Power tool

Abstract

A power tool 1 is connected to a battery pack 5 including an overcurrent detector 533 which detects overcurrent of a battery module 51 and outputs a detection signal, and an overdischarge detector 532 which detects overdischarge of the battery module 51 and outputs a detection signal. The power tool 1 includes a motor 2 which is driven by electric power supplied from the battery pack 5, a trigger switch 31 for setting the power supply from the battery pack 5 to the motor 2 in ON / OFF state, and an FET 410 which sets the power supply to the motor 2 in ON / OFF state on the basis of the detection signal from the overcurrent detector 533 or the overdischarge detector 532.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is based on and claims the benefit of priority from the prior Japanese Patent Application No.2006-243168, filed on Sep. 7, 2006; the entire contents of which are incorporated herein by reference.Technical Field[0002]The present invention relates to a power tool, and particularly to a power tool which prevents overcurrent and overdischarge of a battery pack.BACKGROUNDDescription of the Related Art[0003]Heretofore, there has been known a codeless power tool (hereinafter referred to as a power tool) such as a driver or a circular saw which can operate by supply of electric power from a battery pack. Large electric power is necessary to operate such the power tool. Therefore, recently, in addition to a nickel-cadmium battery and a nickel-hydride battery, a lithium-ion battery is used widely as a charging battery of the battery pack.[0004]The lithium-ion battery has an advantage that large electric power can be supplied and si...

AI Technical Summary

Benefits of technology

[0013]According to the power tool in the first aspect of the invention, the second switch unit which sets the power supply to the drive part in ON / OFF state on the basis of the detection signal is provided not for the battery pack but for the power tool. When the power supply to the drive part is set in ON state, the current flows into the second switch unit. However, in case of a power tool in which large current flows, plural second switch units must be provided in parallel to disperse the current. According to the power tool in the first aspect of the invention, since the second switch unit is provided for the power tool, the necessary number of second switch units may be provided correspondingly to the current which its power tool requires. Hereby, it can be prevented that: Due to a small number of the second switch units, the current amount per a second switch unit becomes excessive, so that the second switch unit breaks down. Further, since it is not necessary to provide the unnecessary number of second switch units, cost can be reduced.

[0014]Further, according to the power tool in the second aspect of the invention, there is provided the retaining unit which retains, in case that the power supply to the drive part is interrupted by the second switch unit in the state where the first switch unit is turned on, the interruption state of power supply to the drive part by the second switch unit while the first switch unit is in the ON state. Therefore, it is possible to prevent the power supply and the power interruption from being repeated in a short period. Namely, though the OFF state of the second switch unit solves the overcurrent state, and thereafter the ON state of the second switch unit restarts the power supply, the overcurrent state is soon produced again, so that the second switch unit is turned off again. The repeat of this ON / OFF of the second switch can be prevented by the retaining unit. Also, while the first switch unit is in the ON state, the interruption state is retained in case that the second switch unit is interrupted so as to present the power tool from being driven again when overcurrent is removed by taking the power tool away from the object. Accordingly, safety improvement can be made.

[0016]According to the power tool in the fourth aspect of the invention, the first signal output unit includes the resistor and the condenser, the second signal output unit includes at least the resistor and the condenser, and time constant by the resistor and the condenser is larger in the second signal output unit than that in the first signal output unit. Therefore, once the second switch unit is turned off, while the first switch unit is turned on, it is possible to prevent the electric power from being supplied again to the drive part by simple construction.

[0018]According to the power tool in the sixth aspect of the invention, there are provided the first signal output unit which outputs the first detection signal for permitting the power supply from the battery to the drive part to the second switch unit, and the second signal output unit which outputs the second detection signal for interrupting the power supply from the battery to the drive part to the second switch unit. Herein, in case that the battery and the drive part are connected in the state where the first switch unit is turned on, the second detection signal is input to the second switch unit. Therefore, once the second switch unit is turned off, while the first switch unit is turned on, it is possible to prevent the electric power from being supplied again to the drive part.

Problems solved by technology

Therefore, depending on the used power tool, the number of the FET's included in the battery pack cannot make the interruption of the overcurrent possible.

Namely, in case that the number of the FET's included in the battery pack is small, and the current flowing in the power tool connected to the battery pack is large, the current flows in the FET's excessively, and the FET's break down.

To the contrary, in case that the number of the FET's included in the battery pack is large, and the current flowing in the power tool connected to the battery pack is small, the FET's become useless.

Therefore, the user can recognize that the second switch unit is turned off and the power tool cannot be operated.

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