L OW COST ELECTRICAL CURRENT SENSORS WITH AUTOMATIC MEASUREMENT RANGE DEIM, University of Palermo, Viale delle Scienze – Building 9, Palermo, Italia The developed measurement system, despite its low cost, can be effectively used to improve measurement precision, and can be used to further investigate methods to raise the S/N ratio in noisy environments with statistical and analytical analysis. We present a simple and low cost Smart Current Sensor with wide automatic measurement range. The logic core system of the sensor is the Atmel AT32UC3C2256C AVR 32-bit microcontroller[1], the wireless core is a Microchip MRF24J40MA IEEE RF transceiver[3]. The MCU features 11 ADC multiplexed channels, with 12-bit resolution and sample rate up to 2Msps, two of which can be sampled in parallel. The sensing board consists of two hall-effect based current sensors[2], assembled on different PCBs in order to perform with different sensitivities. Design, Realization and Working Principle Abstract : Design of a simple and low cost Smart Current Sensor with wide automatic measurement range based on a 32 bit microcontroller system and a IEEE RF transceiver [1] Ziegler, S.; Woodward, R.C.; Iu, H.H.-C.; Borle, L.J. Current Sensing Techniques: A Review. Sensors Journal, IEEE (Volume:9, Issue: 4) 2009 [2] Atmel AT32UC3C2256C datasheet [3] Microchip MRF24J40MA 2.4 GHz IEEE Std RF Transceiver Module datasheet [4] LEM FHS-40P/SP600 datasheet [5] LEM FHS-40P/SP600 design guide [6] Galioto, N.; Lo Bue, F.; Rizzo, D.; Mistretta, L.; Giaconia, C.G. A Novel Wireless Sensor Network for Electric Power Metering. Applications in Electronics Pervading Industry, Environment and Society. Springer, 2014, ISBN Four main techniques are generally used to sense electrical current: Rogowski coil; Current transformer; Low resistor shunt current monitor; Hall effect; In order to design a system with both high galvanic insulation and integration capabilities, the most suitable choice are hall-effect sensors. For our purposes, we relied on the FHS40-P SP600 sensor produced by LEM. Its working principle is straightforward, and requires very few additional electronics components. Hall-Effect Sensor system Fig.1 Magnetic field produced by a current flowing in a long and thin wire conductor Current Measuring Introduction The output of an hall effect sensor is a voltage signal proportional to the sensed magnetic field B. In our context, the magnetic field is proportional to an electrical current flowing inside a long and thin PCB track, multiplied by a factory calibrated constant gain: Sensing characteristic References The system is composed by a 32-bit MCU[2] which samples the sensor output at constant rate and communicate via an radio interface[3] using a custom network layer protocol[6]. The sensing part of the system is composed by two hall-effect sensors with a custom assembly. Future Work Experimental Results Sensors characterization In order to gather information about the sensitivity of each sensor, we run the system with some known flowing current configurations, and annotated the output voltage of each sensor. We defined the sensitivity of the sensor as the output voltage over the current flowing the PCB track, and averaged the different sensitivities obtained so far. We then linearized the output response of the two sensors. In fig.4 it can be seen that the linearized model fits the real measures with a good approximation. Since the MCU can concurrently sample two analog inputs, we can use them to sense both the sensors, having two different measurements for the same flowing current. As a side note, special care must be taken to ensure that the PCB track temperature remains near safety ranges. This is typically achieved by changing PCB track width and thickness. Custom primary conductor design allowed us to sense the current flowing in the conductor with two different sensitivities: the sensor mounted inside the two PCBs gets a strong contribution from the traces on both PCBs, the sensor mounted outside gets a strong contribution from the traces on the bottom PCB only. N.Galioto, F. Lo Bue, L. Mistretta, C.G.Giaconia Fig.2 Functional block diagram of the system Fig.3 PCB trace design and assembled sensors Conclusion Fig.3a Sensors Voltage outputs (500 mV/Div). Both sensors are within their measurement range. Fig.3b Sensors Voltage outputs (1V/Div). One sensor saturated. Fig. 4:The linearized model fits the acquired data with good approximation. We have developed an new electrical current measurement system. It uses two low-cost current sensors, and with a custom PCB design of the primary current conductor, we were able to dynamically and automatically change the measurement range. This system exhibits galvanic isolation and theoretical zero insertion loss characteristics.