| 摘要 |
PURPOSE:To perform bias voltage controlling according to the actual ground potential and make possible the constant-density and sharp copying irrespective of image ratios by regarding the minimum voltage of the latent image potentials detected by boosting and dropping means as ground potential. CONSTITUTION:When the switch RS3 connecting to a boosting resistance R14 is closed for a fixed time before a sensor 11 opposes to the latent image of a photosensitive drum 13, a capacitor C4 is charged up to the voltage by which the maximum bias voltage is applied to a main electrode 13. Thence, when the latent image of the drum 13 and the sensor 11 oppose, the latent image potential detected is impedance-converted by an FETQ1 and is applied to the negative side terminal of a voltage-dropping diode. If the pontential is below the voltage of the capacitor C4, the voltage of the capacitor C4 corresponds to the minimum surface potential through discharging. With the voltage which is regarded as a ground potential as a reference, the bias voltage is controlled by way of a transistor Q4, Zener diode ZD6, etc. Thereby the sharp copy of a fixed density whose image density does not decrease is made irrespective of the large and small of the image ratios. |
