What do a shunt do , anyway ?
I was looking at a meter and it said shunt required , Why ?
Forum Discussion
MrWizard wrote:
the meter it self won't stand up to the 'amps' being pulled
can't carry amps thru the meter
the shunt mounts at the battery and your charger and load wires connect to the shunt
the shunt is a Heavy and calibrated resistor, usually 75 ohm
the load amps passes thru the shunt, the meter measures the voltage drop across the shunt, to give you the reading of the power amps being used
This is essentially correct, except a shunt's resistance is nearly always far, far lower than 75 ohms. Many shunts are sized to have a 75 mV drop at their full current; for a 100A shunt, Ohm's law dictates that the resistance is 0.75 milliohms or, equivalently, 750 microohms. A 10A 75mV shunt would have a resistance ten times higher (7.5 milliohms).
Many ammeters that don't have an external shunt have an internal shunt and operate on the same principle. The main advantages of a separate shunt are that you don't need to route heavy wires through the ammeter itself if it's removed from where the current you're measuring is located, and the meter doesn't need to have suitably heavy connectors (and space for them) or to worry about the power dissipation inside the meter. A 100A 75mV shunt dissipates 7.5W at full scale, which is plenty enough heat to worry about in a small enclosed meter case.
