| 摘要 |
<p>A mode change knob (not shown) allows a user to select one of the three modes in which a hammer drill can operate. When the drill only or combined hammer and drilling mode is selected, a trigger (not shown) is used to open and close switch 24 such that when the trigger is pressed the motor activates and drives a bit either in a rotary drilling action (drill mode) or a rotating action in which impacts are imparted to the bit (hammer and drill mode), and when the trigger is released switch 24 is opened which interrupts the connection between contacts 18 and 20 and deactivates the motor. When the mode change knob is moved to the hammer only position, electronic sensor 30 sends a signal to control logic 26. Control logic 26 then waits for the user to depress the trigger (not shown) closing switch 24. When this happens, control logic 26 applies a voltage either to a relay or to the gate of triac 28 which enables the relay or triac 28 to conduct, thus providing a parallel connection by-passing switch 24. Consequently, when the user releases the trigger to open switch 24, as relay or triac 28 is in parallel with switch 24 a current continues to flow through the motor 22. It can be seen that in hammer only, mode the user can activate the hammer drill by simply tapping the trigger 12 to depress it and does not need to hold down trigger 12 during use of the hammer in operation.
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