Method 1: Shunts
Shunts measure current directly based on Ohm’s law. Current flowing through a resistor causes a voltage across that resistor to drop in proportion to the current. This linear correlation makes it especially simple to process the analog signal into a measurement. A current sensing amplifier processes the measurement voltage so that the input voltage range of the analog/digital converter (ADC) is properly utilized in the microcontroller.
Suitable shunts are available from suppliers such as Vishay, Rohm, KOA, and Panasonic. They range in size from chip resistors to massive metal arches with screw connectors and power losses of up to tens of watts.
If the layout and signal pickup is compliant with the handling conditions stipulated by the manufacturer, the imprecision of the measurement will be dependent on the tolerances of the components used in the signal chain. The information in the data sheets makes it relatively easy to evaluate the precision of the system as a whole.
Advantages with the use of shunts for current measurement are:
- High bandwidth
- Low sensitivity to interference
- Large and diverse range of products
- Can be integrated into busbar (power rail)
Disadvantages:
- Measurement principle with power loss proportional to R and I2
- No galvanic isolation between measurement current and measurement signal
- Expertise in analog signal processing required
Rutronik24.com provides a decent list of suitable shunt resistors. For more informations, please visit: https://www.rutronik24.de/category/precision-chip-resistor/
Method 2: Magnetic Field Sensors
This method involves the magnetic field sensor being placed on the electrical conductor. A cylindrical magnetic field forms around the straight, energized conductor. According to Ampère’s law, the strength of the magnetic field is dependent on the current and the distance from the conductor. A user constructing a current sensor based on this principle must therefore control the mechanical production tolerances and the mechanical stability of the structure as a whole.
The advantages of using magnetic field sensors are:
- Galvanic isolation of measurement current and measurement signal
- Practically zero power loss in measurement
- Digitalization already performed in the magnetic field sensor
- Suitable for measurement of large currents
Disadvantages:
- Potentially sensitive to stray fields
- Mechanical expertise required of user, geometry, and production tolerances reflected in transfer function
- Low bandwidth
Magnetic field sensors are available on the market from suppliers such as Infineon, Micronas, and Melexis, and can be found at rutronik24.com: https://www.rutronik24.de/category/current-sensors/
Method 3: Ready-to-Use Current Sensor ICs or Current Sensor Modules
The use of prefabricated current sensor modules or ICs is much simpler than using magnetic field sensors, as manufacturers will have already solved the mechanical design issues. The transfer curve is known and is shown in the data sheet. Current sensor ICs for soldering onto PCBs where the conductor is fed through the IC package are widespread. For higher currents, there are modules with package (through-)holes through which the power conductor is looped once or several times. Varying the loop count provides a simple method of adjusting the sensitivity of the measurement system.
The advantages of using ready-to-use modules and ICs are:
- Ease of use with curves as described in the data sheet
- Galvanic isolation of measurement current and measurement signal
- Practically zero power loss in measurement
- Digitalization already performed in the sensor module
Disadvantage:
- Few suppliers
The manufacturers of current sensor ICs and modules include Infineon and BYD. The products are available at www.rutronik24.com: https://www.rutronik24.de/category/current-sensors/
Conclusion
As battery-powered devices become more widespread and electrification progresses, there are ever more DC currents to be measured. Depending on the current level and the frequency range of a superimposed AC component, there are various sensors available: shunt resistors, current sensor modules/ICs, or proprietary developments with magnetic field sensors.
Shunt resistors are especially ideal for low voltages due to the lack of galvanic isolation and for low currents due to the unavoidable power loss, which increases in proportion to the square of the current.
Even so, they are used in some electric vehicles with currents of several hundred amperes and voltages of 400V for detecting battery currents.
Because shunt resistors have a low self-inductance, the frequency range is large and is dictated broadly by how analog signals are processed downstream.
Current sensors using magnetic fields are especially well suited to large currents, as they experience practically no power loss, and are ideal for high voltages as they offer galvanic isolation. The bandwidth is usually limited by the sensor anyway and less so by the downstream electronics.
Current sensor modules and ICs are ideal for projects requiring a shorter time-to-market.
For more information about concepts for measuring direct current, please visit our e-commerce platform at: www.rutronik24.com.
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