Quasi-Static Current Measurement with Field-Modulated Spin-Valve GMR Sensors

A non-contact current measurement device comprised of a GMR sensor and a ferrite ring core was investigated. The sensor chip employed a high-sensitivity spin-valve full-bridge GMR sensor of which the direct output has non-negligible hysteresis and a limited linear range. By applying an AC modulation current to modulate the output of the GMR sensor, the hysteresis was reduced, and the linear range was over ±0.5 A. The resolution for DC and quasi-static current measurement was 0.1 mA at a 10 Hz bandwidth. The output in proportion to the measured current was obtained either by demodulating the current-sensitive AC signal or by employing the filtered output of the intrinsically nonlinear spin-valve response. The proposed current sensing scheme is suitable for quasi-static current measurement from DC to over 100 Hz.

Analysis and Suppression of Unwanted Turn-On and Parasitic Oscillation in SiC JFET-Based Bi-Directional Switches

Silicon Carbide (SiC)-based Bi-Directional Switches (BDS) have great potential in the construction of several power electronic circuits including multi-level converters, solid-state breakers, matrix converters, HERIC (high efficient and reliable inverter concept) photovoltaic grid-connected inverters and so on. In this paper, two issues with the application of SiC-based BDSs, namely, unwanted turn-on and parasitic oscillation, are deeply investigated. To eliminate unwanted turn-on, it is proposed to add a capacitor (CX) paralleled at the signal input port of the driver IC (integrated circuit) and the capacitance range of CX is also analytically derived to guide the selection of CX. To mitigate parasitic oscillation, a combinational method, which combines a snubber capacitor (CJ) paralleled with the JFET (Junction Field Effect Transistor) and a ferrite ring connected in series with the power line, is proposed. It is verified that the use of CJ mainly improves the turn-off transient and the use of a ferrite ring damps the current oscillation during the turn-on transient significantly. The effects of the proposed methods have been demonstrated by theoretical analysis and verified by experimental results.

Suppression of High-Frequency Interference in Signal Loop

With the deep and promotion of unattended work in substations, the correct rate of remote signal becomes an important index in automation scheduling system. For remote signal false caused by high-frequency interferences in substation, we propose the use of Ni-Zn ferrite ring to suppress high-frequency interferences. The frequency-impedance characteristics of ferrite ring with different turns are tested. And the inhibitory effect of ferrite ring on interferences below and above 100MHz is measured separately. We installed ferrite rings in remote signal loops, and compared the voltage waveforms before and after installation. Experimental results provide some references to suppress high-frequency interferences by Ni-Zn ferrite ring.

The Development of Fluxgate Magnetometer for Non–Destructive Measurement

Pengukuran tanpa musnah bahan feromagnet dan gangguan medan magnet frekuensi rendah memerlukan penderia medan magnet yang mempunyai kepekaan yang tinggi dan stabil. Magnetometer yang berasaskan prinsip fluxgate dan memenuhi ciri-ciri yang diperlukan telah dibina dengan menggunakan teras gelang ferit. Teras gelang ferit ini dipacukan dengan sumber arus untuk mencapai keadan histerisis. Gelung pengesan yang dililit merentasi gelang ferit akan mengesan perubahan medan magnet yang berlaku pada gelang ferit tersebut. Teknik pengesanan yang digunakan adalah berasaskan pengesanan harmonik kedua oleh suatu pengesan fasa. Penderia fluxgate yang dibina adalah murah mudah dibina, mampu mengesan arah medan magnet dan tahan lasak. Kata kunci: Fluxgate; penderia medan magnet; histerisis; feromagnet Non–Destructive measurements of ferromagnetic material and low frequency magnetic flux distrubance require a highly sensitive and stable magnetic sensor with directional capabilities. A magnetometer based on fluxgate principle that meets the requirement had been developed using a ferrite ring core. The ferrite ring core is excited by an excitation current source to achieve hysteresis condition. The pick up coil that is wound across the ring core will pick up the magnetic flux generated in the ring core. This sensing method is based on the conventional type of fluxgate magnetometer with detection of second harmonics by a phase sensitive detector. Major advantages of the fluxgate magnetometer are low cost, directional, easy to construct, reliable and rugged. Key words: Fluxgate; magnetic sensor; hysteresis; ferromagnetic