Developing device, image forming apparatus, and control method of developing device

A developing device comprising:

• a developing roller disposed to be opposed to a photoreceptor drum, so as to carry toner;
• a magnetic roller disposed to be opposed to the developing roller, so as to form a magnetic brush by carrier in developer so that toner in the developer is supplied to the developing roller and that toner remaining on a surface of the developing roller after development is collected by the magnetic brush;
• a high voltage power supply including a transformer having two or more secondary winding, one of secondary side outputs being connected to the developing roller while the other output being connected to the magnetic roller, so as to apply AC voltages to the developing roller and the magnetic roller;
• a detection portion for detecting occurrence of discharge between the developing roller and the photoreceptor drum;
• a restriction portion for restricting movement of toner from the magnetic roller to the developing roller;
• and
• a moving portion configured to move the restriction portion in a discharge start voltage detection mode in which the high voltage power supply changes the AC voltage to be applied to the developing roller step by step so that a discharge start voltage is detected by using the detection portion, so as to restrict the movement of toner to the developing roller, and to draw back the restriction portion so that the toner can move to the developing roller when printing is executed.