Supply Failure Monitoring Circuitry; Standby Mode - HP 8340B Manual

If
the
+20V
supply fails, only the
+22V
supply
will operate. The
+20V
supply
is
energized
with
a
start-up current source
until
it reaches the voltage required to power the
precision
+10V
reference.
The
+10V
reference serves as a
voltage reference for the +
20V
supply.
If
the instrument
is turned
off,
or
if
the thermal switch temperature threshold
is
exceeded,
the
+20V
supply shuts down.
The reference oscillator supply allows the
10
MHz reference oscillator to be turned off when
an
external
10
MHz
reference
is
used
The 5V secondary
output
is
rectified and regulated to generate
the +5.2V
regulated supply.
SUPPLY FAILURE MONITORING CIRCUITRY
HPUP (high power up) indicates whether all
the instrument
power
supply voltages are operating.
If
a
supply fails or is
low,
HPUP is
low, and
LIPS
(low instrument preset)
is
low.
Pulling
LIPS low causes the instrument microprocessor to reset, protects
the instrument memory,
resets the A3 display processor, and turns on
all
front panel LEDs. LIPS
is
also pulled low when you
press
[INSTR PRESET].
STANDBY MODE
When
line
power
is connected, and the front panel
line switch
is in
the
STANDBY
position, the
following happens:
• Power
is supplied to the rectifiers on the A19 and ASS
assemblies.
•
The
+22V
regulator
(on the
ASS
assembly)
is powered by
the
+20V
rectifier output
(+20V
UNPEG).
•
The cathode of DS1 (front panel
standby
LED)
is
grounded,
lighting the
LED.
•
A62K1 switches (closes) Into standby (fan
remains
off).
•
LSBY (low standby) disables the
+20V
regulator.
WARNING
Although the
+22V
regulator
is the only operational supply, line
power
is
present, the rectifiers are
fully operational, and the filter capacitors are
fully
charged.When the
line
switch is
moved to on,
the relay is discon¬
nected from ground, providing power to the fan. Simultaneously, LSBY
goes from
0
to 22V, allowing the +20V, and other supplies, to operate.
HP 8340B/41B
Power Supplies/Fan Theory of Operation I-5/1-6