Cover story t SponSored FeATUre When current flows in a resistor, Joule heat is
generated proportionally to the square of the current. This will cause not only losses in terms of efficiency, but the self-heating will change the shunt resistive value itself with a consequent accuracy degradation. To limit the self-heating effect, a low value resistance is used. However, when a small resistance is used the voltage across the sensing element is also small and sometimes comparable with the dc offset of the system. In these conditions, achieving the required accuracy on the low end of the dynamic range may not be a trivial task. State-of-the-art analogue front ends, with ultralow dc offset and ultralow temperature drift, can be used to overcome the limitations of small value shunt resistors. However, as operational amplifiers have a constant gain- bandwidth product, a high gain will limit the available bandwidth. Low value current sensing shunts are usually made from specific metal alloys such as manganese-copper or nickel-chrome, which cancel the opposing temperature drifts of their constituents to result in an overall drift in the order of tens of ppm/°C.
Magnetic Field SenSing - indirect current MeaSureMent
Open-Loop Hall Effect The sensor is constructed with a high magnetic permeability ring through which the sensed current wire is passed. This concentrates the magnetic field lines surrounding the measured conductor onto a Hall effect sensor, which is inserted within the cross-section area of the magnetic core. The output of this sensor is preconditioned and usually available in different flavours. The most common are: 0 V to 5 V, 4 mA to 20 mA, or digital interface. While providing isolation and high current range for relatively low cost, absolute accuracies typically do not range below one per cent.
Closed-Loop Hall Effect A multiturn secondary winding on the permeable core driven by a current amplifier provides negative feedback to achieve zero total flux condition. By measuring the compensating current, linearity is improved and there is no core hysteresis with overall superior temperature drift and higher accuracy compared to the open-loop solution. Typical error ranges are down to 0.5 per cent, but the additional compensation circuitry make the sensor more expensive and sometimes limited in bandwidth.
Fluxgate
Figure 6. Thermal EMF in shunts caused by temperature gradient.
Another error contributor in direct
connection dc measurement can be the phenomenon of thermal electromotive force (EMF), also known as the Seebeck effect. The Seebeck effect is a phenomenon in which a temperature difference between at least two dissimilar electrical conductors or semiconductors forming a junction produces a potential difference between the two. The Seebeck effect is a well-known phenomenon, and it is widely used for sensing temperature in thermocouples. In the case of 4-wire connected current
shunts, the Joule heat will form on the centre of the resistive alloy element, propagating whilst the copper sensing wires, which may be connected to a PCB (or a different medium), and which may have a different temperature. The sensing circuit will form a symmetric
distribution of different materials; therefore, the potential at the junctions on the negative and positive sensing wires will approximately cancel. However, any difference in thermal capacity, such as a negative sensing wire being connected to a larger copper mass (ground plane), can produce a mismatch in the temperature distribution, resulting in a measure error caused by thermal EMF effect. For that reason, attention must be reserved to the connection of the shunt and at the distribution of the generated heat.
Instrumentation Monthly August 2021
Is a complex open- or closed-loop system where the current is measured by monitoring the magnetic flux variations of an intentionally saturated core. A coil is wound around a high permeability ferromagnetic core that is intentionally saturated by a secondary coil driven by a symmetric square wave voltage. The inductance of the coil collapses every time the
Figure 7. An open-loop current transducer based on a flux concentrator and magnetic sensor.
Figure 8. An example of the working principle of closed-loop current transducers.
core approaches positive or negative saturation, and the rate of change of its current increases. The current waveform of the coil remains symmetrical unless an external magnetic field is additionally applied, in which case the waveform becomes asymmetrical. By measuring the size of this asymmetry, the intensity of the external magnetic field, and consequently the current that generated it, can be estimated. It provides good temperature stability and accuracy down to 0.1 per cent. However, the complex electronics of the sensor makes it an expensive solution with prices 10 times higher than the other isolated solutions.
Figure 9. DC meter system architecture.
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