Classification of Current Sensors

Classification of Current Sensors

Introduction 
Small power equipment has incorporated more and more new technologies. For example, switching power supply, hard switching, soft switching, voltage stabilization, linear feedback voltage stabilization, magnetic amplifier technology, numerical control voltage regulation, PWM, SPWM, electromagnetic compatibility, etc. The actual demand directly promotes the continuous development and progress of power supply technology. To automatically detect and display current, power supply technology with sensor detection, sensor sampling, and sensor protection has gradually become a trend. Sensors that detect current or voltage came into being and are gradually being favored by the majority of power supply designers. Current sensors are also called magnetic sensors. They can be used in household appliances, smart grids, electric vehicles, wind power generation, etc. Many magnetic sensors are used in our lives, such as computer hard drives, compasses, household appliances, and so on.

Classification of current sensor
With the development of technology, current sensors are constantly innovating and developing. Let's compare the current common current sensors. According to different measurement principles, current sensors can be mainly divided into the shunt, electromagnetic current transformer, electronic current transformer, etc. Electronic current transformers include Hall current sensors, Rogowski current sensors, etc. Compared with electromagnetic current sensors, electronic current transformers have no ferromagnetic saturation. It has wide transmission frequency bandwidth, small secondary load capacity, small size, and lightweight, which are the development direction of current sensors in the future. The fiber-optic current sensor is a new type of current sensor based on the Faraday magneto-optical effect and using optical fiber as the medium.

When linearly polarized light propagates in the medium, if a strong magnetic field is applied parallel to the propagation direction of the light, the direction of light vibration will be deflected. The deflection angle ψ is proportional to the product of the magnetic induction intensity B and the length l of the light passing through the medium. That is, ψ=V*B*l, and the proportional coefficient V is called the Feld constant, which is related to the properties of the medium and the frequency of light waves. The direction of deflection depends on the nature of the medium and the direction of the magnetic field. The above phenomenon is called the Faraday effect. Discovered by M. Faraday in 1845.

Shunt
The resistance shunt is used to measure direct current and is made according to the principle that a voltage is generated across the resistor when the direct current passes through the resistor. The advantage of the resistive shunt is high precision, fast corresponding speed, and low cost, but the disadvantage is that the measuring circuit is not electrically isolated from the measured current. Resistive shunts are suitable for low frequency and small amplitude current measurements. The shunt is actually a resistor with a small resistance value. When a direct current passes through, a voltage drop is generated for the direct current meter to display. The so-called shunt is to divide a small current to drive the meter indication. The smaller the ratio of the small current (mA) to the current in the large loop (1A-tens of A), the better the linearity and more accurate the reading of the ammeter.

Fluxgate current sensor
Under the saturation excitation of the alternating magnetic field, the magnetic induction intensity of the high permeability magnetic core in the measured magnetic field has a nonlinear relationship with the magnetic field intensity. The fluxgate current sensor uses this principle to measure weak magnetic fields. This physical phenomenon seems to be a "gate" to the measured environmental magnetic field. Through this "gate", the corresponding magnetic flux is modulated, and induced electromotive force is generated. Fluxgate current sensor uses this phenomenon to measure the magnetic field generated by the current, so as to achieve the purpose of measuring current indirectly.

Current Transformer
A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction (only for AC testing). The current transformer is composed of a closed core and windings. Its primary winding has a few turns and is stringed in the line of the current to be measured. Under normal working conditions, the voltage drop on the primary and secondary windings is very small, which is equivalent to a short-circuit transformer, so the magnetic flux in the iron core is also very small. At this time, the magnetic potential of the primary and secondary windings F (F= IN) equal in size and opposite in direction. That is, the current ratio between the primary and secondary of the current transformer is inversely proportional to the number of turns of the primary and secondary windings, that is, I1/I2=N2/N1.

When the current transformer is running, an open circuit on the secondary side is not allowed. Because once the circuit is opened, the primary side current becomes the excitation current, so that the magnetic flux and secondary side voltage greatly exceed the normal value and endanger the safety of people and equipment. Therefore, it is not allowed to connect a fuse in the secondary circuit of the current transformer, and it is also not allowed to remove the ammeter, relay, and other equipment without bypass during operation.

The specific reasons are as follows: the current magnetic potential F1=I1N1 of the current transformer to be measured produces a magnetic flux Φ1 in the iron core, the current magnetic potential of the secondary measuring instrument F2=I2N2 produces a magnetic flux Φ2 in the iron core. And the magnetic flux of the current transformer iron core is Φ = Φ1 + Φ2. Φ1 and Φ2 have opposite directions, are equal in size, so Φ=0. If the secondary open circuit, that is, I2 = 0, then Φ = Φ1. The current transformer iron core magnetic flux is very strong, and the current transformer secondary coil N2 produces the high induced potential E. Then a high voltage at both ends of the secondary coil of the current transformer is formed, endangering the life and safety of operators. Therefore, the reason why one end of the secondary coil of the current transformer is grounded is to prevent the danger of high voltage.

The principle of a voltage transformer is similar to that of a transformer. The primary winding (high voltage winding) and the secondary winding (low voltage winding) are wound on the same iron core. According to the law of electromagnetic induction, the relationship between winding voltage U (or electromotive force E), winding turns N, and magnetic flux is U1=-N1dφ/dt, U2=-N2dφ/dt. And then get U1/U2=N1/N2. In the case of negligible no-load current, I1/I2=-N2/N1, so the power of the primary and secondary windings of an ideal transformer is equal to P1=P2. It shows that the ideal transformer itself has no power loss.

Hall sensor
Electronic current transformers include Hall current sensors, Rogowski current sensors, and AnyWay variable frequency power sensors (which can be used for voltage, current, and power measurement) dedicated to variable frequency power measurement. Compared with electromagnetic current sensors, electronic current transformers have no ferromagnetic saturation. It has wide transmission frequency bandwidth, small secondary load capacity, small size, and lightweight, which are the development direction of current sensors in the future.

The essence of the Hall effect is that when the carriers in the solid material move in an external magnetic field, they are affected by the Lorentz force and the trajectory is shifted, and charges are accumulated on both sides of the material, forming a vertical direction to the current. The electric field finally balances the Lorentz force received by the carriers with the repulsion of the electric field, thereby establishing a stable potential difference, or Hall voltage, on both sides.

The Hall current sensor is made according to the principle of the Hall effect and applies Ampere's law, that is, a magnetic field proportional to the current is generated around a current-carrying conductor, and the Hall device is used to measure this magnetic field. Therefore, non-contact measurement of current is possible. The Hall current sensor can measure DC and AC, and the frequency is up to 100KHz, with high accuracy and good isolation; its disadvantage is that the impact speed is slow, and the small current test accuracy is low. It can be used in AC and DC tests with DC-100KHz.

For a given Hall device, when the bias current I is fixed, UH will completely depend on the measured magnetic field strength B. The Hall voltage changes with the change of the magnetic field strength. The stronger the magnetic field, the higher the voltage, the weaker the magnetic field, and the lower the voltage. The Hall voltage value is small, usually only a few millivolts, but it is amplified by the amplifier in the integrated circuit. The voltage can be amplified enough to output a stronger signal.

shows that the magnetic balance current sensor maintains the magnetic balance by generating a compensation current to ensure the closed-loop magnetic flux φ=0. The specific working process is as follows: when a current flows through the main circuit, the magnetic field generated on the wire is gathered by the magnetization ring and induced to the Hall device, and the generated signal output is used to drive the corresponding power tube and turn it on. To obtain a compensation current Is. This current then passes through the multi-turn winding to generate a magnetic field, which is exactly the opposite of the magnetic field generated by the measured current, thus compensating the original magnetic field and gradually reducing the output of the Hall device. When the magnetic field generated by multiplying Ip and the number of turns is equal, Is no longer increases. At this time, the Hall device plays the role of indicating zero magnetic flux, which can be balanced by Is. Any change in the measured current will disrupt this balance. Once the magnetic field is out of balance, the Hall device has a signal output. After the power is amplified, a corresponding current flows through the secondary winding immediately to compensate for the unbalanced magnetic field. From the imbalance of the magnetic field to the balance again, the required time is theoretically less than 1μs. This is a process of dynamic balance.

From an application point of view, the current transformer and the Hall sensor are the same in that they both need a coil to generate a magnetic field. One of the differences is that the transformer requires a changing magnetic field, while the Hall sensor can be a constant magnetic field. Therefore, the former can only be used for AC testing, while the latter can be used for AC and DC testing. The second difference is that the transformer has an iron core, while the Hall sensor does not have an iron core. The former is non-linear in terms of frequency, and the latter is linear. Therefore, the former applies to a narrow frequency band and is generally used for fixed frequency bands (such as 45~66Hz), the latter has a wider frequency band. The third difference is that more transformers are used for electric energy measurement, and the phase index is an important index for measuring transformers. However, Hall sensors are mostly used for control or simple voltage and current independent testing, and generally do not control the phase index, nor provide the phase index (such as the phase error index of 50Hz).

The future development trend of current sensors
High sensitivity. The intensity of the detected signal is getting weaker and weaker, which requires the sensitivity of the magnetic sensor to be greatly improved. Applications include current sensors, angle sensors, gear sensors, and space environment measurement.

Temperature stability. More application fields require the sensor's working environment to become more and more severe, which requires that the magnetic sensor must have good temperature stability. Industrial applications include the automotive electronics industry.

Anti-interference. In many fields, there is no appraisal for the use environment of the sensor, which requires the sensor itself to have good anti-interference. The types of devices include automotive electronics, water meters, etc.

Miniaturization, integration, and intelligence. To achieve the above requirements, chip-level integration, module-level integration, and product-level integration are required.

High frequency. With the promotion of application fields, the working frequency of sensors is required to be higher and higher. Application fields include water meters, the automotive electronics industry, and the information recording industry.

Low power consumption. Many fields require extremely low power consumption of the sensor itself to extend the service life of the sensor. They are used in implanting magnetic biochips in the body, compasses, etc..