Tektronix 7704A User Manual

R egulator
T h
e
Regulator
stage
operates in con j unction with th e
Inverter Control and Inverter Stop stages to regulate t
h
e
Inverter switch ing . Q3102 acts as α switc h controlled by t
h e
Regulator out put of U 3105 (pin 9) or b y t h e Inverter Sto p
stage
.
Wh
en
Q3102
is turne d on, C R 3101 and CR3102 are
forward b iase d .
T h
is
effectively s h ort circuits t he 60-turn
center-ta pp ed
winding
of
Τ 3030 to s hut off t h e Inverter .
F
or
furt h er information, see t he d iscussion of t h e Inverter
stage .
Inverter Sto p
Th e Inverter Stop stage, Q3104, is controlle d by t h e
Stop Trigger output of U 3105 ( p in 8) to s h ut off t h e
Inverter t h roug h t he Regulator stage . During t h e start
perio d , Τ3030 sup plies current to ch arge C3106-C3108
t h ro u g h CR 3106-C R 3108 . Also during t h is time, Q3104 is
reverse
biased by U3105 .
Once triggere d
on by α p
ositive
Sto p Trigger, Q3104 will stay on w h ile C3106-C3108
disc h arge t h roug h t h e base of Q3104 . If U3105 is remove d
from its sock et or is otherwise non-functional, the Inverter
Stop stage will stop t he Inverter after t he first few cycles of
operation . In t h is mo d e, t
h
e d uty cycle will b e muc
h
s h orter because C3106-C3108 will not have sufficient time
to c harge .
L ow- V oltage R ectifiers
T h e rectifiers and filter components in t he secon d aries
of T31 01 provide rectifie d , p re-regulated voltages for
furt h er regulation by t
h
e
L
ow- V oltage Regulators circuit .
E
rror-sense voltages from t
h e +54-
V olt and lamp
p
ower
(+5-
V olt
L
ig
h
ts)
supplies are fed
bac k to U 3105 to control
t
h e amplitude
of t
he Inverter drive
to
t he L ow- V
oltage
Rectifiers stage .
L O W - V O L TAG E RE G UL ATO R S
Th e L
ow- V oltage
Regulators circuit converts semi-
regulate d
voltages
from t he Inverter/ R ectifiers circuit to
stabilize d , low-ripple output voltages .
T h
e -50- V olt Su
pp ly
is referenced to α zener d io de ; t he remaining four su p plies
are reference d to t h e -50- V olt S u pp ly . Each supply
contains α s hort-protection circuit to prevent instrument
damage if α sup p ly is s horted to groun d or to anot her
supply . F igure 2-31 s h ows α d etailed bloc k d iagram of t
h e
L ow-V oltage Regulators
circuit
.
-50-
V
olt Supp
ly
T h e following d iscussion inclu d es t h e d escription of t he
-50- V Series Regulator, -50- V
F
eedbac k Amplifier, -50- V
R
eference, an d -50- V
Current L imiting
stages . Since t
h
ese
stages
are closely
related
in
t
h e production
of t
he
-50-volt
regulate d
output
voltage, t h eir op eration is most easily
understoo d wh en d iscusse d as α unit .
Circ u it Description- 7704A Service
Semi-regulate d -54 volts from t h
e
Inverter/
R
ectifiers
circuit
provid
es t he unregulate d voltage source
for
t h is
supply . Transistors Q32130-Q32115-Q32143 operate as α
fee d bac k -stabilize d regulator circuit to maintain α constant
-50-volt out put level . Q32130 is connecte d as α d ifferential
amplifier to compare t h e fee d bac k voltage at t h
e
base of
Q32130 B against t h e reference voltage at t
h
e b ase of
Q32130A . T h e error output at t
h e collector
of
Q32130 B
reflects t he d ifference,
if
any,
b etween t hese two in p uts .
T h
e c
h ange in error-o u t p u t level at t h e collector of
Q32130
B is always in t h e opposite d irection to t he ch ange
in t h
e fee
d back
i n p
ut at
t h e base
of
Q32130 B (out of
ph
ase) .
Zener dio d e VR 32127 sets α reference level of ab out -9
volts at t he base of Q32130A . Α sample of t
he
output
voltage from t h is supply is connected to t
he
base
of
Q32130 B t h roug
h
d ivi d er
R32132- R 32133- R32134 .
R32133 in t h is divi d er is ad j ustable to
set
t
h e out
p ut
level
of t h is su pp ly . N otice t hat t h e fee d b ac k voltage to t
h
is
divi d er is obtained from α line la b ele d -50 V
Sense . If
t
h e
feed b ack voltage were obtained at t h e su pp
ly,
t
h e
voltage at
t h e loa d woul d not stay constant, d ue to t
he in h erent
resistance of t h e interconnecting
cable b etween t he supply
and its load (as t
he load current varies, t he voltage drop
along
t
h
e ca
b le also varies) . T he Sense configuration
overcomes t h is p roblemby sensing t h e voltage at t
h
e loa d .
Since t h e cu rrent in t he Sense line is
small
and
constant, t h e
loa d voltage is h el d co nstant
regar d less
of
t he load current .
Regulation occurs as
follows : If t he output level of t
h is
su pp ly d ecreases
(less negative) d ue to an increase in loa d
or
α decrease in in p ut
voltage (as α result of line-voltage
ch anges or rip p
le), t h e voltage across divi d er
R32132- R 32133-
R
32134
d ecreases also . T h is results in α
morep ositive
feed
bac
k level at t he base of Q32130
B
t
h
an
t h at esta b lis
h
ed
by t he -50- V R eference stage at
t he base
of
Q32130A . Since t h e transistor with t h
e more p ositive
base controls t he conduction of t
he d ifferential amplifier,
t h e output current at t
h e collector of Q32130 B increases .
T
h is increase
in out p ut from Q32130 B increases t
he current
t
h roug h Q32115 . T h is allows more
current
to
flow t hrough
Q32143 to result in i n crease d
conductio n of t he -50- V
Series Regulator Q32139 . T he loa d
current i n creases an d
t h e outp u t voltage of t
h
is
sup p ly also increases (more
negative) . As α result, t
h e fee d bac k voltage from t he -50- V
Sense line increases an
d t h e base of Q32130 B returns to t
h
e
same
level
as
t h e b ase of Q32130A . Similarly, if t h
e
outp
u
t
level of t
h is su pp ly increases (more negative),
t he out p ut
current of
Q32130 B decreases . T he feed back t
h roug h
Q32115 and
Q32143 reduces t h e co n duction of
t h e -50- V
Series
Regulator to decrease t he output
voltage of t h is
supply
. T h e -50 V olts ad j ustment R32133
d etermines t h e
divi d er
ratio to t h e base of Q32130 B an d t
h ereby deter-
mines t h
e
feed
back voltage .
2- 37

Tektronix 7704A Specifications

General

Brand	Tektronix
Model	7704A
Category	Test Equipment
Language	English