Charging device and image forming apparatus provided therewith

1. A charging device comprising:

a charging member opposed to an outer circumferential surface of a rotating object to be charged; and

a voltage applying section for applying a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes between the charging member and the object to be charged, wherein a relation of

Vpp≦|Vth|

is satisfied, where

Vpp is a peak-to-peak voltage value of the pulsating voltage, and

Vth is a charging start voltage value of the object to be charged.

2. The charging device as claimed in claim 1 , wherein a relation of

|Vth|≦|Vdc|

is satisfied, where

Vdc is the voltage component of the direct current in the pulsating voltage.

3. The charging device as claimed in claim 1 , wherein

the charging member has a shape of a roller, a pad or a sheet.

4. A charging device comprising:

a charging member opposed to an outer circumferential surface of a rotating object to be charged; and

a voltage applying section for applying a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes between the charging member and the object to be charged, wherein a relation of

| V max− Vo|>|Vth|>|V min− Vo|

is satisfied, where

Vth is a charging start voltage value of the object to be charged,

Vo is each voltage of the charged object surface located both upstream and downstream of the charging member in relation to a movement direction of the outer circumferential surface of the object to be charged,

Vmax is a maximum value of the pulsating voltage, and

Vmin is a minimum value of the pulsating voltage.

5. The charging device as claimed in claim 4 , wherein a relation of

|Vmin|≧|Vo|

is satisfied.

6. The charging device as claimed in claim 4 , wherein

the charging member has a shape of a roller, a pad or a sheet.

7. A charging device comprising:

a charging member opposed to an outer circumferential surface of a rotating object to be charged; and

a charge control member opposed to the object to be charged in a position located upstream of the charging member in relation to a movement direction of the outer circumferential surface of the object to be charged; and

a voltage applying section for applying voltages for controlling a surface voltage of the object to be charged, between the charge control member and the object to be charged and between the charging member and the object to be charged, respectively, wherein the voltages are applied so that a relation of

|Vo(i)|<|Vo(ii)|<|Vo(iii)|

is satisfied, where

Vo(i) is a voltage of the charged object surface located upstream of the charge control member,

Vo(ii) is a voltage of the charged object surface located downstream of the charge control member and upstream of the charging member, and

Vo(iii) is a voltage of the charged object surface located downstream of the charging member.

8. The charging device as claimed in claim 7 , wherein

the voltage applying section applies a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes between the charge control member and the object to be charged and/or between the charging member and the object to be charged, wherein a relation of

| V max− Vo|>|Vth|>Vpp

is satisfied, where

Vth is a charging start voltage value of the object to be charged,

Vo is any one of the surface voltages Vo(i), Vo(ii) and Vo(iii) of the object to be charged,

Vmax is a maximum value of the pulsating voltage, and

Vpp is a peak-to-peak voltage value of the pulsating voltage.

9. The charging device as claimed in claim 7 , wherein

a plurality of the charge control members are provided in positions located upstream and downstream mutually in relation to the movement direction of the outer circumferential surface of the object to be charged,

all or part of the plurality of charge control members is selectively applied by the voltage applying section with a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes, wherein a relation of

|Vo(α)|<|Vo(β)|

is satisfied, where

Vo(α) is a voltage of the charged object surface located upstream of one of the charge control members to which the pulsating voltage is applied,

Vo(β) is a voltage of the charged object surface located downstream of the charge control member to which the pulsating voltage is applied and upstream of another adjacent charge control member, and wherein a relation of

| V max− Vo|>|Vth|>Vpp

is satisfied with regard to the charge control member to which the pulsating voltage is applied, where

Vth is a charging start voltage value of the object to be charged,

Vo is any one of the surface voltages Vo(α) and Vo(β) of the object to be charged,

Vmax is a maximum value of the pulsating voltage, and

Vpp is a peak-to-peak voltage value of the pulsating voltage.

10. An image forming apparatus of an electrophotographic system, comprising:

a charging device for uniformly charging an outer circumferential surface of an electrostatic latent image carrier that serves as a rotating object to be charged, the charging device comprising:

a charging member opposed to the outer circumferential surface of the electrostatic latent image carrier; and

a voltage applying section for applying a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes between the charging member and the electrostatic latent image carrier, wherein a relation of

Vpp≦|Vth|

is satisfied, where

Vpp is a peak-to-peak voltage value of the pulsating voltage, and

Vth is a charging start voltage value of the electrostatic latent image carrier,

an exposure device for forming a latent image by exposing the outer circumferential surface of the electrostatic latent image carrier;

a developing device for developing the latent image by making toner adhere to the latent image; and

a transfer device for transferring the toner onto an object to receive transfer.

11. The image forming apparatus as claimed in claim 10 , wherein a relation of

|Vth|≦|Vdc|

is satisfied, where

Vdc is the voltage component of the direct current in the pulsating voltage.

12. The image forming apparatus as claimed in claim 10 , wherein

the charging member has a shape of a roller, a pad or a sheet.

13. The image forming apparatus as claimed in claim 10 , wherein

the developing device has a toner collecting section for collecting the toner that remains on the surface of the electrostatic latent image carrier after transfer by the transfer device.

14. An image forming apparatus of an electrophotographic system comprising:

a charging device for uniformly charging an outer circumferential surface of an electrostatic latent image carrier that serves as a rotating object to be charged, the charging device comprising:

a charging member opposed to the outer circumferential surface of the electrostatic latent image carrier; and

a voltage applying section for applying a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes between the charging member and the electrostatic latent image carrier, wherein a relation of

| V max− Vo|>|Vth|>|V min− Vo|

is satisfied, where

Vth is a charging start voltage value of the electrostatic latent image carrier,

Vo is each voltage of the electrostatic latent image carrier surface located upstream and downstream of the charging member in relation to a movement direction of the outer circumferential surface of the electrostatic latent image carrier,

Vmax is a maximum value of the pulsating voltage, and

Vmin is a minimum value of the pulsating voltage,

an exposure device for forming a latent image by exposing the outer circumferential surface of the electrostatic latent image carrier;

a developing device for developing the latent image by making toner adhere to the latent image; and

a transfer device for transferring the toner onto an object to receive transfer.

15. The image forming apparatus as claimed in claim 14 , wherein a relation of

|Vmin|≧|Vo|

is satisfied.

16. The image forming apparatus as claimed in claim 14 , wherein

the charging member has a shape of a roller, a pad or a sheet.

17. The image forming apparatus as claimed in claim 14 , wherein

the developing device has a toner collecting section for collecting the toner that remains on the surface of the electrostatic latent image carrier after transfer by the transfer device.

18. An image forming apparatus of an electrophotographic system comprising:

a charging device for uniformly charging an outer circumferential surface of an electrostatic latent image carrier that serves as a rotating object to be charged, the charging device comprising:

a charging member opposed to the outer circumferential surface of the electrostatic latent image carrier;

a charge control member opposed to the electrostatic latent image carrier in a position located upstream of the charging member in relation to a movement direction of the outer circumferential surface of the electrostatic latent image carrier; and

a voltage applying section for applying voltages for controlling a surface voltage of the electrostatic latent image carrier between the charge control member and the electrostatic latent image carrier and between the charging member and the electrostatic latent image carrier, respectively, wherein the voltages are applied so that a relation of

|Vo(i)|<|Vo(ii)|<|Vo(iii)|

is satisfied, where

Vo(i) is a voltage of the electrostatic latent image carrier surface located upstream of the charge control member,

Vo(ii) is a voltage of the electrostatic latent image carrier surface located downstream of the charge control member and upstream of the charging member, and

Vo(iii) is a voltage of the electrostatic latent image carrier surface located downstream of the charging member,

an exposure device for forming a latent image by exposing the outer circumferential surface of the electrostatic latent image carrier;

a developing device for developing the latent image by making toner adhere to the latent image; and

a transfer device for transferring the toner onto an object to receive transfer.

19. The image forming apparatus as claimed in claim 18 , wherein

the voltage applying section applies a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes between the charge control member and the electrostatic latent image carrier and/or between the charging member and the electrostatic latent image carrier, wherein a relation of

| V max− Vo|>|Vth|>Vpp

is satisfied, where

Vth is a charging start voltage value of the electrostatic latent image carrier,

Vo is any one of the surface voltages Vo(i), Vo(ii) and Vo(iii) of the electrostatic latent image carrier,

Vmax is a maximum value of the pulsating voltage, and

Vpp is a peak-to-peak voltage value of the pulsating voltage.

20. The image forming apparatus as claimed in claim 18 , wherein

a plurality of the charge control members are provided in positions located upstream and downstream mutually in relation to the movement direction of the outer circumferential surface of the electrostatic latent image carrier,

all or part of the plurality of charge control members is selectively applied by the voltage applying section with a pulsating voltage obtained by superimposing a voltage component of a direct current on a voltage component that periodically changes, wherein a relation of

|Vo(α)|<|Vo(β)|

is satisfied, where

Vo(α) is a voltage of the electrostatic latent image carrier surface located upstream of one of the charge control members to which the pulsating voltage is applied,

Vo(β) is a voltage of the electrostatic latent image carrier surface located downstream of the charge control member to which the pulsating voltage is applied and upstream of another adjacent charge control member, and wherein a relation of

| V max− Vo|>|Vth|>Vpp

is satisfied with regard to the charge control member to which the pulsating voltage is applied, where

Vth is a charging start voltage value of the electrostatic latent image carrier,

Vo is any one of the surface voltages Vo(α) and Vo(β) of the electrostatic latent image carrier,

Vmax is a maximum value of the pulsating voltage, and

Vpp is a peak-to-peak voltage value of the pulsating voltage.

21. The image forming apparatus as claimed in claim 18 , wherein

the developing device has a toner collecting section for collecting the toner that remains on the surface of the electrostatic latent image carrier after transfer by the transfer device.