an average hole, the
headlights a large hole, and the starter a huge, gaping crevasse.)
In our analogy, there are only two sources of water behind the dam: the
battery and the alternator (or generator.) The voltmeter essentially
measures how high the water is behind a 15-foot high dam. (assuming your
voltmeter stops at 15 on its scale.)
When the ignition key is turned to the "on" position, but before the car is
started, the voltmeter will read the battery voltage. (If you give it long
enough to come up to the reading ... TR6 voltmeters have a very slow
response to changes.) This will vary significantly with the charge of the
battery, but should be greater than about 11.5 volts. In our
simple metaphor, this means that this is roughly equivalent to the water
behind the dam being 11.5 feet deep.
When the car is running, the alternator starts to push water, as
well. A working alternator will push more water than the battery,
and raises the water level behind the dam to about 13.0 - 14.0 feet.
(13-14 volts.)
What makes this all really complicated is determining how the voltmeter
reacts when a "hole is punched in the dam". This is analogous to turning
something on, (such as the starter, lights, radio, spark plugs, etc.)
Unfortunately, the "water behind the dam" metaphor starts to get real
complicated when that happens, as various electrical components will punch
holes of various sizes at various heights in the dam, and require varying
amounts of current (amperage) to do anything useful, so I'll try to stop
whilst we may still be ahead, unless somebody really wants me to try to
continue... (or pick it up themselves at this point...)
--ken
An amateur Triumph mechanic who took a few EE classes...
Kenneth B. Streeter | EMAIL: streeter@sanders.com
Sanders, PTP2-A001 |
PO Box 868 | Voice: (603) 885-9604
Nashua, NH 03061 | Fax: (603) 885-0631
|