HP LaserJet 9040/9050 Service Manual - Page 96

Low-voltage power supply unit The low-voltage power supply unit consists of the fuser control circuit and the low-voltage powersupply circuit. Figure 27. Power-supply-block diagram Fuser control circuit Three heaters heat the fuser roller and pressure roller. The main heater (H1) heats the center of the fuser roller, the sub-heater (H2) heats both ends of the fuser roller, and the lower heater (H3) heats the pressure roller. Two thermistors monitor the surface temperature of the rollers. The upper thermistor (TH1) monitors the fuser roller surface temperature and the lower thermistor (TH2) monitors the pressure roller surface temperature. Two thermal switches guard against overheating the three heaters by turning off power to the heaters when the temperature rises abnormally. The printer is designed to prevent flicker. Lights attached to the same ac power source as the printer can flicker as a result of high-frequency noise. To control flicker, the printer gradually increases and decreases power to the heaters when they are turned on and off. Low-voltage power-supply circuit The low-voltage power-supply circuit converts ac voltage from the power source to dc voltage when the printer is turned on. The ac power is converted as follows: z +24 Vdc for motors, solenoids, clutches, and the high-voltage power supply circuit z +5 Vdc for the laser/scanner PCA, the beam detect PCA, and the formatter z +3.3 Vdc for the formatter, sensors, and the ICs on the DC controller PCA 80 Chapter 5 Theory of operation