Abstract: An insulation resistance detection method and circuit for an ungrounded DC power supply systems. When the ratio of the voltage of the positive bus to that of the negative bus exceeds a preset range, the DC current is injected into the ungrounded DC power supply systems. DC power supply systems have essentially some grounded stray and external capacitors, so some of the injected DC current will flow into the grounded capacitor, increasing the voltage of the fault bus and the leakage current. The proposed method increases the leakage current by the injection of DC current, so the current sensor can detect the leakage current from which the insulation resistance can be calculated. The DC current injection method can significantly improve the detection of ground insulation faults in ungrounded DC power supply systems.

Abstract: A structure of an automatic speed change device for bicycle gearbox is disclosed in this present invention. The above-mentioned design can be applied to a bicycle with a rear wheel speed change device. According to this invention, the automatic speed change device is able to control the allotting device of the rear wheel speed change device to transfer the chain into suitable gear by the treading rat of the user. This design can simplify the manual gear shifting process in the prior art for controlling the rear wheel speed change device. Therefore, this invention can make riding bicycle more easily and widely for all age.