Tektronix 2213 - Page 50

Cur r ent from one si de of
t he ac-power-source in pu t will
go
t h
ro
u g h L 925 ( α current-limiti ng impe d ance) and triac
0925
. Dio d es C R 931 an d C R 933 (on t h e M ai n board) an d
C
R 932 an dC R 934 (on t he Current L imit b oa r d) form α f u ll-
wave bridge rectifier circuit . T h e rectifie r con verts t h e ac-
input voltage into do pulses t h at c h arge C937 . Su r ge
a
rr estor VR 938, conn ected in parallel wit h C937, condu cts
to p rotect t he following circ u itry sh ould t h e Pr ereg u lato r
out p ut voltage b ecome too h ig h .
T h e
two-transistor circuit composed of 0933,
0938, an d
associated compone n ts pr ovi d es
ove r -cu r rent p rotection in
t he event of triac misfi r ing or ac- p
ower-source t r an sie n ts .
Tran sistor Q938 is an i n sulatd-gate
FE T u se d as α switc h in
t
h
e c
h arging p at h of C937 . Tran sistor
Q933 co n tr ols t he
FE T b ias to limit t he c u rrent under a
b normal firi n g con d itio n s
of
Q925 . In n ormal p owe
r
-su pp
ly o p eration, t he voltage
d evelo p ed across R 937 is
not
s
u fficie n t to b ias 0933 i n to
conduction . T he gate-to-source
voltage of Q938 is set to
10 V b y VR 934 and R
938,
so
t h e FE T presen ts α low
resistance to t he ch arging c
urr ent to C937 . If tr iac Q925
s h o u l d misfire to cause
excessive curre n t, 0933 becomes
forward biase d a
n
d
0938 is switc hed off to reduce the
cur r ent . Wh en 0938
switc h es off, t he cur r en t t
h
at
was
flowing t h roug
h Q938 flows t h roug h R 939 . T he voltage d ro p
develo p ed across
R 939 causes cu rr en t to flow t
h roug h
VR 933 and
R 933, w h ic h h ol ds Q933 on fo r most
of t he
r emai
n
d
er of t h e ac- p ower-source
in pu t cycle . R esisto r
R 939
limits t he rate of collapse of t
h
e
fiel d a r ound L 925 to
prevent damage to Q938 . T h ermistor
R T935 ad j usts t h e
b ias of Q933 over varying am b ie n t temperat
ures .
PRERE G UL ATO R CO NTRO
L . Th e ac-sou r ce voltage is
full-wave rectified by C R 903 t
h
roug
h C R 906 an d app lied to
α
voltage d ivider composed of
R 911, R 912, and R 915 .
O u tput from t h is divider serves as
α reference voltage fo r α
ram p -and- pe d estal com p a
r ator u tilizi ng α prog r ammable
un i j un ctio n transistor (PUT),
Q921 . Capacitor C912 filte
r
s
t he line noise to
preve n t false tr iggeri ng of t he PU
T .
V oltage-dropping
resistor R 914 provi des current
for zener
dio d es VR 914 a
n
d
VR 915 to p rod uce con stant voltages
d uring eac h h alf of t
h
e ac- p
ower-source cycle .
Wh en
t h
e
instrument is first turned on,
C917
is
not
ch a r ge d . Capacitor
C915 c h a r ges t hrough C R 917 to
t h
e
voltage of VR 915
minus t he d io ded r op of CR 917 .
Wh
e
n t he
anode
voltage of 0921 is greater t han t
h
e gate
voltage,
0921 will fire and
C915 will d isc h arge t hr oug
h
t
h
e p
rimary of
Τ 925 . T
h
is
event will h ap pen afte r t he pea
k
of
t he voltage
waveform .
P
ulse
tra nsformer Τ 925 is con n ected to t
h
e gate
of Q925, a nd t he disc
harge of C915 t h roug h t
h
e
Τ 925
p rimary winding is cou p
led to t he secondary to cause
triac
Q925 to con du ct
. After firing, t he triac will tur
n off agai n
w h en t he si n usoidal source
voltage crosses t h roug
h zero .
As C917 ch arges t h roug
h 8917, Q918 cu rr en t increases
proportionally to ch arge C915
more rapidly . Wh en C915
c h arges
at α faster rate, t h e anode
voltage of 0921 rises
ab ove t he gate voltage ea r lier in t
h
e
ac-source cycle and
t he r e by causes Q925 to co
n duct fo r α lo n ger perio d of
time .
REV N O
V 1982
Th eo r y of Operation-2213
Service
Th e p ortion of t he cycle p recedi
ng t he ze r o-c r ossing p oint
ove r w h ic h t h e triac is conducti
n g is
called
t he con du ctio n
angle . T h e co ndu ctio
n angle will i n crease from n early zero
(at turn on) to an
angle sufficie n t to s upp ly t h e energy
n eeded b y t he inverter . F eed b ac
k from t h
e
inverter t h ro u g h
o p tical isolato r U 931
h
olds
t h e co r rect co nd uction a n gle by
shu nting cu rr ent
from R 917 . T h is shu nting actio n co n t r ols
t h e voltage on
C917, t here b y co n tr olling t he inc r ease in
base voltage on
0918 . T h is action controls t he c h argi ng
rate of C915 a
n d t h erefore t h e conductio n angle of 0925 .
Th e P reregulator
circuit can h a n dle α wi de range of
i np
u
t
voltages by c
h anging t h e con du ctio n an gle of t h e tr iac as
t h e
in p
ut
voltage ch a n ges . As t he inp u t voltage i n creases,
t
h
e co
n
d
u ctio n angle will d ecrease to maintai n t h e
P re r egulator out
pu t voltage at α constan t level . T he voltage
d ivider com
p osed of R 911, R 912, a nd R 915 p rod uces an
output
voltage pr oportional to t h e input li n e voltage
t h
at
is
a p
plied to t h e gate of 0921 . Since VR 914 and
VR 915 h ol d
b ias levels on Q918 consta n t regardless of
i npu t voltage,
t he point on t he cycle at w h ic h 0921 fires
will
vary
wit h
c h anges in t he ac-so u rce votlage . T h is feed-forwar
d ,
toget h e r wit h t he fee db ack from t he
I n
verter
t hr
o
ugh op tical
isolato r U 931, ensures α constant
P
rereg
u lato r outp ut to
t he Inverter .
Pr eregulator B oa r d Co n fig ur atio n
T he P ower Input circuit co n verts t h e i n p ut ac-source volt-
age to filtere d do fo r use by t he Pre r egulato r .
T h e P O WER switc h (S901) con
n
ects
t h e ac-su pp ly
source t h ro u g h fuse F 901 to b ri d ge
rectifier C R 904 . The
b ridge f u ll-wave rectifies t
h
e so ur
ce voltage, an d its output
is filte r e d b y C909 .
In p
ut su rge
curre n t at t he time of
instr u ment power-up is
limited b y t h ermisto r s R T901 and
R T902 . I n itially t h
eir
resistances
are h ig h , b ut as t hey warm
up, t h eir resistances
d ecrease and t hey dissi p ate less
p ower . T he
instr u me n t is protecte d f r om large voltage
transients b y
suppresso r VR 901 . Cond u cted EMI is
atten u ated b y li n e filter FL 9001, common-mo de tr a
nsformer
Τ
901,
differential-mo d e transforme r Τ 907, and ca p acitors
C901,
C903, C904, and C905 . Capacitors C907, C908, and
C910 form α
h ig h -fre q uency by p ass network to preven t t he
diodes
in
C
R 904 from generati ng EMI .
T h
e
P reregulator provides α regulated dc-ou t p ut voltage
for u se by
t h
e
I n verter circ u itry .
Wh en t
h
e
instrument is t u rned on, voltage develo ped
across
C909 will ch arge C913 t h roug h R 911 .
Wh
e
n t he
voltage
h
as
risen to α level h ig h enoug h t h at U 920 can
relia b ly drive
Q933, U 920 will receive its V cc voltage
t h roug h 0915 . Th is level is set by
zener dio de VR 917 i n t he
emitter circuit of 0917 and b y t he voltage divider
consisti n g
of R 912 and R 913 . T he ze
n
e
r
dio de
will keep 0917 off un til
t he voltage at
its b ase reac hes approximately 6 .9 V .
T h
e
n
3-
2 3