Small-size current transformers are often used as power-supply for electronic current transformers（ECTs）. As a current-measurement equipment， ECT is expected to have a wake-up current as small as possible. However， the power supply will not gain enough energy from the power line if the wake-up current is too small. Aiming at achieving the maximum power output from the power-supply， this paper analyzes the relationship between the wake-up current and turns of the secondary winding. Based on a simplified model of current transformer， a model is developed for this purpose. This model uses Simulink Scanning between the minimum and maximum turns to find the best secondary winding. The experimental results fit well with the simulation.