HP 3478A Technical Manual page 117

3478A
connected to ground (by the switch). This state shows
that the Front/Rear Switch is in the rear position.
7-F-83. Power Supplies
7-F-84. General. The 3478A has one set of power
supplies for the Floating Common Circuitry and another
set of supplies for the Chassis Common Circuitry. The
Floating Common Circuitry has three supplies: + 15SV, -
I5V, and + SV. The Chassis Common Circuitry has two
+ SV supplies. One of the supplies is used by the CMOS
RAM (US 12) and the other supply is used by the rest of
the chassis common logic circuitry. Refer to Schematic
4 for the following explanation on the 3478A's power
supplies.
7-F-85. 15V and -15V Power Supplies (Floating
Common). A full-wave bridge rectifier, consisting of
CR701 through CR706, develops the raw (unregulated)
voltages for the supplies. The regulated + 15V is
developed by voltage regulator U703 and the -15V by
U702. Breakdown diodes CR711 and CR712 are used
for overvoltage protection. Overvoltage protection of
regulators U702 and U703 is by diodes CR715 and
CR714, respectively. The diodes conduct if the raw
(unregulated) voltage is too large. Capacitors C702,
C703, and C706 are filter capacitors. Temperature
sensitive resistor RT706 and RT707 are used to protect
supplies from excessive output currents. The protection
circuitry operates as follows:
a. If the output current of the + 15V supply is
excessive, RT707 heats up. If the -15V has excessive
current, RT706 heats up.
b. Since RT706 and RT707 have positive
temperature coefficients, their resistance increases to a
large value due to high temperature.
c. The high resistance causes most of the supply
voltage to drop across RT706 and RT707, shutting down
the respective supply.
d. The large resistance value remains until RT706
or RT707 cool down (the supply draws normal current).
7-F-86. +5V Power Supply (Floating Common). This
power supply receives its raw (unregulated) voltage from
full-wave rectifiers CR703 and CR704. The + 5V is
developed by voltage regulator U701. Breakdown diode
CR715 is used for overvoltage protection. Capacitor
C711 is a filter capacitor.
7-F-87. +5V Power Supply (Chassis Common). This
power supply receives its raw (unregulated) voltage from
full-wave rectifier CR760 and CR761. The + 5V is
developed by voltage regulator U760. Breakdown diode
CR766 is used for overvoltage protection and capacitor
C766 and C761 are filter capacitors. Comparators
U850C and U8SOD are used to reset the Chassis
Common CPU (USO1). Refer to paragraph 7-F-59 for
information on the comparators operation.
7-F-88. +5V Power Supply (CMOS RAM). When the
3478A is turned on, the CMOS RAM receives its supply
voltage (+ SV) from regulator transistor Q701. The raw
(unregulated) voltage for Q701 also comes from CR760
and CR761 (see previous paragraph). The reference
level for Q701 comes from the Chassis Common + SV
power supply. When the 3478A is turned off, the
Chassis Common + SV goes low and transistor Q701
turns off (stops conducting). Diode CR764 then starts to
conduct and the voltage from battery BT701 is used to
supply the CMOS RAM.
7-F-22
TM 11-6625-3071-14