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What is a current sensor?

A current sensor is a current detection device that detects information about the measured current and converts it proportionally into a voltage or current signal that meets the standard to meet the requirements of information transfer, processing, storage, display, recording and control. This chapter provides a brief introduction to the different types of current sensors and gives the accuracy calculation formulae for engineering design reference.

01 Current sensors

Shunt.

As a common current detection element, it has the advantages of high accuracy, good linearity and high temperature stability, and is commonly used for small current DC applications. Shunts are often used for high current applications as they are directly connected in series with the circuit and have insertion losses and heat problems.

Current transformers.

The current signal is scaled by using the different turn ratios of the primary and secondary sides, and can only be used as an AC signal detection.

Hall current sensors.

The Hall effect is the deflection of moving charged particles in a magnetic field caused by Lorentz’s magnetic force. This deflection results in the accumulation of positive and negative charges in the direction of the perpendicular current and magnetic field, thus forming a transverse electric field, which is converted into the required signal to meet the standard output by signal amplification. The Hall effect can therefore be used to achieve non-contact current detection, with the advantages of no insertion loss, isolation, high detection accuracy and simple structural circuitry.

Open-loop current sensors.

The electromagnetic signal generated by the original side current is converted into a voltage signal and output through an amplifier. There are SMD products for small current detection and also modular ones for high current detection.

Closed-loop current sensors.

The secondary coil is spared on the core, and when current flows on the primary side, the compensation flux generated by the secondary line current is equal in size and opposite in direction to the flux generated by the primary current Ip, making the total flux in the core zero. The sub-side compensation current generated by the Hall device and the auxiliary circuit accurately reflects the magnitude of the primary side current, with the original and secondary side current magnitude being the coil turns ratio relationship.

Closed loop current sensors have the advantages of high accuracy, good linearity, low magnetic detuning and good dynamic performance, with relatively high cost and high power loss.

Fluxgate current sensors.

Similar to Hall current sensors, both detect the current signal by detecting the size of the magnetic flux in the air gap, only the induction element in the air gap becomes a fluxgate probe.

The following is a comparison of several commonly used current sensors.

Magnetoresistive current sensors.

The development of magnetoresistive technology has led to the further expansion of current sensor sensing elements, where the development of anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR) technologies has led to the realisation of higher accuracy, better temperature stability and higher bandwidth for current sensors, with the finished product now mainly being a chip type current detection product.

The phenomenon of a change in the resistance of a substance in a certain magnetic field is called the “magnetoresistance effect”, magnetic metals and alloys generally have this magnetoresistance phenomenon, usually, the resistivity of the substance in the magnetic field only produces a slight reduction; under certain conditions, the resistivity reduction is quite large, than the usual magnetic metal and alloy material magnetic resistance value of about 10 times higher than the Under certain conditions, the reduction in resistivity can be quite large, more than 10 times higher than the magnetic resistance of the usual magnetic metals and alloys, known as the “giant magnetoresistance effect” (GMR).

As the GMR effect has become more researched, the TMR effect has begun to attract attention. Although metallic multilayers can produce high GMR values, the strong antiferromagnetic coupling effect leads to high saturation fields and low magnetic field sensitivity, which limits the practical application of the GMR effect.

In magnetic tunnel junctions (MTJs), there is no or essentially no interlayer coupling between the two ferromagnetic layers, and only a small external magnetic field is required to reverse the magnetisation direction of one of the ferromagnetic layers, resulting in a large change in tunneling resistance, so MTJs have a much higher magnetic field sensitivity than metallic multilayers. At the same time, MTJs are inherently high resistivity, low energy consumption and stable performance.

Therefore, MTJs have unparalleled advantages whether as readout heads, various types of sensors, or as magnetic random memories (MRAMs), and their application prospects are very promising, attracting high attention from research groups around the world.

Accuracy calculation.

The most core parameter of current sensors, namely current detection accuracy, current accuracy is mainly considered linearity, zero point and zero temperature drift, closed-loop products add a gain error.

Accuracy calculation for open-loop products.

Accuracy calculation for closed-loop products.

Usually the specification is given for the accuracy corresponding to the rated current, the algorithm of all errors, where the denominator is the value corresponding to the current test current, so the linearity error, for example, the rated current 100A current sensor, the linearity error of 1%, in the actual current 10A, the linearity error reaches 10% (1% * 100/10), the same reason zero point error and zero point drift error also The zero point error and zero drift error also increase. The accuracy calculation of the current sensor is the core parameter of the selection, and the error calculation under extreme conditions is the important basis for product stability and consistency.

The small current accuracy of the current sensor is the real test of the sensor, so in order to meet the accuracy needs of different currents, there are currently dual range output current sensors on the market, as well as Hall current sensors and shunt combination, where Hall is responsible for large currents, the shunt is responsible for small currents. The introduction of magnetoresistive technology has also increased the options available for high accuracy requirements.

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