Shunts for ammeters

[Night Train]

I have just won some ammeters on ebay, Linky.

I am only really interested in the DC ones that seem to peak at 700A.
When they arrive what is the right way to determine the correct shunt size to get that 700A FSD?

I will be using one on my electric tractor and the other on my electric car when it gets built.

Thank you

 

[TheDoctor]

Generally speaking, something reading 700A will actually be a voltmeter measuring the voltage drop across a known resistor - usually in the order of a milliohm or less. If I is 700A, and R is 0.01 milliohm, then V (your voltage drop) is 7 millivolts. Your problem here may well be getting a sufficiently accurate resistor that'll pass 700A, especially if you're going to connect it permanently.
Of course, those ones you've bought may well have the shunt built in, in which case you're laughing.
If not, I'd look out for a DC clampmeter and use that.

 

[Night Train]

Cheers TD, I already have a clamp on DC ammeter but I was hoping to use these for the vintage look.
I guess I need to work out the FSD mV required and take it from there.

I know one way is to put a known load in circuit with a large copper bar and then use the ammeter terminals as a wiper to measure the voltage along the copper bar until it reads the expected, or known, current. The length of bar between the wiper contacts is then the shunt. That is fine for smaller currents, a few 10s of amps, but I would struggle to find a known 700A load to put on my 48V battery pack to reasonably calibrate it.

 

[fossyant]

ABC ZXY la la la Might as well be speaking Martian !

 

[dan_bo]

I'd be buggered I was messing around with ~ 700A.

 

[Garz]

Do you have your own substation?

 

[subaqua]

Do you have your own substation?

700A DC isn't that large, especially at lower voltages.

 

[Night Train]

At 48V it is 33.6kW and I still don't have a suitable known load.
My tractor should only peak at 400A in its current set up, 19.2kW.

I will test the ammeter for its FSD voltage and then see if I can find a shunt that fits. Ebay has 600A 75mV shunts so I will need to see what 75mV gives me on the ammeter.

 

[subaqua]

enjoy playing

 

[Night Train]

I've worked out I get FSD to 600A with 36mV.
I haven't managed to measure the internal resistance of the ammeter as it is too high, nor the resistance of the wire I dropped 36mV across as it was too low!
May try using my DMM as an ammeter to measure the current flow through the ammeter.