Image forming device

An image forming device capable of executing an image forming operation that forms an image on a sheet, the image forming device comprising:

• an image carrier that is rotated, and has a surface that allows an electrostatic latent image to be formed and carries a toner image which is the electrostatic latent image manifested by a toner;
• a charge device that charges the image carrier to a specific charge potential;
• an exposure device that is arranged on a more downstream side than the charge device in a rotational direction of the image carrier, and that exposes, according to specific image information, the surface of the image carrier charged to the charge potential to thereby form the electrostatic latent image;
• a development device that is arranged to face the image carrier in a specific development nip portion on a more downstream side than the exposure device in the rotational direction, and includes a development roller that has a peripheral surface, which is rotated and carries a development agent composed of the toner and a carrier, and that forms the toner image by supplying the toner to the image carrier;
• a transfer portion that transfers, to the sheet, the toner image supported on the image carrier;
• a development bias application portion capable of applying, to the development roller, a development bias in which an AC voltage is superimposed on a DC voltage;
• a current detection portion capable of detecting a DC component of a development current that flows across the development roller and the development bias application portion;
• and
• a bias condition determination portion that, based on the DC component of the development current detected by the current detection portion when a specific measurement latent image is developed with the toner by applying the development bias to the development roller corresponding to the specific measurement latent image formed on the image carrier, executes a bias condition determination mode that determines a reference peak-to-peak voltage which becomes a reference for a peak-to-peak voltage of the AC voltage of the development bias applied to the development roller in the image forming operation,
• wherein
• in the bias condition determination mode, the bias condition determination portion executes each of:
• a first approximation expression determination operation that respectively acquires the DC component of the development current under a condition that the peak-to-peak voltages of the AC component of the development bias are respectively set to at least three first measurement peak-to-peak voltages included in a specific first measurement range, and determines a first approximation expression which is a primary approximation expression showing a relation between the first measurement peak-to-peak voltage in the first measurement range and the DC component of the acquired development current,
• a second approximation expression determination operation that respectively acquires the DC component of the development current under a condition that the peak-to-peak voltages of the AC component of the development bias are respectively set to at least three second measurement peak-to-peak voltages included in a second measurement range set to have a minimum value larger than a maximum value of the first measurement range, and determines a second approximation expression which is a primary approximation expression showing a relation between the second measurement peak-to-peak voltage in the second measurement range and the DC component of the acquired development current, and
• a reference voltage determination operation that determines, as the reference peak-to-peak voltage, the peak-to-peak voltage at an intersection where the first approximation expression determined by the first approximation expression determination operation and the second approximation expression determined by the second approximation expression determination operation intersect each other.