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Are current shunt resistor values usually based on the max. current that will be seen and the smallest amount of current change that can be detected?
I'm wondering if the current shunt resistance value is independent of the voltage on the rail you're measuring. If the current across multiple rails in a system is the same and you want the same resolution on each rail, will the current shunt resistance be the same for a 1 to 3 V rail and for a 1 to 12 V rail?
The choice of a shunt resistance value is always a compromise. You have to make the resistance value high enough to get a good signal for your measurement system. But then you have to make sure it is not so high that it causes excessive voltage drop. For higher currents, the power dissipation of the shunt resistor can become a serious issue also, causing the shunt to overheat, or simply generating too much heat in the localized area where the shunt is.
One solution that can sometimes be employed is to use an amplifier to amplify the voltage across the shunt. Specialized amplifiers exist called "current shunt amplifiers."
Amplification can provide the best of both worlds by increasing the signal amplitude without increasing voltage drop or power dissipation in the shunt itself.
The voltage of the power supply whose current is being measured by the shunt is not really a part of the calculation. Although if you want to use a high-side shunt with a current sense amplifier (CSA) then you need to select a CSA that can accommodate the high-side voltage. For a low-side shunt, no special requirements exist for the CSA since the sensing voltage is near ground potential.
It appears that you are mixing up series resistors, intended for measuring higher voltages, with shunt resistors, intended for measuring higher currents.
The range of a milliammeter may be increased by passing the excess current through a parallel resistor, having a lower resistance value than that of the meter.
The parallel resistor is known as the meter shunt.
Shunt resistance = Meter resistance / (n-1) where 'n' is the number of times by which the full scale reading is to be increased.
For example, the range of a 45 Ω meter may be increased from (0 - 1 mA) to (0 - 10 mA) with the help of a 5 Ω shunt resistor (45 / (10-1) = 5).
In practice, a meter shunt is a conductor that has low resistance and is capable of carrying the maximum current that is to be measured.
Here's a drawing of a high current meter shunt.
Holes for shunt / cable mounting and screws for meter wires are shown.
Shunts are classified, not by their resistance values, but by the voltage drop across them at rated current.
Standard values are 50, 60, 75, 100 and 150 mV to pair them with millivoltmeters having full scale readings 50, 60, 75, 100 and 150 mV but graduated in amperes.
