Power supply apparatus capable of switching output resistance

An image forming apparatus comprising:

• a photosensitive member;
• a charging member configured to charge the photosensitive member;
• a light source configured to expose a surface of the photosensitive member to light to form an electrostatic latent image;
• a developing member configured to develop the electrostatic latent image to form a toner image;
• a primary transfer member configured to transfer the toner image from the photosensitive member to an intermediate transfer member;
• a secondary transfer member configured to transfer the toner image from the intermediate transfer member to a sheet;
• a fixing unit configured to fix the toner image on the sheet;
• and
• a power supply apparatus configured to supply a transfer promoting voltage to the primary transfer member which is a load,
• wherein the power supply apparatus includes
• a first power supply configured to apply a DC voltage with a first polarity to the load,
• a second power supply connected in parallel with the first power supply, the second power supply being configured to apply a DC voltage with a second polarity, opposite the first polarity, to the load via a first resistor connected to the load,
• a resistance switching circuit including a second resistor forming an output resistance of the first power supply, the resistance switching circuit being configured to switch between a state in which the second resistor forms part of the output resistance and a state in which the second resistor does not form part of the output resistance, and
• a control circuit configured to control the first power supply, the second power supply, and the resistance switching circuit,
• the control circuit is configured to
• in a case where a DC voltage with the first polarity is applied to the load from the first power supply, control the resistance switching circuit such that the second resistor does not form part of the output resistance, and
• in a case where a DC voltage with the second polarity is applied to the load from the second power supply, control the resistance switching circuit such that the second resistor forms part of the output resistance, and
• a resistance value of the output resistance when the second resistor is formed as part of the output resistance is greater than a resistance value of the output resistance when the second resistor is not formed as part of the output resistance.