Image forming apparatus with function of setting appropriate development bias

An image forming apparatus, comprising:

• an image carrier including a circumferential surface for carrying an electrostatic latent image and a toner image;
• a developer carrier including a magnetic member therein and a circumferential surface for carrying a developer containing toner in the form of a layer of magnetized filaments to supply toner from the layer of magnetized filaments to the image carrier for development of the electrostatic latent image;
• a bias applying section for applying to the developer carrier a development bias including a direct current voltage VmagDC and an alternating current voltage VmagAC that are superposed on each other;
• a leakage detecting section for causing a discharge between the image carrier and the developer carrier and detecting a leakage voltage when the discharge occurs;
• a bias controlling section for controlling the development bias for the image formation and the leakage voltage detection, and
• a storage section storing a table allocated with frequencies of the alternating current voltage VmagAC according to varied amplitudes of the alternating current voltage VmagAC, the table identifying in advance a zone that indicates the combination of the alternating current voltage VmagAC and the frequencies that allow acquisition of a high quality image while preventing both fogging and ghosting and a correction coefficient corresponding to an environmental condition including atmospheric pressure, wherein
• the leakage detecting section performs the leakage voltage detection in a blank region of the image carrier, the blank region bearing no electrostatic latent image,
• the bias controlling section controls the development bias to change based on a leakage voltage by determining an alternating current voltage VmagAC according to a detected leakage voltage and determining a frequency of the alternating current voltage VmagAC according to the alternating current voltage VmagAC and the coefficient, and
• the bias controlling section causes, when the leakage detecting section performs the leakage voltage detection, an amplitude of the alternating current voltage VmagAC and a frequency of the alternating current voltage VmagAC to be changed in accordance with the table, and wherein
• the bias controlling section sets:
• a frequency of the alternating current voltage VmagAC by multiplying a relational expression (1) by the correction coefficient in a case where the developer is a magnetic one-component developer,
• Frequency f(kHz)=(1.15˜1.65)×Peak-to-Peak Voltage Vpp (kV) (1),
• and
• a frequency of the alternating current voltage VmagAC by multiplying a relational expression (2) by the correction coefficient in a case where the developer is a magnetic two-component developer,
• Frequency f(kHz)=(2.0˜4.5)×Peak-to-Peak Voltage Vpp (kV) (2),
• as Vpp being peak-to-peak voltage of the alternating current voltage VmagAC.