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fedico94(at)mchsi.com
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 Posted: Wed Oct 24, 2012 5:32 pm Post subject: Compatable Shunt |
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The 60 amp alternator reads now on the G3X system. I do not have a reading on the Auxillary alternator even though I have installed another 50 mv/100 amp shunt. When I get the cowling off then I will switch the wires. The auxillary alt ammeter still has a red "x" through it. How did things work out with the folks at Garmin ? Any new insights ?
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nuckolls.bob(at)aeroelect
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| Posted: Thu Oct 25, 2012 8:28 am Post subject: Compatable Shunt |
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At 08:30 PM 10/24/2012, you wrote:
| Quote: |
The 60 amp alternator reads now on the G3X system. I do not have a
reading on the Auxillary alternator even though I have installed
another 50 mv/100 amp shunt. When I get the cowling off then I will
switch the wires. The auxillary alt ammeter still has a red "x"
through it. How did things work out with the folks at Garmin ? Any
new insights ?
|
Do I recall correctly that a local individual
correctly diagnosed/rectified a similar difficulty
with the main alternator shunt?
Aside from a reasoned deduction that the G3X system
is calibrated to read 1 mv of shunt signal as 1 amp
of measured current, I don't have enough data to
suggest a root cause for the problem repeating on
the aux alternator shunt.
You spoke of 'switching wires' . . . exactly what
is the proposed experiment. It MIGHT be that the
'x' is in response to a minus reading . . . so if
there are any wires to be swapped, it would be only
the small wires that run from the shunt to the G3X
ammeter input pins.
I won't be face-to-face with the Garmin folks I
mentioned until next spring or perhaps later. As
a paid user of the Garmin products it would seem
that you're better advised to strike up a conversation
with Garmin yourself. You might inquire of the individual
who fixed the other shunt as to what he discovered and
the rationale for his actions that solved the problem.
Discussions on forums like this are most productive
when an individual with specific knowledge and experience
can contribute. All the rest of us can only hypothesize
on possible causes . . . those hypotheses are based
on the depth and purity of observations offered.
It seems the path to understanding and resolution has
already been demonstrated. I'm not trying to be obtuse my
friend - but neither I or anyone else can do more than
make educated guesses without having special knowledge
of the circumstances.
Bob . . .
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bicyclop(at)pacbell.net
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| Posted: Thu Oct 25, 2012 8:49 am Post subject: Compatable Shunt |
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Speaking of shunts, B&C doesn't sell a 40A shunt to match their 40A alternator. The next size up is 75A and the next size down, 30. What to do?
Ed Holyoke
On 10/25/2012 9:26 AM, Robert L. Nuckolls, III wrote:
[quote][b]
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JLuckey(at)pacbell.net
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| Posted: Thu Oct 25, 2012 10:15 am Post subject: Compatable Shunt |
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Ed,
Shunt rating is not directly related to alternator output. It is directly related to how the ammeter is calibrated. Therefore you must match the shunt w/ the ammeter. Also, there is nothing magic about manufacturer/ammeter/shunt compatibility. A shunt is essentially a highly-calibrated resistor. So a shunt from manufacturer A will work just fine w/ an ammeter from B as long as their “mili-volt” rating is the same. The ammeter will tell you, for example, that it requires a 50 amp, 50mV shunt. (You want to have an ammeter/shunt current-rating that exceeds the alternator output).
Ed, in your case you need to decide what range ammeter works for your application. I might recommend a 50 amp ammeter, or maybe a 60 amp is a little more common. Then buy the appropriately-rated shunt for that ammeter (from any manufacturer) and you will be good-to-go. You may never get full deflection on the positive/charge side of the ammeter (because the alternator only puts out 40 of the 60 amp the gauge can display) but that’s not a big deal. (BTW – I always recommend a zero-center ammeter because it can show a lot of useful info, however, the shunt must be placed in the correct location in the battery circuit. But that discussion is beyond the scope of this message)
I recently shopped for ammeter hardware and I decided on a 60A VDO ammeter (they are a little pricey, but I like the way they look and the 270 degree needle sweep is really cool!). Then I bought a shunt from whoever had a good price. I think I got the shunt from AC Spruce and was about $35 (the VDO shunt was over $70 and was gigantic) – It all works just fine.
You might look at ACSpruce as they have several ammeters & shunts available.
In the attached photo, the shunt is the thing just right of center and has a light-green base. Don’t forget to fuse BOTH wires that go from the shunt to the ammeter. They are both hot B+. This is the “power panel” for my RV-7. It’s handy because it sticks all the high-current components in a 6x6 inch space and makes wiring very easy.
I know – too much information,
Jeff Luckey
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Ed Holyoke
Sent: Thursday, October 25, 2012 09:48
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Compatable Shunt
Speaking of shunts, B&C doesn't sell a 40A shunt to match their 40A alternator. The next size up is 75A and the next size down, 30. What to do?
Ed Holyoke
On 10/25/2012 9:26 AM, Robert L. Nuckolls, III wrote: | Quote: | | http://www.matronics.com/Navigator?AeroElectric-List |
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nuckolls.bob(at)aeroelect
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| Posted: Thu Oct 25, 2012 1:33 pm Post subject: Compatable Shunt |
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At 11:48 AM 10/25/2012, you wrote:
| Quote: | | Speaking of shunts, B&C doesn't sell a 40A shunt to match their 40A alternator. The next size up is 75A and the next size down, 30. What to do? |
It depends on what you want the shunt to do . . .
What is the proposed display for data offered by
the shunt? If it's a 'steam gage' then the shunt
must be sized to millivolts/amp that are matched to
that gage.
The legacy shunt/instrument configuration suggests
that a shunt driven ammeter have a full scale indication
when fed with a 50 millivolt DC signal. This 50 mv 'standard'
is what drives the shunt manufacturing community to
offer ratings like 60A/50mv and 100A/50mv. These same
shunts paired with an 40 mv instrument would provide full
scale displays at 48 and 80 amps respectively.
The shunt is a 4-terminal resistor with a very low
temperature coefficient of resistance. The loadmeters
I offer are actually 53mv full scale devices. Further,
the instruments are intended to be loadmeters that display
a PERCENTAGE of rated load for the circuit being monitored.
http://tinyurl.com/dx9sz3e
If the customer wants 100A full scale loadmeter, the shunt
would be specified as 100A/53mv. I custom-scale the shunt
to the match the instrument in conformity with customer
design goals.
We've been having some discussions about shunts used with
the Garmin G3X system where the manual calls for 100A/50mv
shunts. This is a constant because the instrument is scaled
to display 2A per millivolt . . . irrespective of the size
of the alternator or other limits that would never produce
so large a display.
If you are driving a loadmeter scaled to your 40A alternator,
then you'll need a 40A/50mv shunt . . . which I can craft for
you. If your ammeter is a steam gage, then the shunt needs
to be paired with the instrument's full scale value irrespective
of the alternator's output. If you're driving an electronic display
that can be calibrated by the installer, then a 30A or 60A shunt can be
used . . . and compensated for by the calibration procedure.
So we need to know more about how the shunt is used and what
the instrument expects.
Bob . . . [quote][b]
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