Tektronix 7704A User Manual

Circuit Description -7704A
Service
Reg u lator output d rives t h e
Regulator stage t h roug h pin
9- U3105 . U nd er normal operating
conditions,
only the
E
Sense input at p
in 15 h as control over t he ou t put pu lse
wid t h for p re-reg u lation . H owever, an error detected by any
of t he sensing in p uts will affect t
h e
output p
ulse wi d t h and
will also pro d u ce α Stop
Trigger
to t
he Inverter
Stop
stage .
T h e op eration of eac
h individual function of t h e Inverter
Control stage
is described in t he following d iscussions .
P re- R
egulator
T h
e
P re- R egulator portion of
U 3105, in con j unction
with t
h e Regulator stage, mai
n tains constant voltages at t
he
outputs of t he L ow- V oltage
Rectifiers .
Transformer Τ3035 provides
Inverter power a nd p h ase
information to U3105 .
T
h e p
h ase information is connecte
d
to
t he trigger in p ut of t
he Inverter Control multivibrator
via
p ins 10 an d 11 t h roug
hC3125 an d C3126 . Bri d
ge rectifier
C R 3130-C R 3131-C
R 3132-C R3133 p rovides p
ositive an d
negative op erating voltages
to U 3105 . Α s h unt
regulator in
U 3105 regulates t
h e +7 .5-volt out p ut of t
h e b ridge rectifier
connected to p
in 6 . T he -2-volt
(nominal) out put
connected to
pin 7 is unreg
u
late
d . VR 3147 provides α
sta b
le reference voltage for t
he sensing- d ivider resistors
R3142-133143-133144-133146
.
In
t h is divi d er,
R3143
ad j
usts t he voltage
level
at
t he Ε Sense input to
t h e
P re- R egulator
(pin
15- U
3105) to set the out put voltage
of
t
h
e
L ow- V oltage Rectifiers .
In
t he sta b le state of t
he Inverter Control multivibrator,
t he Regulator out put at
p in 9 is near groun d to turn
off t he
Regulator stage .
After t h e Inverter c u rrent passes
t h roug h
zero, eit h er p
in 10 or pin 11 will go p ositive to
trigger t he
Inverter Control
multivibrator into its asta b le state .
Wh ile
t he multivi b rator
is in t h e astable state, t h e Reg
u
lator
output voltage level is
positive to turn on t h e Regulator
stage . T he d u ration of t
he astable state is determine db y
t he
voltage level at t
he Ε Sense input at p in 15 .
If
t
h is voltage
level is low,
t h e du r ation is s h ort . As t
h is voltage level
increases, t he d u ration increases .
F ault
P
rotection
T h
e
fault- p rotection p ortions of U 3105
protect t h e
p
ower-s u pp ly com p onents from d amage d
ue
to
s hort
circuits, turn-on surge currents, an d ot
h er malf unctions .
Wh en α fault is d etecte d at t
h e B alance or Ι Sense i np uts
( p ins 2 a n d 13
res
p
ectively), α current outp u t from t
h
e
Sam p le
P erio d Timer outp u t
( p
in
1)
ch arges C3114
. If
t
he
d
etecte d fa u lt lasts longer t h an about 30 millisecon d s,
C3114 c
h arges positive enoug h to p rod uce α Stop Trigger
out put at
pin 8 to turn t he Inverter off . Wh en t h e Inverter
is s h ut off, t he current c h arging C3114 is interrupted an
d
C3114 will d isc harge . After about 450 millisecon d s, t
he
Stop Trigger o u tp u t will return to α
near zero-volt level to
2- 36
allow t he Inverter to ru n . T h is cycle re p eats until t
h
e fa
u
lt
is correcte d ; t he Inverter is on for about 30 milliseconds,
t h en off for ab o u t 450 millisecon d s .
I n verter Cur r ent L imiter . T h e Inverter Current
L imiter
provi d es protection for t he Inverter
com p onents from
damage d ue to excessive current .
Operation
of
t h is stage is
similar to t h e
P
re-
R egulator
(voltage
regulation) . T h e
Inverter Current
Limiter ta k es control of t he Inverter
Control Reg u lator output p ulse wi d t h du ring turn-on or
w h enever an overload causes t
h
e Inverter
current to reac h
t he limit value .
R3137 is t he
current-sensing resistor . T h e voltage at t he
j unction of
R3137- R 3136-C R3138 is t he negative rectifie d
Inverter current . T he 1 Sen se input at p in 13 is
normally
h eld positive t hrough R3135 .
If
t
h e Inverter cu rrent
increases, t h e voltage at t h e Ι
Sense
input
will become more
negative . T he I n verter Control
Regulator output p u lse
widt h increases until t h e Inverter
current
reac
h es α value to
h old p in 13 near the zero-volt
level . If t he voltage at p in 13
remains near zero for more t
h
an
approximately 30 milli-
secon d s, t he Stop Trigger output at
pin
8
will go positive to
trigger t he Inverter Stop stage
.
B
alance .
T h e B alance portion of U3105 provid es s h ort-
circuit
protection for t h e +5- V olt and -17- V olt out p uts of
t h e
L
ow-
V oltage Rectifiers . V oltage divider
R3152-133154-133155
biases t he Balance Sense input at p in
2 of U 3105 near α
zero-volt level . If o n e of t he out put
voltages ch anges
sufficiently
to
cause t he voltage level at
p in 2 to vary
approximately 200 millivolts ( p ositive or
negative), α
positive Stop Trigger out p ut is p ro d uce d at
pin
8 of
U3105 .
L ine Sto p . T he L ine Sto p portion of
U 3105 p rotects
t h e Line-Input components
from d amage d ue to turn-on
surge current .
T
h is is ac
h ieved b y triggering t h e Inverter
Stop stage to stop t
he Inverter w h en t h e P O
WER
switc
h
is
turne d
off . T h e L ine Stop stage will also stop t
h e Inverter if
t
he AC line voltage falls below α
minim u m value . ( F or
furth er information,
refer to t he discussion of t h e L ine
Input stage .)
T he line-fre q
uency signal from transformer
Τ
3001
is
connected to t
he L ine Stop Sense in put of U 3105 at
pin 4 .
During normal
operation, t h e line-fre q
uency signal causes
t h e
L ine Sto p Timer terminal (pin 3) to b
e near α zero-volt
level (groun d ) . T
h is zero-volt level keeps C3117
from b eing
charged towar d +7
.5 volts t h roug h
R
3117 .
Wh en t h e
li n e-fre q uency
signal is interrupted or
falls b elow α
mini m um val
u e, C3117 will begin to c
harge to +7 .5 volts .
Wh en t h e voltage
at p in 3 reach es
approximately +0 .7 volt,
t he L ine Stop
stage will pro d uce α positive Sto p
Trigger
output at pin 8
of U 3105 to trigger t he Inverter
Stop stage .
Ο

Tektronix 7704A Specifications

General

Brand	Tektronix
Model	7704A
Category	Test Equipment
Language	English