Ammeter & Shunt Question

[Norman_E]

The 100 amp reading ammeter on my boat has been disconnected because a 110 amp alternator is fitted. Can I re-connect it if I fit a shunt that will pass more of the current? I cannot change the ammeter because it is built into a panel, but I could re-graduate it, for example changing the present 20, 40, 60, 80, & 100 markings to 25, 50, 75, 100 & 125.
My question is how do I work out what shunt to fit?

 

[VicS]

You need to know the sensitivity of the meter movement (ie what current it requires to actually pass through it to give fsd) and its resistance

It is however very unlikely that you will ever get the full 110 amps and even if you do the meter should withstand a 10% overload for short periods. i take it the shunt is still in circuit and being subjected to the overloads if any.

I would simply reconnect the meter to the shunt. That way you wont have to worry about any recalibration. You might very occasionally send the ammeter off scale, but I doubt it.

You might find some helpful info in the electrical section on http://www.tb-training.co.uk/

 

[savageseadog]

Just halve the value of the shunt. somehow. Then your meter readings will double. One way would be to connect antother identical shunt in parallel. Other way would be to move the tappings closer together ie halve the distance, if you can do it.

 

[Stoaty]

I go along with VicS here. Just connect it back up. You will probably find it very unlikely that you will see more than 95 amps continuous out of a 110 amp alternator. You would have to have a very dead, very large, battery bank, with the charge just started and nothing else switched on, even then it would only put out maximum charge for a brief period.

 

[Norman_E]

Thanks all. Very interesting. I will try re-connecting it when I re-launch the boat. I have not fully investigated the wiring but there are two meters. The one in question shows amps charge, and the other one (connected) shows consumption.

 

[Stoaty]

Let us know how it all performs.

 

[jimbaerselman]

[ QUOTE ]
I have not fully investigated the wiring but there are two meters. The one in question shows amps charge, and the other one (connected) shows consumption.


[/ QUOTE ] If, by 'consumption' you mean A/H, then both meters almost certainly use the same shunt. A/H reading is just a time summing (integration) of the amps reading.

 

[Norman_E]

No, it is not showing amp hours consumption, but an instant readout of amps discharge. I suspect that there are diodes hidden somewhere because this meter is does not seem to be affected by charging either from the alternator or the battery charger, though I have not studied it when the engine is running, but notice it showing a reading if the cabin lights are on or if there is a pump running. It will be April before I am able to investigate the wiring.

 

[VicS]

[ QUOTE ]
I suspect that there are diodes hidden somewhere

[/ QUOTE ] No reason for there to be diodes, at least not unless they are both wired to the same shunt.
If each has its own one is probably just in the alternator output wiring while the other is in the wiring going to all the services. Then one reads the alternator output and the other reads what the ship is consuming.

BTW it would be wise to have small fuses in both leads to both meters located close to the shunts. Then if a fault occurs on any of the four connections the fuse will blow rather than the wiring catching fire!