HP LaserJet 4 Service Manual - Page 143

Power System When the main power switch is turned ON, the AC Power Supply provides AC voltage to the DC Power Supply and the fuser assembly heat lamp. The paper path doors must be closed to activate the AC power supply door switches before voltage is provided to the heat lamp. The DC Power Supply generates +5V DC, and +24V DC for use by the Formatter PCA and the DC Controller PCA. The DC Controller PCA distributes voltages to the remaining electrical assemblies. Over-current protection for the DC Power Supply is provided by a current monitoring circuit (crowbar circuit). To reset the crowbar circuit, the printer's power must be turned OFF for at least ten minutes. The power system block diagram is illustrated in Figure 5-20. Note that both AC and DC power supplies are contained within the Power Supply Assembly. The high voltage power supply is a discrete assembly located on the underside of the printer. Functional Overview 5-31