Image forming apparatus

An image forming apparatus comprising:

• a plurality of image bearers to carry toner images of a plurality of colors;
• a movable intermediate transferor to secondarily transfer the toner images, which have been primarily transferred from the plurality of image bearers, onto a transfer medium;
• a plurality of primary transferors to primarily transfer the toner images from the plurality of image bearers onto the movable intermediate transferor at primary transfer sections at which the plurality of image bearers contact an outer circumferential surface of the movable intermediate transferor;
• a primary transfer power source to apply voltage to the plurality of primary transferors;
• a primary transfer current detector to detect a current value representing an amount of current flowing through the plurality of primary transferors when a specified voltage is applied to the plurality of primary transferors;
• and
• processing circuitry configured to:
• apply a constant voltage to the plurality of primary transferors by the primary transfer power source;
• and
• determine a primary transfer voltage value by substituting a constant current value detected by the primary transfer current detector as a variable into a conversion formula for calculating the primary transfer voltage value.

An image forming apparatus comprising:

• a plurality of image bearers to carry toner images of a plurality of colors;
• a movable intermediate transferor to secondarily transfer the toner images, which have been primarily transferred from the plurality of image bearers, onto a transfer medium;
• a plurality of primary transferors to primarily transfer the toner images from the plurality of image bearers onto the movable intermediate transferor at primary transfer sections at which the plurality of image bearers contact an outer circumferential surface of the movable intermediate transferor;
• two primary transfer power sources to apply voltage to the plurality of primary transferors, the two primary transfer power sources including:
• a first primary transfer power source coupled to a first primary transferor corresponding to a first image bearer for a black toner image;
• and
• a second primary transfer power source coupled to second primary transferors corresponding to second image bearers for color toner images of three colors other than black;
• a primary transfer current detector to detect a current flowing through the first primary transferor when a specified voltage is applied to at least one of the plurality of primary transferors;
• and
• processing circuitry configured to
• apply a constant voltage to the first primary transferor by the first primary transfer power source, and
• determine a primary transfer voltage value by substituting a constant current value detected by the primary transfer current detector as a variable into a conversion formula for calculating the primary transfer voltage value.