This paper presents a power supply of self-powered online monitoring systems for power cords. The proposed power supply obtains energy from the magnetic field induced by wire-carrying currents using a specially designed coil-based magnetic energy harvester (CMEH). The CMEH reduces the influence of the magnetic flux leakage and can harvest enough energy, especially when the wire-carrying current is small. The magnetic core of the CMEH contains two special C-cores and all end faces are tooth profile. The relationship between the magnetic flux leakage and the number of teeth has been analyzed. The design details of the power supply circuit including the unit for power management and overvoltage protection are given in this paper. Test results show that the overvoltage protective unit can protect the supercapacitor from over its max operating voltage. With the low dropout regulator unit function, the power supply is capable of providing a stable output voltage.