DC Ammeter Shunt Sizing
Sizing and reading a DC current-measuring shunt -- the precision low-value resistor a DC panel meter, battery monitor, or PV/DC combiner uses to measure current. A shunt is rated as a millivolt drop at a rated current (a '50 mV, 100 A' shunt), so R = rated mV/1000 / rated current = 0.5 milliohm, and the meter reads current = rated current x (measured mV / rated mV), so 25 mV on a 50 mV/100 A shunt is 50 A. At rated current it dissipates rated current x rated volts = 5 W, which is why shunts derate to ~2/3 for continuous use. Put it in series in the return leg and sense at the potential (voltage) terminals so lead resistance does not add to the reading. A design aid; the shunt accuracy class, temperature coefficient, and the meter's input range and calibration govern.
Formula and source
shunt_resistance_ohm = (rated_millivolt / 1000) / rated_current_a; measured_current_a = rated_current_a x (measured_millivolt / rated_millivolt); power_dissipation_w = rated_current_a x (rated_millivolt / 1000).
DC current-shunt sizing by Ohm's law, by name; the shunt's accuracy class and temperature coefficient and the meter's input range and calibration govern the measurement.
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