Dan :
I was the author of the recent post which included the idea of using a
shunt to increase the range of the factory ammeter. I don't know that I
can claim to be the first person to think of it, but I _can_ say I used
this idea around 1982 <g>
About your web page :
1) Nice diagrams and explanations !
2) There is a simple method to measure the resistance of the ammeter : run
a known current through it and measure the voltage drop. By Ohm's law, the
resistance is the voltage divided by the current. (Volts/Amps = Ohms) So,
for example, if you run 10 amps through the meter and measure 7 millivolts,
the meter has a resistance of 0.7 milliohms. Even a cheap digital meter
should be good to a millivolt. Where can you find a 10 amp current, you
ask ? Try turning on the headlights ! <g> The lighting load on a TR
(engine off) is close enough to 10 amps for this exercise. For a better
value, read the ammeter itself (which still isn't very good). Best would
be a DVM in series, many of them will handle 10 amps.
3) I took an easier way : using the headlights as a test load, I used "cut
and try" to get a 50% reading. Since the factory ammeter is a simple
"moving vane" type, it has a few turns of fairly heavy copper wire. So, an
inch or so of 12 gauge copper should get you pretty close. In my case, I
used steel baling wire (22 gauge ?) that was handy, and added multiple
strands to get the ratio I wanted. Two strands were pretty close, and
that's what I use. By the time I was finished, the battery was run down
enough to see that the full alternator output just about moved the needle
to the 30 mark.
4) I can't see that the exact ammeter reading is of any real value anyway.
As long as the maximum current is near the limit of the meter, who cares
if it's 40 amps or 80 amps ? The important thing is that the reading is
near zero, except when charging the battery (or running those pesky horns
<g>)
BTW, does Alcoa TN have anything to do with the ALuminum COmpany of America
?
Randall
On Thursday, February 25, 1999 3:29 PM, DANMAS@aol.com
[SMTP: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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