Dual RFID Tag System for AC Current Sensing

We present a wireless and battery-free sensing system operating at the UHF band to measure domestic ac current ranging from 0 to 13 A in a power cable. The proposed sensing system uses two tag antennas operating at 868 MHz, each connected to a tuning circuit. The reactance of the tuning circuits varies as a function of the dc bias from a current transformer, causing a shift in the resonance frequency of the tag antennas. The Magnus S2 RFID transponder chip is used to compensate for differences in the applied reactance, and transmits the extent of the tuning capacitance parameter in the form of a 5-bit sensor code which can be correlated to the current drawn by an electrical load. Results indicate currents can be measured reliably over 0 to 12 A representing a significant improvement over a single diode version of the technology.