Ammeter resistors are put back in the spotlight as the rise of renewable energy. The increase in the number of solar, wind, wave, and tidal power sources has pushed the need for a highly accurate method of measuring current. The solution is required for several reasons: to ensure accurate billing, have a more granular level of control, and, perhaps most importantly, to ensure the efficiency of the distributed power system.


There are a few ways to measure current. However, the one most suited for renewable applications looks to be conventional ammeters, using shunt resistors to handle the high currents. Renewable energy travels a more complex route to the power grid. Photovoltaic panels generate pure dc energy, which must use an inverter to be converted to ac to feed into the grid. Some wind turbines also generate dc, especially the smaller installations seen near homes, farms, and industrial units. In larger wind-farm installations, it’s possible for the turbines to produce ac power. However, connecting the power to the grid requires the voltage and frequency to be compatible with the grid. In many cases, though, this isn’t the case. The voltage and frequency of the output power is correlated with the rotational speed of the wind turbine, which depends on the wind speed.

One solution is to use a gearbox to sync the turbine speed with mains frequency. Some measurement tools based on Hall-effect current transducers that can measure both ac and dc current, and they too provide galvanic isolation. The main problem with this type of measurement is the accuracy—it’s normally impossible to get the required accuracy of 0.1% (or better) using Hall-effect current transducers. Then there comes the third option: use ammeters with precision shunt resistors, while this type of measurement doesn’t provide galvanic isolation, so this must be included elsewhere.

Ammeter shunt resistor operation is well-understood and used extensively for current measurement in the past. The resistors are used to “shunt” most of the current around the measurement instrument. This technique allows measurement of greater currents than is possible using normal ammeters. The low-value resistor is situated in series with the load, creating a small voltage drop across the resistor, which can be measured using a voltmeter. The current is then calculated using Ohm’s law.