Figure 3-10. Computer Power Distribution - HP 330 Service Information Manual

--12
Vdc
Voltage from the power transformer feeds directly to a -12 V dc Post Regulator. The output
from the Post Regulator is nominally -12.29 V dc ±0.43 V dc. In addition to being distributed
in the computer (or expander), the
+
12 V dc output is also fed to the Overvoltage Protection
Circuit and Reset Circuit. External to the power supply, the -12 V dc is connected to the other
side of the Power-On LED on the front panel.
Protection and Reset Circuits
Functions of the Protection and Reset Circuits are:
• Overvoltage Protection .
• Reset Signal Generation.
()vervoltage Protection Circuit
All output voltages are inputs to the Overvoltage Protection Circuit. If the voltage goes high,
the circuit will shut down the power supply, protecting its components as well as components in
other areas of the cornputer (or expander). The output of the Overvoltage Protection Circuit
feeds the Switching Control Circuit.
Reset Circuit
1When ac line voltage is applied, the power supplies start building up their output voltages.
The +5 and + 12 V dc supplies input the Reset Circuit. As these inputs reach their operating
voltages, the processor board circuits are also being powered up. When normal operating levels
are reached, a Power-Up RESET (PURESET) signal is sent to the CPU to initialize it and start
the Boot ROM sequence.
Power Distribution
Power Distribution routes power to the assemblies installed in the unit. Figures 3-10 and 3-11
shows how power is distributed in the computer and expander.
I/O BACKPlANE
Figure 3-10. Computer Power Distribution
Functional Description
69