01040.
The
base
of 01022
is
pulled
negative through
R1042,
and
the
+15
V supply for
the
Preregulator
IC
turns off
(see
Preregulator Control description).
The
power
supply will try to restart itself; but, as long
as
the
excessive-current condition persists, the current-limit cir-
cuit will
keep
shutting the supply down, protecting the
instrument.
Rectifiers
The
rectifiers convert the alternating current from the
secondary windings of inverter output transformer T1060
to the various
de
supply voltages required
by
the
instru-
ment. Rectification
is
done
by
conventional diode rectifier
circuits,
and
filtering
is
done
by
conventional
LC
networks.
The
+
87
V unregulated supply
is
produced
by
a
voltage-doubler circuit. The positive plate of C1130 at the
anode of CR1132 is referenced at approximately
+45
V
through diode
CR1131
(to the +
42
V unregulated supply).
As the positive half cycle from the
42
V secondary winding
(actually about
+45
V peak)
is
applied to the negative
plate of C1130, the positive plate is elevated to a peak
value of approximately
+90
V. Diode CR1132 becomes
forward biased
and
storage capacitor C1132
is
charged to
about + 90
V.
Following cycles replenish the charge
drawn off
by
the loads
on
the
+87
V supply
line
.
Line Signal
A sample of the ac line voltage is coupled to the Trigger
circuit
by
transformer T1229
and
provides the
LINE
TRIG
signal to the Trigger hybri
d.
Transformer current
is
limited
to a safe value
by
resistors R1014
and
R1015 placed
in
series with the primary winding leads. The transformer's
output characteristics
are
matched to the input of the
Trigger circuit hybrid
by
R1208
and
C1208.
Line
Up
Signal
The
circuit composed of 01029, opto-isolator U1029,
and
their associated components, detects when power has
been
applied to the instrument
and
the Preregulator Con-
trol power supply
is
functioning properly.
When
the
rectified line voltage reaches proper operating voltage, the
voltage divider composed of R1027
and
R1028 forward
biases 01029. As soon as the Preregulator Control power
supply turns
on,
current flows through
R1029,
01029,
and
the opto-isolator
LED.
The illuminated
LED
saturates
transistor U1029
and
the
LINE
UP
signal to the Power-Up
Delay
circuit (diagram
1)
is
pulled
HI,
indicating that the
Preregulator Control circuit should now
be
functioning
properly.
POWER DOWN.
When
instrument power is turned off,
the voltage across the primary storage capacitors
(C1021
Theory
of Operation-2445A/2455A Service
and
C1022) begins to fall as the capacitors discharge. As
the voltage drops, the bias current through R1027 to the
base of 01029 also drops until the
bias
voltage across
R1028 reaches a point about 2 V above the average
transformer drive
level
at
pin
2 of U1029. At this point,
01029 turns off,
and
the LINE
UP
signal to the Power-Up
Delay
circuit goes
LO.
This
LO
signals the Microprocessor
that it should start its power down routine.
The
Line
Up circuit tells the Microprocessor that the pri-
mary capacitors
have
started discharging while there
is
still a stored charge (set
by
R1027
and
R1028) about 40%
in
excess
of
that required to keep the power supply volt-
ages
in
regulation. This allows the Microprocessor to com-
plete the power -down sequence before the supplies drop
below their normal operating
level.
Further information
about the power-down sequence is given
in
the Micropro-
cessor Reset Control description.
Fan Circuit
Fan
motor 810 is driven by adjustable three terminal
regulator U1110.
The
fan's speed is determined
by
the
voltage supplied
by
U1110
and
varies with ambient tem-
perature.
As the ambient temperature in the cabinet increases,
the resistance of thermistor RT1110 decreases causing
more current to flow
in
R 1112. This causes the voltage at
pin 2
and
therefore the voltage at
pin
3 of U 1110 to
increase,
and
the
fan
motor speed increases to provide
more cooling capacity.
LOW-VOLTAGE REGULATORS
The
Low-Voltage Regulators remove ac noise
and
rip-
ple
from the various unregulated
de
supply voltages.
Each
regulator output is automatically current limited if the out-
put current exceeds the requirements of a normally func-
tioning instrument. This limiting prevents
any
further com-
ponent damage.
+ 10 Volt Reference
Each
of the power-supply regulators control their
respective outputs
by
comparing their output voltages to a
known reference
level.
In
order to maintain stable supply
voltages, the reference voltage must itself
be
highly stable.
The
circuit composed of U1290, U1300C
and
associated
components establish this reference.
Resistor R1400
and
capacitor C1400 form
an
RC
filter
network that smooths the unregulated +
15
volt supply
3-43
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