Model
5340A
Theory of Operation
4
-
276.
In the event that the
+15
volt reference fails or falls below
11
volts,
A32CR3
conducts to
turn off
A32Q2
thereby disabling the supply. This prevents
A32U1
from locking in the off
position. The series pass transistor in
A32U1
drives
A32Q5
to handle the larger current require
-
ments of the regulator.
A32R17
sets the current limit point and
A32R16
and
R18
set the current
foldback characteristics.
4
-
277.
+5
VOLT
DIGITAL
REGULATOR (PART
OF
A32).
The
+5
volt digital regulator (Figure
8
-
34)
is
similar
to
the
+5
volt
RF
regulator except for no preregulator, additional series
pass
transistors, and two current limit circuits. In order to handle the
3
amp load, an external series
pass
transistor
(Q2)
and driver
(A32Q3)
are provided.
A32R9
serves
as
the collector load for
A32Q3.
The operation of
U2
is
similar to that described in Paragraph
4
-
270. A32R10
provides
current limiting for the
IC.
For example, if
Q2
opens, the series
pass
transistor in
A32U2
and
A32Q3
would be seriously overloaded. In this case, the drop across
A32R10
drives
A32U2
into
current limiting.
A32Q4
provides current limiting for loads above
3
amperes. When the drop
across
A32R13
and
R15
forward biases
A32Q4, A32U2
is
driven into the current limit mode. The
collector of
A32Q4
connects to the compensation input (see Figure
4
-
52)
which
is
in parallel
with the current limit amplifier in the
IC.
The current foldback circuit
is
made up of
A32Rll
and
R12. A32R14, A32CR6,
and
A32C10
provide regulation for the current foldback point to prevent
line voltage variations from changing the foldback characteristics.
A32CR5
and
A32C9
provide
ripple filtering for the current foldback circuit.
4
-
278. A29 +15
Volt Regulator
Assembly, 05340
-
60025
4
-
279.
The
+15
volt regulator (Figure
8
-
33)
is similar to the
+5
volt
RF
regulator described in
Paragraph
4
-
275.
The supply provides approximately
170
milliamperes. The preregulator is
composed of
A29Q1, A29Q2,
and
A29CR1-CR3. A29Q3
is the series pass transistor.
Current
limiting
is
set by
A29R7
and
A29R10. A29R6
and
A29R9
set the current foldback characteristics.
A29R5
adjusts the output voltage.
4
-
280. A28
-
15
Volt and
t175
Regulator
Assembly, 05340
-
60022
4
-
281.
The
-
15
volt regulator (Figure
8
-
33)
supplies about
180
milliamperes and consists of
a
preregulator, regulator, series pass transistor and driver, and current limit circuits.
A28Q1
and
A28Q3
serve
as
a constant current source for
A28CR2.
Zener diode
A28CR2
establishes
a
con
-
stant
19.6
volts for the regulator reference input.
R5
sets the proper current input for the
regulator reference.
A28U1
is
a
negative supply voltage regulator similar to the positive voltage
regulator previously discussed. The reference current input at
A28U1
flows internally through the
IC
to pin
1
and
A28R6
and
A28R7.
Adjusting
A28R7
sets the output supply voltage. The boost
output at
A28U1(7)
drives
Q7
and
QlO
to regulate the output voltage.
A28C4
provides com
-
pensation to prevent high
-
frequency oscillation. Current limiting for the
IC
is determined by
A28Rll.
A28R16
is
the current limit resistor and works in conjunction with
A28Q8.
When the
current limit point is reached, the drop across
A28R16
turns on
A28Q7
which diverts drive
current from
A28Q10. A28R13
and
A28R14
determine the current foldback limiting
characteristics.
I
4
-
282.
The
+175
volt regulator provides
+175
volts
at
about
28
milliamperes to drive the display
tubes in
A25.
Since no filter capacitors are used for rectifier
A30CR1,
a
pulsating dc
is
developed
for driving the display tubes.
A28Q2, A28R2,
and
A28CR1
serves
as
a
current source for
A28CR3
and
A28CR4.
A28CR3
and
A28CR4
provide the
+175
volt reference for
A28Q4. A28Q4
drives
A28Q6
which serves as the series pass transistor.
Current limiting
is
set by
A28R15
and the
current foldback limiting, chgracteristics are determined by
A28R8
and
A28R10.
When the drop
across
A28R15
is
sufficient
'to
turn on
A28Q9, A28Q9
diverts current from
A28Q4
thereby
reducing the output voltagb
to
maintain
a
constant current. With overloads, the voltage across
A28R8
and
A28RlO bias3siA28Q9
to
conduct further to reduce the output current below the
current limit point set ,by
A28R15.
This
is
foldback action and prevents excessive power dis
-
sipation in the regulator.
A28R9
suppresses
as
oscillations.
A28C6
filters out the pulsating dc from
A28Q9
to prevent
A28Q9
from conducting
at
the peak levels of the pulsating dc.
-I
4
-
53