In a message dated 2/25/99 8:01:35 PM Eastern Standard Time,
gprtech@frontiernet.net writes:
> Current measurement circuits I've designed have used resistance's of 0.005
> ohms to
> measure currents of up to 20 amps.
>
> There's also the possibility that the ammeter is not a shunt type. Some
> ammeters
> just wrap a few loops of heavy wire around a metal bar to generate the
> magnetic
> field to move the meter needle. In a case like this, the resistance of the
> wire is
> not particularly relevant, since it's not the voltage drop across the wire
> that
> matters, but the current flow. A shunt may not affect the meter in a linear
> fashion.
George,
That is true of the ammeters used in the Triumphs - they are of the current
loop/metal bar type. Even so, even with a "loop" resistance of only 0.005 ohms
or less, a shunt with an equal resistance will bypass half the current. Even
if the resistance is only a micro-ohm, as long as they are equal. In all
reality, though, it is not very likely that we could come up with a good
"repeatable" value for the shunt resistance. There might be enough variation
from one loop of wire in one meter to another, and in the length and
installation techniques of the shunt itself to make it impractical. Variations
in the installation technique alone would probably be enough to kill the whole
concept.
As for the linearity, I really don't know. You may well be right on that too.
I assumed a linear relationship, as the meter scale is linear. Like I said,
it's an excellent idea in theory, but it may well not work out in practice. It
can certainly be done, but it might take a one-on-one laboratory construction
and calibration to make it work, rather than an owner installed modification.
Good points!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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