Steve,
As Bob alluded to earlier, the amount of heat generated in a resistance is
given by the square of the current (in amps), multiplied by the resistance
value in Ohms. Components do not "cause" resistance to the wiring, and I
completely agree with Bob that while the symptom you had (hot connections
apparently originating at a poor junction near the alternator) can be
explained (just on the basis of the bad connection), the fact that the hot
connection went away when you removed the ammeter is kind of a mystery to
me.
The other important electrical equation that you normally need to deal with
is Ohm's law, which says that the voltage across a resistor equals the
current (in amps) multiplied by the resistance.
Let's say you were charging a hypothetical battery, from an alternator
that's putting out 14 Volts, through a ten foot long, 10 gauge wire, with a
resistance of 1/20 ohm (for instance - I'd have to look up the resistance
tables for AWG copper sizes to get the real value), and you were running 20
amps through it. The power lost due to resistance heating of the wire is 20
x 20 x 1/20 = 20 watts, and the voltage drop across the wire is 20 x 1/20 =
1 volts, so the actual terminal voltage at the battery would be 13 volts.
If you then connected an ammeter in series, whose shunt resistor was 1/20
ohm (for example - I'm just making the arithmetic easy for myself), you now
have two resistors in series between the alternator and the battery. The
total resistance is 1/20 + 1/20 = 1/10 ohm, and (assuming the battery
voltage remains constant even though we're going to change the charging
current), the charge current will now be 1 V / (1/10 ohm) = 10 amps. As a
result, the power lost in the wire will be 10 x 10 * 1/20 = 5 watts, and the
power lost in the ammeter is also 5 watts. This is important: Adding
resistance in the circuit REDUCES the total power lost due to heating. What
does happen, though, is that instead of having 20 watts dissipated over 10
feet of wire, we now have five watts being lost (and five watts of heat
being generated) in the small confines of the ammeter. And that is why
things get hot at connections - localized resistances that have a lot of
current flowing through them generate a LOT of heat. The main complaint that
the Mopar guy was voicing (besides the ammeter itself) was the quality of
the connections at the ammeter and the firewall bulkhead, which were the
sources of the localized heating.
My guess would be that at least one third of the 'hot' ammeters encountered
by Margaret at Mo-Ma are due to bad (loose or corroded) connections at the
ammeter. Another third would be caused by wiring accessories directly to the
positive terminal of the battery (or on the Tiger, by wiring accessories to
the batt connection of the starter solenoid), which requires the ammeter to
pass a lot more current than just what is required to charge the battery, as
well as giving false indication of the charging process. Only a third or
less probably have any genuine issues that are not caused by
nut-behind-the-knob mistakes.
Best regards,
Theo
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