Patent ID: 7486049

Claim:
A protection circuit for a lithium battery pack serving as a power source of a power tool, adapted to connect to a reversal/forward rotation circuit, a speed variation circuit, and a motor of the power tool, comprising: an abnormality control integrated circuit, a first resistor, a second resistor, a power transistor, a first diode, and a second diode, wherein the abnormality control integrated circuit has a power terminal connected to an anode of a lithium battery set constituting in part the battery pack, and connected to a first detection terminal of the abnormality control integrated circuit via the first resistor, the first detection terminal being also connected via the second resistor to a negative terminal of the protection circuit, which is further connected to an emitter terminal of the power transistor, the power transistor having a base terminal connected to an output control terminal of the abnormality control integrated circuit, a collector terminal of the abnormality control integrated circuit being connected through the first diode to the anode of the lithium battery set, the second diode being arranged between the emitter and collector of the power transistor, the collector of the power transistor being connected to corresponding throw terminal of a single-pole-tripe-throw switch constituting in part the speed variation circuit of power tool, the abnormality control integrated circuit also having a second detection terminal connected to a pulse width modulation generation circuit that constitutes in part the speed variation circuit of the power tool, the grounding terminal of the protection circuit being connected to second and third throw positions of the single-pole-triple-throw switch of the speed variation circuit.