Trace Resistance sensing

The intrinsic resistance of a conducting element, usually a copper trace in Printed circuit Board (PCB) can be used as sensing element instead of a shunt resistor.^[4] Since no additional resistor is required this approach promises a low-cost and space saving configuration with no additional power losses either. Naturally, the voltage drop of a copper trace is very low due to its very low resistance, making the presence of a high gain amplifier mandatory in order to get a useful signal. There are several physical effects which may alter the current measurement process: thermal drift of the copper trace, initial conditions of the trace resistance etc. Therefore, this approach is not suitable for applications that require a reasonable accuracy due to the large thermal drift. In order to overcome the problems associated with the temperature drift, a digital controller can be used for thermal drift compensation and calibration of the copper trace.^[5] A significant drawback of this kind of current sensor is the unavoidable electrical connection between the current to be measured and the sense circuit. By employing a so-called isolation amplifier, electrical isolation can be added. However, these amplifiers are expensive and can also deteriorate the bandwidth, accuracy and thermal drift of the original current sensing technique. For these reasons, current sensing techniques based on physical principles that provide intrinsic electrical isolation deliver a better performance at lower costs in applications where isolation is required.

Current transformer

Current transformers used as part of metering equipment for three-phase 400A electricity supply

The CT is based on the principle of a transformer and converts a high primary current into a smaller secondary current and is common among high AC current measurement system. As this device is a passive device, no extra driving circuitry is needed in its implementation. Another major advantage is that it can measure very high current while consuming little power. The disadvantage of the CT is that a very high primary current or a substantial DC component in the current can saturate the ferrite material used in the core ultimately corrupting the signal. Another problem is that once the core is magnetized, it will contain hysteresis and the accuracy will degrade unless it is demagnetized again.

Rogowski coil

Rogowsky coil

Rogowski coil is based on Faraday’s law of induction and the output voltage V_out of the Rogowski coil is determined by integrating the current I_c to be measured. It is given by,

${\displaystyle V_{\rm {out}}=-k{\frac {NA\mu _{0}}{2\pi r}}I_{\rm {c}}+v_{\rm {out}}(0)}$

where A is the cross-sectional area of the coil and N is the number of turns. 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. However, this can be compensated for by adding more turns on the Rogowski coil or using an integrator with a higher gain k. More turns increase the self-capacitance and self-inductance, and higher integrator gain means an amplifier with a large gain-bandwidth product. As always in engineering, trade-offs must be made depending on specific applications.

Hall effect

Hall effect sensors are devices based on the Hall-effect, which was discovered by Edwin Hall in 1879 based on the physical principle of the Lorentz force. They are activated by an external magnetic field. In this generalized device, the Hall sensor senses the magnetic field produced by the magnetic system. This system responds to the quantity to be sensed (current, temperature, position, velocity, etc.) through the input interface. The Hall element is the basic magnetic field sensor. It requires signal conditioning to make the output usable for most applications. The signal conditioning electronics needed are an amplifier stage and temperature compensation. Voltage regulation is needed when operating from an unregulated supply. If the Hall voltage is measured when no magnetic field is present, the output should be zero. However, if voltage at each output terminal is measured with respect to ground, a non-zero voltage will appear. This is the common mode voltage (CMV), and is the same at each output terminal. The output interface then converts the electrical signal from the Hall sensor; the Hall voltage: a signal that is significant to the application context. 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. Temperature compensation is also easily integrated.

Fluxgate sensors

Fluxgate Technology principle

Fluxgate sensors or Saturable inductor current sensors work on the same measurement principle as Hall-effect-based current sensors: the magnetic field created by the primary current to be measured is detected by a specific sensing element. The design of the saturable inductor current sensor is similar to that of a closed-loop Hall-effect current sensor; the only difference is that this method uses the saturable inductor instead of the Hall-effect sensor in the air gap.

Saturable inductor current sensor is based on the detection of an inductance change. The saturable inductor is made of small and thin magnetic core wound with a coil around it. The saturable inductor operates into its saturation region. It is designed in such a way that the external and internal flux density will affect its saturation level. Change in the saturation level of a saturable inductor will alter core’s permeability and, consequently, its inductance L. The value of saturable inductance (L) is high at low currents (based on the permeability of the core) and low at high currents (the core permeability becomes unity when saturated). Fluxgate detectors rely on the property of many magnetic materials to exhibit a non-linear relationship between the magnetic field strength H and the flux density B.^[6]

In this technique, high frequency performance is achieved by using two cores without air gaps. One of the two main cores is used to create a saturable inductor and the other is used to create a high frequency transformer effect. In another approach, three cores can be used without air gap. Two of the three cores are used to create saturable inductor, and the third core is used to create a high frequency transformer effect. Advantages of saturable inductor sensors include high resolution, high accuracy, low offset and gain drift, and large bandwidth (up to 500 kHz). 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.

Magneto-resistive current sensor

A magneto-resistor (MR) is a two terminal device which changes its resistance parabolically with applied magnetic field. This variation of the resistance of MR due to the magnetic field is known as the Magnetoresistive Effect. It is possible to build structures in which the electrical resistance varies as a function of applied magnetic field. These structures can be used as magnetic sensors. Normally these resistors are assembled in a bridge configuration to compensate for thermal drift.^[7] Popular magneto resistance-based sensors are: Anisotropic Magneto Resistance (AMR), Giant Magneto Resistance (GMR), Giant Magneto Impendence (GMI) and Tunnel Magneto Resistance (TMR). All these MR-based sensors have higher sensitivity compared to Hall-effect sensors. 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). GMI and TMR sensors are even more sensitive than GMR based sensors, and are now in volume production at a few manufacturers.(TDK, Crocus, Sensitec, MDT)^[8]