Image forming apparatus having a plurality of developing devices

Claim 2

An image forming apparatus comprising: a rotatable photosensitive member having a photosensitive layer over its surface; a charger for charging the photosensitive member; exposing means for exposing the photosensitive member to form an electrostatic latent image thereon; a first developing device opposed to the photosensitive member for developing the electrostatic latent image formed on the member; a second developing device positioned downstream from the first developing device with respect to the direction of rotation of the photosensitive member for developing the electrostatic latent image formed on the member; and for controlling the output of the charger so as to elevate the surface potential of the photosensitive member by a predetermined potential when the second developing device is used, the predetermined potential being equal to a decay potential occurring while the surface of the photosensitive member rotates from the position opposed to the first developing device to the position opposed to the second developing device.

Claim 3

An image forming apparatus as claimed in claim 1, wherein said charger includes a wire electrode and a grid electrode for controlling the potential on the photosensitive member and said controlling means controls the voltage applied to the grid electrode of the charger.

Claim 4

An image forming apparatus as claimed in claim 1, wherein said exposing means includes a light source and a scanning optical system for scanning an original.

Claim 5

An image forming apparatus as claimed in claim 1, further comprising: first application means for applying a bias voltage to the first developing device; and second application means for applying to the second developing device a bias voltage equal to the output from the first application means.

Claim 6

An image forming apparatus comprising: a rotatable photosensitive member having a photosensitive layer over its surface; a charger for charging the photosensitive member; exposing means for exposing the photosensitive member to form an electrostatic latent image thereon; a first developing device having a rotatable developing roller for developing the electrostatic latent image formed on the photosensitive member; a second developing device positioned downstream from the first developing device with respect to the direction of rotation of the photosensitive member and having a rotatable developing roller for developing the electrostatic latent image formed on the photosensitive member; first application means for applying a bias voltage to the developing roller of the first developing device; and second application means for applying to the developing roller of the second developing device a bias voltage lower than the output from the first application means by a predetermined voltage, the predetermined voltage being equal to a decay potential occurring while the surface of the photosensitive member rotates from the position opposed to the first developing device to the position opposed to the second developing device.

Claim 7

An image forming apparatus comprising: a rotatable photosensitive member having a photosensitive layer over its surface; a charger for charging the photosensitive member; exposing means for exposing the photosensitive member to form an electrostatic latent image thereon; a first developing device having a rotatable developing roller for developing the electrostatic latent image formed on the photosensitive member; a second developing device positioned downstream from the first developing device with respect to the direction of rotation of the photosensitive member and having a rotatable developing roller for developing the electrostatic latent image formed on the photosensitive member; first application means for applying a bias voltage to the developing roller of the first developing device; and second application means for applying to the developing roller of the second developing device a bias voltage lower than the output from the first application means by a predetermined voltage, the predetermined voltage being equal to a decay potential occurring while the surface of the photosensitive member rotates from the position opposed to the first developing device to the position opposed to the second developing device.

Claim 8

An image forming apparatus as claimed in claim 5 further comprising: means for selecting one of the first and second developing devices to be used; and means for rotating either the developing roller of the first developing device or that of the second developing device corresponding to the selection by the selecting means.

Claim 9

An image forming apparatus as claimed in claim 5, wherein color of toner stored in the first developing device is different from that of toner stored in the second developing device.

Claim 10

An image forming apparatus comprising: a rotatable photosensitive member having a photosensitive layer over its surface; a charger for charging the photosensitive member; means having a light source for locally erasing the charge from the photosensitive member by the light from the light source to thereby form an electrostatic latent image on the member; a first developing device opposed to the photosensitive member for developing the electrostatic latent image formed on the member; a second developing device positioned downstream from the first developing device with respect to the direction of rotation of the photosensitive member for developing the electrostatic latent image formed on the member; and control means for increasing the amount of light from the light source of the image forming means by a predetermined amount when the first developing device is used the predetermined amount being an amount required for lowering the surface potential of the photosensitive member by the same amount as potential decay occurring while the surface of the member rotates from the position opposed to the first developing device to the position opposed to the second developing device.

Claim 11

An image forming apparatus comprising: a rotatable photosensitive member having a photosensitive layer over its surface; a charger for charging the photosensitive member; means having a light source for locally erasing the charge from the photosensitive member by the light from the light source to thereby form an electrostatic latent image on the member; a first developing device opposed to the photosensitive member for developing the electrostatic latent image formed on the member; a second developing device positioned downstream from the first developing device with respect to the direction of rotation of the photosensitive member for developing the electrostatic latent image formed on the member; and control means for increasing the amount of light from the light source of the image forming means by a predetermined amount when the first developing device is used the predetermined amount being an amount required for lowering the surface potential of the photosensitive member by the same amount as potential decay occurring while the surface of the member rotates from the position opposed to the first developing device to the position opposed to the second developing device.