Developing apparatus.

摘要

In a developing apparatus wherein two-component developer consisting of toner (T) and carrier (C) is used and the rotation number of a developing roller (22) selected under normal mode can be changed under another mode under which the amount of toner used is different from that under normal mode, a magnetic sensor (30) is used as means for detecting toner concentration in a developer (D) containing section. Input voltage applied to the magnetic sensor (30) is adjusted and determined in such a way that the output voltage of the magnetic sensor (30) which is previously set relative to a reference toner concentration in the developer (D) becomes equal to a reference voltage, on which it is determined whether the adding of toner is started or stopped, even though the rotation number of the developing roller (22) selected under one mode is changed under another mode. The output voltage of the magnetic sensor (30) rises as the toner concentration in the developer (D) becomes low. The reference voltage is set 4V relative to the reference toner concentration of 4%, for example, and when the output voltage becomes larger than the reference voltage, the adding of toner is started. The output voltage becomes lower as the toner concentration becomes higher and when the output voltage becomes smaller than the reference voltage or equal to it, the adding of toner (T) is stopped. When the rotation number of the developing roller (22) is changed in this manner to change one mode to another, such input voltage that corresponds to the rotation number of the developing roller (22) is applied to the magnetic sensor (30) to control the toner density in the developer (D) by comparing the output voltage of the magnetic sensor (30) with the reference voltage selected under this another mode.