US Patent 7427842 Series motor and method of controlling the same

A circuit for a series motor for a power tool is disclosed, wherein reliable switching is achieved by means of two electronic switches, without the use of a mechanically disconnecting switch. To this end, two electronic switches, preferably triacs, are preferably connected in series to each other, the voltage drop across the triacs being continuously monitored by a monitoring circuit. A check for faults can be carried out before switching on the power tool. Alternatively, a fusible cutout can be tripped by means of a protective switch connected in parallel thereto.