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Ammeter shunt

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island time
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Joined: Mon Sep 28, 2015 9:11 pm

Ammeter shunt

Post by island time » Mon Feb 12, 2018 7:13 pm

I'm working on the 23 Commander and getting near to rewiring. All wiring will be replaced, I am going to make a new wiring harness using all the original factory wire colors. Several changes have been made electrically during the engine replacement including replacing the old Motorola alternator with external regulator with a new Delco 1-wire alternator with internal regulator. With all I know about electricity, admittedly there are some things I don't understand about DC such as...
When it comes to the ammeter, I am using the original dash gauge, is the shunt required? why? I don't understand it's function. Also with this particular device comes a requirement for a "very" specific wire length between it and the gauge. I can only imagine this relates to resistance?
As always I thoroughly appreciate all help.

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

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Bilge Rat
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Joined: Thu Jul 16, 2015 4:02 pm

Re: Ammeter shunt

Post by Bilge Rat » Wed Feb 14, 2018 8:13 am

The gauge shunt's purpose is to reduce the current (amperage) to a more easily readable value. If say your alternator can put out 40 amps at high charge, a direct series reading ammeter would have 40 amps passing through it's windings. Typically, that would require at least a 10 gauge conductor for the gauge windings, that's quite large and difficult to make the gauge accurate. Also, that much current passing through a gauge with a metal shell presents a number of safety issues to ensure the positive current cannot accidentally short out to the shell's ground. There have been some spectacular short circuits because of gauge insulation failure trying to conduct the full battery ampacity (hundreds of amps!). This is known as a fault condition. The shunt essentially is a calibrated resistor that is "tapped" to measure how much voltage reduction is passing through it. This is a tiny current value that is proportional to the amount of current passing through at the given moment. The gauge then reads this smaller proportional current value. This is much easier and safer for the gauge to display and will be more accurate. This technology is at least 100 years old, but works quite well.

Not sure the reasoning for the wire length limitation but I suspect you are correct about total resistance of the circuit. Like any conductor carrying current, too small a conductor will cause the wire to overheat as it's carrying too many amps. That overheating leads to conductor failure and burn-down. Not a good day of pleasure boating. Also, too small a conductor can cause a phenomenon known as voltage drop; the conductor's resistance is too great (at the current it's trying to carry) to guarantee a minimum of loss of voltage at the end of the circuit. That Ohms law thing; voltage, current and resistance all figure into the equation.
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

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island time
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Re: Ammeter shunt

Post by island time » Wed Feb 14, 2018 9:13 pm

Many thanks, your detailed explanation makes good sense.
I will share a little detail found in the wiring diagram obtained from the museum, the notes on the drawing say wire size and lengths are as follows- #16 gauge wire to be exactly 11', #14 Gauge-18' and #12 Gauge 28'. The note says to coil and tie excess wire near shunt.

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

User avatar
Bilge Rat
Posts: 153
Joined: Thu Jul 16, 2015 4:02 pm

Re: Ammeter shunt

Post by Bilge Rat » Thu Feb 15, 2018 7:30 am

That makes sense, the wire's resistance is critical in the metering portion of the circuit to maintain accuracy. Been chasing electrons for over 40 years but I haven't caught them yet!
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

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