An office machines such as a multifunction peripheral and an image scanning apparatus becomes an essential electronic device in the office. For continuously feeding a stack of documents to increase the scanning speed, the multifunction peripheral and the image scanning apparatus usually has an automatic document feeder.
FIG. 1 is a schematic view illustrating an automatic document feeder disclosed in U.S. Pat. No. 6,792,241. As shown in FIG. 1, the automatic document feeder 1 comprises a sheet pick-up arm 10, a sheet pick-up roller 11, a separation pad 12, a separation roller 13 and a stopper 14. The stopper 14 is disposed in the sheet feeding path C. The stopper 14 has a stopping surface 14a. 
The sheet pick-up arm 10 of the automatic document feeder 1 is used for feeding the sheets D into the inner portion of the automatic document feeder 1. The sheet pick-up arm 10 may be switched between a standby position and a sheet feeding position. As shown in FIG. 1, the sheet pick-up arm 10 is in the standby position. When the sheet pick-up arm 10 is lifted to the standby position, the stopping surface 14a of the stopper 14 is vertically inserted into the sheet feeding path C to stop the sheets D from entering the inner portion of the automatic document feeder 1.
FIG. 2 is a schematic view illustrating the automatic document feeder of FIG. 1, in which the sheet pick-up arm is in a sheet feeding position. When the scanning operation is activated, the sheet pick-up arm 10 is lowered to the sheet feeding position. At the same time, the stopper 14 is pressed down by the sheet pick-up arm 10, and the stopping surface 14a is distant from the sheet feeding path C. Under this circumstance, the sheets D are no longer stopped by the stopping surface 14a, and thus the sheets D will be successively fed into the internal portion of the automatic document feeder 1 by the sheet pick-up roller 11.
The conventional automatic document feeder 1, however, still has some drawbacks. For example, since the sheet pick-up arm 10 should be maintained in the standby position when no sheet feeding operation is performed, proper magnitude of electricity is required to have the sheet pick-up arm 10 stay in the standby position. For achieving a power-saving purpose, the office machine having an automatic document feeder 1 (e.g. a multifunction peripheral or an automatic sheetfed scanner) is operated in a sleeping mode or a power-saving mode if the office machine is not used. In a case that the automatic document feeder 1 is in the sleeping mode or the power-saving mode, the electricity magnitude is not sufficient to have the sheet pick-up arm 10 stay in the standby position. Due to insufficient electricity magnitude, the sheet pick-up arm 10 fails to continuously stay in the standby position when the automatic document feeder 1 is in the sleeping mode or the power-saving mode. Meanwhile, due to the gravity of the sheet pick-up arm 10, the stopper 14 is pressed down by the sheet pick-up arm 10, and the stopping surface 14a is distant from the sheet feeding path C. Under this circumstance, the sheet stopping function of the stopper 14 is lost. If the user forgets that the automatic document feeder 1 is in the sleeping mode but forces a stack of sheets to be introduced into the automatic document feeder 1, many sheets may be erroneously inserted into the sheet feeding path C. After the automatic document feeder 1 is powered on and the automatic document feeder 1 is in a working mode, the stopper 14 fails to normally stop the sheets D because too many sheets are erroneously inserted into the sheet feeding path C. When the sheet feeding operation is performed, a so-called double feeding problem occurs because too many sheets are jammed in the sheet feeding path C. As such, the automatic document feeder 1 fails to be normally operated.