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From: Ken Streeter
Subject: Re: Voltmeter vs Ampmeter
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
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From: jsnyder@icds5.dseg.ti.com (James A. Snyder)
Subject: Re: Voltmeter vs Ampmeter
(Ken Streeter's Post Quoted-Deleted Here)
Lawrence, I think both of these guy were doing a pretty good job, but I would like to give you another perspective. The analogy of voltage to water pressure and current (amperage) to water flow is a good one. Higher water pressure means that more water will flow through a given size pipe. Similarly, higher voltage means a higher flow of electrons (cur- rent, measured in amperes or "amps" as they are affectionately known) through a given resistance. What you are probably interested in is what your car's voltmeter or ammeter says regarding the proper functioning of the electrical system. Either meter will tell you if your charging system is generally functioning properly when you understand what they mean.
As Ken said, if your voltmeter reads above 13 V. when the engine
is at running speeds your generator/alternator is probably OK. The
voltage measured is(someone correct me here, if I am wrong) at the
generator/alternator output. If the voltage there is HIGHER than the
battery voltage, the battery is being charged. :-) If the voltage is
lower than the battery voltage, your gen/alt. or voltage regulator is
not working properly and the battery will be discharged. :-( An
Ammeter measures the current flow INTO the battery. Thus, positive
flow implies that all is well, the battery is being charged. Negative
flow means that your gen/alt. is not supplying enough current and
your battery is discharging. Summation: Voltage higher than 13 V :)
Voltage lower than 13 V :( Current positive :) Current negative :(
This is at driving speeds. At idle, it is normal for voltage to be
slightly low and current slightly negative. Hope this helps to clear
things up.
Jim Snyder
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From: "Richard R. Olson"
Subject: Re: Voltmeter vs Ampmeter
Hello again, .....nice water analogy deleted....... If (as someone
just mentioned) the TR6 voltmeter is slow to respond, then the
ammeter is overly sensitive. Turn blower on, ammeter goes -, then +,
slightly, hit brakes, same result, turn on headlamps, hear needle
ping on - side, then center. Neat to watch!!
Rick "who owned a '72 TR6 for a month before he realised it had an
ammeter, and not a voltmeter like his '73" Olson
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From: David Rupert <75701.242@CompuServe.COM>
Subject: Voltmeter vs Ampmeter
Lawrence, Well, the stupid answer is that a voltmeter measures volts, and a ampmeter measures amps! ;>) Seriously, the voltmeter will measure the voltage potential across the electrical system, from positive to negative, while the ampmeter will measure the amount of current flowing through the electrical system. What this means for a voltmeter equipped car is that you should be seeing in excess of 13 volts when the car is running. The alternator (or generator) should produce somewhere between 13-16 volts in order to keep the battery charged. If, for some reason, you only get a reading of 12 volts or less, something in your charging system has packed up and gone south. A voltmeter is always hooked up from positive to negative. On an ampmeter equipped car, the meter is measuring the amount of current flow that is being drawn from the charging system.
Now, just to make it more confusing, the meters in cars usually read positive and negative, with zero in the middle. The idea here is to measure how much current the various electrical thingys in the car are drawing vs. how much current the charging system is producing. If all is well, the meter should be reading zero or slightly positive (the extra current should be flowing into your battery to charge it). If the ampmeter is reading negative, the car is drawing more current than the charging system is producing. You're headed for a dead battery. Ampmeters are always put in series from the charging system to the rest of the car. I've also seen the term ammeter, which may be the correct term for the device I described. The difference would be that the ammeter measures differential current, whereas the ampmeter measures total current draw through the system. Or I could just be full of *&$#, which is probably more than likely. Maybe the automotive engineers just left out the 'p'. Waiting on error correction from the rest of the gang.
David Rupert BSEE GMI Engineering & Mgt. Institute '89
75701.242@compuserve.com
1967 TR4A (rigid axle)
1980 TR7 convertible
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From: cloughbt@batman.flight.wpafb.af.mil (Bruce T. Clough)
Subject: Re[nth]: Voltmeter vs Ampmeter
Uh folks, There is no such thing as an Ampmeter, it is an ammeter. I originally thought this, so I scanned my EE references and could find plenty of ammeters, but no ampmeters. In reality it doesn't matter since we all knew what an ampmeter was. More useless trivia... BTW: The Nissan died hard this week (can you say destroyed lower end), so the TR7 is now the daily driver until I save enough money for the XJ6. Not that I'm complaining with warm sunny weather outside, December might be a bit different!
Donations to the "Bruce wants a large, four-door Jag Fund" can be
sent to the author. Please email to
cloughbt@batman.flight.wpafb.af.mil . Any donations accepted in any
currency.
Bruce Clough BSEE, MSEE, PhDBCE(Briefing Chart Engineering, from the
School Of Hard Knocks)
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From: Thomas Howard
Subject: Re: Voltmeter vs Ampmeter
On the Volt vs Amp question. I have a good understanding of the issue but I don't think that it is relevent here. The real value of the meters on the car dashboard is to show a CHANGE from the norm. If you have a working system then all you have to do is watch for a change in reading. Understanding if the indication is part of the repair process. Establish that base line and roll on down the road.
Thomas Howard LUSD, (619) 390-2627
('72 Triumph GT-6; and a good bit of USA iron)
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