Ammeter - Shunt question

[ray_mcavoy]

Originally Posted by RMBLFSH

Ray, thank you for the further investigation.

You're welcome!

From what you have learned would you agree that a shunt with a rating of 70 amps and 180 millivolts would work in this application?

Yes, based on what I have measured, a shunt with a 70A, 180 mV rating would in theory give a full-scale reading with 70A flowing through the circuit.

I say in theory because I believe most commercially available ammeter shunt ratings are calculated for use with a high impedance meter movement. As I posted above, the coil inside the factory ammeter is only about 0.1 Ohms. That will allow a considerably higher amount of current to flow through the meter itself than what you'd get with a high impedance movement.

Also, assuming those exact values are not available, how close does the shunt need to be, would a 100 amp, 200 millivolt shunt work?

As far as how close it needs to be, I don't think it is highly critical. Like VetteVet said earlier, the factory ammeter is more or less just a charge/discharge indicator and the original shunt was just a set length of 12ga wire. If the shunt resistance is too low, you won't get very much needle movement on the ammeter. If the shunt resistance is too high, too much current will flow through the meter itself and blow the inline fuses. Basically, I'd consider anything in between those two extremes to be acceptable.

A 100A, 200mV shunt will have a little lower resistance (as compared to a 70A, 180mV shunt) so I'd expect that to give a slightly lower reading on the ammeter for the same current. But again, those shunt ratings are most likely calculated for use with a high impedance meter, not the 0.1 Ohm factory ammeter so that's going to change the calculations.