HP 7925D Service Manual page 213

Appendix A
A-70. VOLTAGE PROTECTION CIRCUITS.
Circuits 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 in-
corporated into each disc drive supply. Diodes on power
and motor regulator PCA-A9 provide reverse polarity pro-
tection 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 addi-
tional 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 possible overvoltage condition and guards the -12
volt regulated supply from a possible connection to a posi-
tive supply. A fuse (A9Fl) provides overload protection for
the +36 Vdc supply. The -24 volt regulated supply is also
protected from overload by a fuse (A9F2).
A-71. 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 inter-
connect cables, the main harness, and motherboard
PCA-A7. The regulated supplies are used to provide the
A-32
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operating voltages for the controller and the disc drive
circuitry. The unregulated + 10 volt supply is used to pro-
vide the operating voltage for the carriage unlatched sole-
noid and pack chamber door unlocked solenoid. In addi-
tion, it provides the input to the +5 volt voltage regulator.
Ground 1 is used as logic ground throughout the controller
and disc drive circuitry. The +20 volt supply is used to
evaluate the line voltage condition (brownout protection).
A power distribution list is provided in table A-12. The
outputs 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
unregulated 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.