Image forming apparatus having contact charger

An image forming apparatus comprising:

• a photosensitive member;
• a contact charging unit having a charging rotary brush in contact with a surface of the photosensitive member;
• exposure means for exposing the surface of the photosensitive member to an image over an area thereof, charged by the charging unit, to form an electrostatic latent image;
• a developing unit for developing the latent image into a visible image and collecting residual developer remaining on the surface of the photosensitive member after the visual image is transferred to a transfer material;
• and
• a power source unit for applying to the charging unit an oscillating voltage fulfilling the requirement of:
• .DELTA.t1<L/Vpc and .DELTA.t2>3.times.L/Vpc
• wherein said oscillating voltage has a plurality of peaks and a central voltage value, wherein .DELTA.t1 is a time taken for the oscillating voltage to vary between .+-.300 V above and below the central voltage value between the peaks of the oscillating voltage, .DELTA.t2 is a time taken for the oscillating voltage to fall between .+-.300 V above and below the central voltage value between the peaks of the oscillating voltage, L is a printing pitch, and Vpc is the speed of movement of the surface of the photosensitive member.

An image forming apparatus comprising:

• a photosensitive member having a surface;
• a contact charging unit having a charging rotary brush in contact with the surface of the photosensitive member;
• exposure means for exposing the surface of the photosensitive member to an image over an area thereof, charged by the charging unit, to form an electrostatic latent image;
• a developing unit for developing the latent image into a visible image and collecting residual developer remaining on the surface of the photosensitive member after the visual image is transferred to a transfer material;
• and
• a power source unit for applying to the charging unit an oscillating voltage having a frequency and a sinusoidal waveform fulfilling the requirement of:
• (1/.pi.f).multidot.arcsin(600/Vp-p)<L/Vpc
• or
• (1/ f).multidot.arcsin(600/Vp-p)>3.times.L/Vpc
• wherein f is the frequency of the oscillating voltage, Vp-p is a peak-to-peak voltage of the oscillating voltage, L is a printing pitch, and Vpc is a speed of movement of the surface of the photosensitive member .