Tektronix T921 User Manual

Circuit
Description-T921/T922/T922R

III .
1
POWER
SUPPLY
CIRCUT DESCRIPTION
POWER
INPUT
Ac power is applied to the
primary of T700 through line fuse
F700, POWER switch S700, Line Selector
switch S701, and Range
Selector switch S705 .
The Line Selection switch,
S701, connects the split primary
windings of T700 in parallel
for 120 V operation or in series for
240 V . When changing the
nominal line voltage, also change the
line fuse . See parts list for
correct fuse falues .
The Range Selector
switch, S705, selects either LO (100 or
220
V) or HI (120 or 240 V) nominal line-voltage
range
.
SECONDARY CIRCUITS
The secondary circuit
supplies four regulated voltages :-8 V,
+8 V, +33
V,
and
+100 V .
Operational amplifiers U742A (+8
V supply) and U742B (-8 V
supply) have differential
inputs that monitor output voltage
variations and provide correction
signals to the series-regulating
transistors . For
example, suppose the +8 volt supply drops .
This
negative change is coupled to the inverting input of
U742A
through
sense resistor R756, causing pin 7 to go
positive . Since
the voltage across VR746 remains
essentially constant, Q754 and
0756 follow this change
and
raise
the output voltage back to +8
volts . In the +100
volt supply, Q726 acts as the feedback amplifier
with its base being the inverting input . The regulating
action is the
same as
in the +8 and -8 volt supplies . Zener diode,
VR762,
provides a 5 volt reference for the -8
V supply, which in turn
provides the reference for the +8 and
+100 volt supplies . The
series regulating elements in the +100
V and +8V supplies are
transistors Q734-0736 and
0754-0756 . The series regulating
element in the-8 V supply isa
modified Darlington configuration
consisting of Q774 and Q776 .
Current limiting circuits provide
short-circuit protection
for each regulated supply . The following
describes
the +8V current-limiting circuit . The other
current-
limiting circuits operate similarly .
In the +8 V supply, Q752 is
normally biased off . Under normal
conditions, the base of
Q752 is set at about +8V . As the supply
current
increases,
the voltage drop across R754 increases .
Since
the 0756
emitter-base diode voltage difference remains
constant,
the increasing voltage on Q756 emitter
due to the R754 voltage
drop causes a corresponding increase
at the base of 0756 . This
voltage is applied to voltage
divider R752 and R753, causing the
base of
Q752 to go more positive . When the supply
current
increases
sufficiently beyond the normal operating current,
0752
turns on . The collector of Q752 moves
in the negative direction,
which begins turning off Q754-0756
and creates a foldback
condition (see Fig . 7-1) .
Transistor Q756 continues to conduct
some current
when the supply is limited, dropping enough
voltage
across R754
to
keep 0752 biased on .
REV . B DEC 1976
Regulated +33V is provided by
Zener diode VR784 from the
+100 V supply . A
different circuit provides a similar voltage for
the T922R (see T922R circuit
description at the end of this
section) .
Current divider, R741, R742, R2102 (see
diagram 5)
provides
a sample of the line voltage for line triggering .
POWER-ON LAMP CIRCUIT
The ON lamp, DS796,
remains
on as
long as the line voltage
does not vary more
than approximately 10% from the nominal .
selected
line voltage (100, 120, 220, or 240
V) . When . the line
voltage is not within the 10% limit, the ON
lamp blinks .
As
long as Q796 is conducting, DS796 remains on .
If
Q796
is
biased off, DS796 goes out, allowing C796 to
charge through
R796 and R797 . When C796 reaches
about 80 V, it discharges
through DS796 causing it to turn on
momentarily . Capacitor C796
again is charged through
R796-R797 and discharged through
DS796 . This cycle repeats,
causing the ON lamp to blink until the
line voltage is within
the 10% limit and Q796 conducts .
When the 100
V unregulated supply at voltage divider
R792
and
R793 increases to more than about 10% above
the nominal
value, Q792 turns on and Q796 turns off, causing
DS796 to blink .
When the 100 V unregulated supply at voltage
divider R794 and
R795 decreases to less than
about 10% below the nominal value,
0796 turns off, causing DS796 to
blink .
0752
Fig . 7-1 . Foldback
circuit action .
10,
,
8
.
I
NOMINAL
LOAD
BEGINS
CONDUCTING
a
6-
0
4-
DEAD
2-
SHORT
0
80 -- 0
0 100
200 300 400 500 600 700
LOAD CURRENT (mA)
1982-38

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English