Patent ID: 8913908

Claim:
An image forming apparatus comprising: a photosensitive member; a scorotron charger including a wire and a grid, and configured to charge the photosensitive member; a developing device configured to supply a developer to the photosensitive member; a voltage application circuit configured to apply a voltage to the scorotron charger; a constant-voltage circuit configured to set a grid voltage of the grid to a constant voltage between the grid and a ground; a control device configured to control an output voltage of the voltage application circuit; and a deboost circuit configured to deboost the grid voltage between the grid and the ground to generate a developing voltage to be applied to the developing device, wherein: the deboost circuit has a circuit configuration in which a resistor and a control transistor are connected in series with each other, and is configured to deboost the grid voltage by a voltage drop of the resistor to generate the developing voltage, the deboost circuit including a developing voltage detection circuit configured to detect the developing voltage, the constant-voltage circuit is connected to the ground through a current detecting section, the control device is further configured to: calculate a grid current flowing in the grid by: calculating a first branch current in the grid current branching into the constant-voltage circuit from a detection value of the current detecting section, calculating a second branch current in the grid current branching into the deboost circuit based on a voltage difference between the grid voltage and the developing voltage and the resistor, and totaling the calculated first branch current and the calculated second branch current as the grid current, control the output voltage of the voltage application circuit such that the grid current becomes a target current value, and control an output of the deboost circuit, by providing a control signal to the control transistor, to control a current flowing in the resistor such that a detection value of the developing voltage detection circuit becomes a target voltage value of the developing voltage, and the deboost circuit is directly connected to the ground.