Ammeter & Shunt Question

Norman_E

Norman_E

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My boat has a 100 amp ammeter built into the electrical panel above the chart table and matching the other instruments. It has however been disconnected because the engine has been fitted with a 120 amp alternator.

Is it possible to work out how to fit a shunt (i.e. what size) so that the ammeter will read close to full scale at the full 120 amp output, and not be damaged. I have in mind arranging for it to read 90 at full output and pro rata, e.g reading 30 when the output is 40 amps. I have not looked behind the panel, so I dont know if the original shunt is still in place.
 
I can understand how you'd want all the kit to match...... but how often (and for how long) does your alt actually output over 100A ? If the meter is moving coil the needle will have a little extra 'reach' past the end of the scale anyway, before it hits the little pin - could you just take this to be your "100 to 120A" area?

The shunt may well be marked with its resistance - like 0.xR..... A few mins with Ohms law will allow you to specify a shunt which will make the scale read anything you like. If you don't have the shunt you'll need to find out the spec for the meter itself. Often they are marked on the panel, like

FSD 10mA (FSD= full scale defection)

or just

10mA

Once you know this you can again specify the shunt.

Hope that helps
Andy
 
A good reason for fitting a digital ammeter I would have thought. You could halve the resistance of the shunt or install a resistor in series with the meter, either + or - side. Without knowing the meter spec I would suggest lashing a 1 Megohm variable and adjust to read as you wish. Remove and measure with a meter and replace with the nearest fixed resistor.
Safety note: As you will be dealing with sources of high current, do not short out!
 
Thank you both, I will have to do a bit of research into the boat's wiring. The ammeter in question has a switch beneath it to switch between different battery banks but does not work in any position of the switch. It is the middle one in the left hand bank of meters and switches in this photo.
ChartTable.jpg

I suspect that the meter itself has been disconnected, and the label in German under the switch says "Nicht Beschaltet" or something similar.

P.S. Owners of new HR 31s please note, this is what a chart table looks like!
 
Norman_E said:
Thank you both, I will have to do a bit of research into the boat's wiring. The ammeter in question has a switch beneath it to switch between different battery banks but does not work in any position of the switch. It is the middle one in the left hand bank of meters and switches in this photo.
ChartTable.jpg

I suspect that the meter itself has been disconnected, and the label in German under the switch says "Nicht Beschaltet" or something similar.

P.S. Owners of new HR 31s please note, this is what a chart table looks like!
Click to expand...

If you have several banks of batteries then there may actually be a shunt adjacent to each bank and the switch selects which shunt it wants to read.

The shunts will not look like anything sophisticated and may just be a short length of flat conductor in the negative lead with a small terminal at each end for the meter feeds.
 
Amp meter

Firstly I think the 100A range would be fine for a 120 amp alternator. As said a 20% overload of the meter will not hurt the meter and I would bet you would never see 100A out of the alternator. And if you did it would be for a very short time until very flat batteries recovered.
The advantage of an amp meter with remote shunt which is almost always the case for high current meters is that the heavy wiring that might carry 100A does not have to be taken up to the meter but rather the shunt can be next to the battery or alternator and light wires carry the tiny current being measured.
As said the shunt will be in the form of a bar of metal like 5 to 10 cms long with heavy bolts each end for the large wires and 2 smaller wires going off to the meter. The bar may be under an insulating cover.
If the switch has a selection of metering options then there should be or have been a shunt for each option. Each small wire to the meter should have a small fuse near the shunt. This is because any short to -ve of the small wires will potentially carry all the main circuit current so smoking the small wires.
So one at each battery and one for the alternator.

If you really want to increase the range for the meter a small resistor in series with the meter will reduce the current through the meter so increasing range. One trick is to measure the resistance of the meter it might typically be 500 or 1000 ohms. If you fit a similar resistance in series you will double the current range. You could fit a small toggle switch in to short the resistor so reverting to original range. So switch would be 100A in one position and 200A in the other. Or any other range you choose the relationship of resistance of the meter to is in direct proportion. So 20% of the meter resistance will increase the range by 20%. Good luck tracking down that wiring. olewill
 
Shunts are normally specified as number of amps for a drop of number of Milli volts.

My main battery shunts, and I have 3, are 500 Amps for 50mV. I have a bow thruster that can is specified to 600 Amps max and I have a DVM with max range of 20mV and all works fine except when I operate the bow thruster I get an over range on the DVM which os OK by me.
 
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