In order to make an ammeter that won't get very hot in operation or cause a
significant voltage drop across it, the resistance of the meter must be very
small.
To keep the voltage drop across a 30 amp meter to 1/10 of a volt, the resistance
of the meter must be 0.003333 ohms. This might be even lower than the contact
resistance of your test probes.
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 Richardson
DANMAS@aol.com wrote:
> Listers,
>
> Someone on this list suggested using a shunt to increase the range of ammeters
> to allow the use of higher amperage alternators without having to lose the
> ammeter function. Unfortunately, I missed the original post (you won't believe
> how busy I've been of late - especially for an old retired coot), so I don't
> have the individual's name.
>
> A couple of people have contacted me off list to ask about it, so I thought I
> would put together one of my long winded dissertations on the subject, and
> ammeters in general, using some material I had already put together, and post
> it on the web.
>
> Using a shunt is an excellent idea, but as usual, there are a couple of
> "gotchas" to be aware of. The write-up can be found at:
>
> http://members.aol.com/danmas2/ammeters.htm
>
> Thanks,
>
> 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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