t
supplied through
R1283.
The
current through 01281 pro-
vides
base
drive for series-pass transistor 01280.
When
voltage-sense amplifier U13008 detects that the
output voltage
has
reached
+5
V,
it begins shunting
base-drive current away from 01281
via
CR1281
and
holds the output voltage constant.
Current limiting for the
+5
V supply is done
by
U1300A
and
associated components. Under normal current-
demand
conditions, the output of U1300A
is
high
and
diode CR1282
is
reverse
biased.
However, should the
current through the current-sense resistor
R1281
reach
approximately 2
A,
the voltage developed across
R1281
will
raise
the
voltage at
pin
2 of U1300A
(via
divider R1282
and
R1286)
to a
level
equal
to that at
pin
3. This causes
the output of U1300A to go low, forward biasing CR1282.
This sinks base drive current away from 01281
and
lowers the output current
in
series-pass transistor 01280.
-15
V Regulator
Operation of
the
-15
V Regulator, composed of three-
terminal regulator U1330, operational amplifier U1270C,
and
their associated components,
is
similar to that of the
+
15
V Regulator with the following major changes.
The
control voltage
at
the three-terminal regulator's reference
pin
(pin
1)
is established
by
the current through series-
resistors R1333
and
R1334.
The
reference
pin
is
clamped
by CR1332 at about
-5.6
V should a failure
in
the sens-
ing
network occur. (Clamping also prevents latchup of the
operational amplifier during start-up of the power supply.)
Finally,
the sensing divider formed by
R1331
and
R1332
is
referenced to
the
+ 1 0 V reference instead of ground to
enable sensing of negative voltage.
- 8 V Regulator
Operation of
the
- 8 V Regulator
is
similar to that of
the
+87
V
and
+42
V Regulators.
Due
to the lower
operating voltages of the -
av
Regulator the common-
base transistor present
in
both
the
+87
V
and
the
+42
V
is
not required. Current limiting
in
the
-8
V supply occurs
at about 480
mA.
- 5 V Regulator
Operation of
the
-5
Volt Regulator is similar to that of
the
+5
V Regulator. Current limiting
in
the
-5
V supply
occurs at about 2
A.
+ 5 V Inverter Feedback
Operational amplifier
U1371C
and
associated com-
II
Theory
of
Operation-2445A/2455A Service
ponents
are
configured
as
a frequency-compensated
voltage-sensing network. The circuit monitors
the
+5
V
digital power supply
line
from the rectifiers
and
provides
feedback to the Preregulator Control
IC
(U1030)
via
opto-
isolator U 1
040
(both
on
diagram
9).
The
feedback is
used
to slightly vary
the
voltage-sensing characteristics of the
Preregulator Control circuitry. The feedback
(FB)
signal
slightly varies
the
voltage to the Inverter output
transformer
and
holds the output of the 5 V secondary
windings at
an
optimum
level.
Output levels
of
the other
secondary windings are related to the + 5 VD
level
and
are
also
held
at their optimum values. This technique mini-
mizes power losses
in
the series-pass transistors
and
increases regulator reliability .
Power-Up Delay
The
Power-Up
Delay
circuit, composed of 01370,
01376, U1371D,
and
the associated components, ensures
that the various regulated power supplies
have
time to
reach
their proper operati
ng
voltages before signaling the
Microprocessor that the power supplies are up.
When
power
is
first applied, a
LINE
UP
signal from the
Preregulator Control circuit goes
HI,
indicating that the
power switch has
been
closed
and
that ample supply volt-
age
is available for driving the Inverter transformer. The
HI
is applied
to
the base of 01370, but since the collector
is not property
biased
yet,
no
transistor current
will
flow.
As
the Inverter begins to
run,
the various voltages from
the secondary rectifiers
begin
coming
up
to their proper
levels. A
+2
.5 V reference voltage
is
applied to opera-
tional amplifier U13710
pin
12
and
forces the output high,
biasing 01376
on
.
Before
any
of
the
Low-Voltage Regulators
may
function
properly, the + 10 V reference voltage must
be
established
as
previously described.
When
the +
15
V Regulator turns
on,
current flows through 01370,
and
pin
13
of U1371D is
pulled above the
+2.5
V reference through divider R1370
and
R1372.
The output of U1371D goes low, turning off
01376.
When
power to the instrument
is
turned off, the
LINE
UP
signal goes
LO
(as
explained
in
the
Line
Up
Signal
description). The falling LINE
UP
signal turns 01370 off
and
drives the output of U1371D
high.
The output
level
from U1371D turns
on
01376
on
and
pulls the
PWR
UP
signal to the Microprocessor
LO
. This
LO
initiates the
power-down
sequence
used
to shut down the instrument
in
an
orderly fashion. The delay between the time that the
PWR
UP
signal goes
LO
and
when the regulated power
supplies
fall
below their normal operating levels provides
ample
time
for the Microprocessor to complete the power-
down
sequence
.
3-45
To Next Page
Loading...
