Power Distribution System; Power Panel Assembly; Power Supply Assembly; Voltage Regulator Circuits - HP 7925D Service Manual

Theory of Operation
1-62. POWER DISTRIBUTION SYSTEM
The power distribution system primarily consists of
the power panel assembly, power supply assembly,
voltage regulator and protection circuits, and the associated
switches, fuses, lamps, and wiring. The purpose of the
power distribution system is to develop the various
operating voltages from the primary power source and
distribute these voltages throughout the disc drive cir-
cuitry. Figures 4-22 and 4-23 provide the wiring dia-
grams for the power panel assembly and disc drive
mainframe assembly, respectively. Tables 4-3 and 4-4
provide the signal distribution list for motherboard
PCA-A7 and the power distribution list for the disc
drive mainframe assembly, respectively. Together
these diagrams and listings provide all of the wiring
information for the disc drive.
1-63. POWER PANEL ASSEMBLY. The power
panel assembly (22, figure 6-1) is located in the lower rear
section of the disc drive cabinet. This assembly consists of a
circuit breaker, a three-receptacle outlet strip, and an air
exhaust fan. The power panel assembly equips the enclosure
with an electrical package to control, protect, and distribute
single-phase mains power. The exhaust fan is used to help
maintain the internal cabinet temperature at the proper operat-
ing level.
The disc drive is supplied with an appropriate power cord.
The various power cords available are shown in the HP
7925D Installation Manual, part no. 07925-90912.
1-64. POWER SUPPLY ASSEMBLY. The power
supply assembly consists of a power transformer (T1), three
bridge rectifiers (CR1 through CR3), five filter networks (R1
through R5 and C1 through C5), and eight fuses (F2
through F9). This assembly develops five unregulated dc
voltages (+20V, -20V, +lOY, +36V, and -36V) from the
primary power source. The terminal strip (TB1) on the
primary side of the power transformer (T1) permits strap-
ping of the primary windings to match the available prim-
ary power source. Refer to the HP 7925D Installation Man-
ual, part no. 07925-90912, for the various strapping con-
figurations of TBl. Fuses F2 through F6 provide overload
protection for the five unregulated secondary voltages.
Fuses F7 through F9 provide overload protection for bridge
rectifiers CR1 through CR3. All power supply assembly
grounds are brought to a common ground block (TB2). It is
important that these connections be made as shown be-
cause each has a specific assignment, i.e., ground 1 serves
as logic ground, ground 2 serves as linear motor ground,
and grounds 3 and 4 serve as spindle motor grounds.
1-65. VOLTAGE REGULATOR CIRCUITS. The
unregulated dc voltages developed by the power supply
assembly are routed to the four voltage regulator circuits
on power and motor regulator PCA-A9 via the power sup-
1-32
7925
ply harness. The unregulated output from the
+
10 volt
supply is routed to the input of the +5 volt voltage reg-
ulator. The +5 volt regulated supply is sampled at
motherboard PCA-A7 so that the proper voltage level will
be maintained at that point. The unregulated output from
the +20 and -20 volt supplies are routed to the inputs of
the + 12 and -12 volt voltage regulators. The unregulated
output from the -36 volt supply is routed to the -24 volt
voltage regulator. Test points are provided on power and
motor regulator PCA-A9 to monitor the outputs from the
+ 5, + 12, -12, and - 24 volt regulated supplies.
1-66. VOLTAGE PROTECTION CIRCUITS. Cir-
cuits are provided in the disc drive to detect over and
under voltage conditions in the +5, +12, -12, +36 and -36
volt supplies. Reverse polarity protection is also incorpo-
rated into each disc drive supply. Diodes on power and
motor regulator PCA-A9 provide reverse polarity protec-
tion at the input of the +5, +12, -12, and -24 volt regulated
supplies and +36 and -36 volt unregulated supplies. Diodes
on motherboard PCA-A7 provide additional reverse
polarity protection for the +12 and -12 volt regulated
supplies. In addition, a crowbar circuit on motherboard
PCA-A7 guards the +5 volt regulated supply from a pos-
sible overvoltage condition and guards the -12 volt regu-
lated supply from a possible connection to a positive
supply. A fuse (A9F1) provides overload protection for the
+36 Vdc supply. The -24 volt regulated supply is also pro-
tected from overload by a fuse (A9F2).
1-67. SUPPLY VOLTAGE DISTRIBUTION. The
outputs from each of the four regulated supplies, the +10
volt unregulated supply, the +36 volt supply, and ground
1 are routed throughout the disc drive via the main har-
ness and motherboard PCA-A7. The regulated supplies
are used to provide the operating voltages for the disc
drive circuitry. The unregulated +10 volt supply is used to
provide the operating voltage for the carriage unlatched
solenoid and pack chamber door unlocked solenoid. In
addition, it provides the input to the +5 volt voltage regu-
lator. Ground 1 is used as logic ground throughout the
disc drive circuitry. The +10 volt supply is used to evalu-
ate the line voltage condition (brownout protection). A
power distribution list is provided in table 4-5. The out-
puts from the +20, -20, +36, -36 volt unregulated supplies
and grounds 2, 3, and 4 are not included in this listing
because they are used exclusively on power and motor
regulator PCA-A9. As long as PSF equals 1, the PSU
LED on drive control PCA-A4 will be on indicating that
the +5, +12, and -12 power supplies are working and
within tolerance. As previously mentioned, the unregu-
lated outputs from the +20, -20, and -36 volt supplies are
used to develop regulated voltages, ground 2 is used as
linear motor ground, and grounds 3 and 4 are used as
spindle motor grounds. The unregulated ±36 volt
supplies are used to provide the operating voltages for the
linear motor power amplifier and the four spindle motor
current switches which are located on power and motor
regulator PCA-A9.