My daughter has an old car 1977, the local (australian) version is a "holden gemini" the australian versions were not fitted with a factory ammeter but some international models were. She has obtained a factory ammeter from another country which fits perfectly in an instrument cluster. The problem is it does not have an internal shunt resistor and of course running the entire current draw thru the little meter would be disasterous. The range of the meter is 50 amps (+ and -) Can anyone suggest a shunt resistor that would come anywhere near being correct? There's no need to be highly accurate, just close enough to be convincing. Looking forward to any help I may get, thanks.

 

Assuming your meter has a standard 50 mV deflection, you will need a 0.001 ohm resistor. Alternatively, you can use copper wire ( e.g. : 1.2 inches of 20 awg solid copper wire. That is, 0.1 foot (1.2 inches) between the connecting lugs or solder joints. The total length will likely be longer so you can secure it.)

The problem with the copper wire is that 20 awg diameter copper wire might start getting warm with 50 amps so a longer piece of fatter wire will be better, i just dont know the size/space limitations you have. Also, the size of the mounting lugs (bigger is better) will influence how well the heat is managed. You can expect 2.5 watts of power from the resistor (copper wire) at full 50 amp current. You can check wire resistance on google and select a proper wire thickness or, better, a real resistor.

 

Hey GopherT, thanks heaps for the reply. I am worried that maybe 20 awg may not handle the "potental" 40- 50 amp load in an automotive situation. Any comments?

 

Simply buy a 50 Amp 50mv shunt.. not a resistor..not a piece of wire.. an actual shunt.
Like this
http://www.cshunt.com/dc_ammeter_shunts.html
of course it could be 100mv scaled too..