Xerox WorkCentre Pro 412 Service Manual page 63

5-8-2 AC Input Stage
AC input power path is consist of the fuse (F501) for AC current limit, the Varistor (TNR501) for by-passing
high voltage surge, the discharge resistor(R508), the AC impulse noise filtering circuit (C501, LF501, C503),
the common mode grounding circuit (C504, C505), the second noise filter (LF502), and the thermistor
(TH501).
When power is turned on, TH 501 limits inrush current by it's high resistance, and when it's temperature rises,
it's resistance reduces to approximately zero ohms.
5-8-3 SMC (Switched Mode Control)
The AC input voltage is rectified and filtered by BD552 and C507 to create the DC high voltage applied to the
primary winding of T501. TR01 pin #1 is driven by the SMPS device U502. U502. auto-starts and chops the
DC voltage. The U502 is PWM SMPS IC and has internally a SMC (switched mode control) IC and a MOS-
FET output stage. The SMC IC has a Auto-restart without a Power Supply for the IC and a Thermal Shutdown
function and so on. C509, R512, C510, D505 clamp leading-edge voltage spikes caused by transformer leak-
age inductance.
The power secondary winding (pin # 11-12) is rectified and filtered by D507, C552, L551, and C554 to create
the 5V output voltage. The bias winding (pin # 4-5) is rectified and filtered by D506 and C511 to create U502
bias voltage. The secondary output 5V is regulated through the path of the voltage divide by R553, R556-
U503 switching PC252-the bias voltage of U502-U503 PWM duty cycle-T501 secondary voltage. C508 filters
internal pin, determines the auto-restart frequency, and together with R506, compensates the control loop.
U552 of the secondary stage -12V is the Low Power-loss Regulator with built-in over-current protection func-
tion.
5-8-4. Fixed Temperature Control
5-8-4-1. Fixed Lamp Control Circuit
AC Neutral
CON502
WorkCentre Pro 412
Launch Issue
R503
C502
THY501
R502
Fixed Lamp Control Circuit
August 2001
AC Power Live
L501
Zero crossing circuit
4
2
R505
1
6
U501
Circuit Description
Logic Unit
Fuser On
SMPS Unit
DC Power
5-37