[DeTomaso] Amp gauge

[Jeff Detrich]

In my experience, all the ammeter was supposed to do is measure whether the
battery is in a charge, nuetral, or discharge state. Back in the old days
when cars had amp gauges, that was pretty important since there wasn't much
sophistication in the electrical components, ie, they were on or off or you
had a short somewhere. As to the Voltmeter vs Amp gauge issue, it's sort of
like HP vs Torque.

Jeff
6559

___

>From Wikipedia Answers: (interesting point on the use of a shunt)

There are two main wires that normally come off the battery, one extremely
large one that feeds the starter, and one reasonably thick wire (usually
around 10 ga) that feeds everything else in the
car<http://wiki.answers.com/Q/How_does_an_electric_ammeter_work#>
.

A classic style ammeter runs in series with the battery wire that feeds
everything else in the car. It samples which way the current is flowing and
how much current is flowing. If the alternator is pushing current into the
battery (charging the battery) the gauge reads positive. If current is
flowing out of the battery, it should read negative, and usually means that
your charging system is not keeping up with the electrical demands.

Since the classical ammeter is hooked up in series, it involves very thick
wire (like 10 ga) to be routed all the way to the ammeter in the cockpit of
your car. With all that current flowing through the ammeter, a short
circuit on the ammeter in the cockpit can be fairly dramatic, and some
classic car guys avoid ammeters for the possibiilty of causing a fire.
There is also an ammeters out there that have an external shunt. An
external shunt is basically a very low value resistor that is in series
just as the ammeter was in the previous example, and then the ammeter is in
parallel to the shunt. Basically MOST of the current goes through the shunt
and only a small percentage goes through the ammeter. By knowing the
resistance of the shunt and the meter, the meter is able to calculate the
total current by measuring only the small current that goes through the
meter. This kind of setup allows small wires to be run to the ammeter in
the cockpit, and the high current to remain in the engine compartment. The
danger in these setups on some old
cars<http://wiki.answers.com/Q/How_does_an_electric_ammeter_work#>is
there isn't a fuse on that small ammeter wire, So if the shunt is
damaged or a connector breaks, it would try to flow all the current through
the remaining path which is the small ammeter wire and that would melt that
wire (along with whatever else it is next to).

Read more:
http://wiki.answers.com/Q/How_does_an_electric_ammeter_work#ixzz1vKfrSMNK

On Sat, May 19, 2012 at 10:46 AM, Guido deTomaso <guido_detomaso at prodigy.net
> wrote:

> We call an "amp gauge" on a single wire in and out of the battery an
> "ammeter",
> but the same gauge on the single wire out of the alternator a
> "loadmeter".  It
> sounds like Kirby's GT40 "ammeter" was wired as a "loadmeter".  Gary's
> comments
> below seem aimed at a loadmeter ... but there's no one voltage that's a
> line
> between discharging and charging, and unless you've got some electronics
> that
> need at least some minimum voltage, I don't see any "critical info" coming
> from
> a voltmeter.  IIRC small aircraft only have ammeters.
>