Current sensing
The selection of a current sensing method depends on requirements such as magnitude, accuracy, bandwidth, robustness, cost, isolation or size.
The current value may be directly displayed by an instrument, or converted to digital form for use by a monitoring or control system.
The generated signal can be then used to display the measured current in an ammeter, or can be stored for further analysis in a data acquisition system, or can be used for the purpose of control.
The parasitic inductance present in the shunt affects high precision current measurement.
The intrinsic resistance of a conducting element, such as a copper trace on a printed circuit board can be used as a sensing resistor.
Accuracy is limited by the initial tolerance of manufacturing the trace and the significant temperature coefficient of copper.
However, these amplifiers are expensive and can also limit the bandwidth, accuracy and thermal drift of the original current sensing technique.
Faraday's Law of induction – that states: the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit – has been largely employed in current sensing techniques.
Two major sensing devices based on Faraday’s law are Current transformers (CTs) and Rogowski coils.
The Rogowski coil has a low sensitivity due to the absence of a high permeability magnetic core that the current transformer can take advantage of.
The Hall voltage is a low level signal on the order of 30 μvolts in the presence of one gauss magnetic field.
This low-level output requires an amplifier with low noise, high input impedance and moderate gain.
A differential amplifier with these characteristics can be readily integrated with the Hall element using standard bipolar transistor technology.
In the simplest configuration, a Hall-effect magnetic field sensor can be placed adjacent to the conductor and its output measured but there are limitations.
Also the Hall voltage produced will be tiny so very high amplification would be required with its associated thermal instability and noise issues.
Drawbacks of saturable inductor technologies include limited bandwidth for simpler design, relatively high secondary power consumption, and risk of current or voltage noise injection into the primary conductor.
A magneto-resistor (MR) is a two terminal device which changes its resistance parabolically with applied magnetic field.
Despite this, these sensors (GMR, CMR, and TMR) are still more expensive than Hall-effect devices, have serious drawbacks related with nonlinear behavior, distinct thermal drift, and a very strong external field can permanently alter the sensor behavior (GMR).
