[DeTomaso] Shunts, fuses, was Amp gauge

[Guido deTomaso]

Think this came up 6 or 7 years ago, GM products like my truck seem to use a 
shunted ammeter, with a fuse at both ends of the wire, possibly for the reasons 
cited below (fire).  In the Stewart Warner shunt arrangement, they provide a 
table of the allowed wire gauge and length.  The length does not need to be 
evenly divided on each side of the gauge.  Adding a fuse at each end would throw 
off the reading, I predict, but maybe not enough to notice.

For the same reason (fire), I've added a fuse to the wire that goes to the 
voltmeter on one car I own where an unshunted ammeter isn't practical.  It's a 
long ~22 gauge wire going a long way to the battery ... could set some things on 
fire if it were to heat up.

The huge wires going to an unshunted ammeter won't be as great a fire issue if 
the whole system is protected by a fusible link.  That's exactly how I burned up 
a fusible link once.

GD



"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.

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 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. "
>