Tektronix FG 504

+15 V Su pp ly . Th
e
+15
V su pply is series regulate d an d
r efere nced to α 6 .2 V ze n er
co
n
nected i n t he emitter of
Q1610 . P otentiometer R 1605 a dj usts t he +15
V o u tput .
R esistor R
1602
p
rovides α voltage correction fee d
bac
k
p at h . S h oul d t h
e loa d on t he +15 V sup p ly i n crease, t he
voltage at t h e base
of Q1610will drop slig h tly . T h is causes
t he base of Q1620 to
go slig h tly positive, reduci n g
co n ductio n . T h e base of Q1622
now goes slig h tly
n
egative, t hus i n creasi n g co nd uctio n in t h
e series p ass
tra n
sistor located i n t he p ower module . T h is su pp lies
more curre n t for
the +15 V Ι oa d andrestores ! h
evoltage to
t he p reset level . T h
e p ower module su pp lies +33 .5 V do
t h roug h rear interface co n tacts 12 Α
and 12 Β for t he
operatio n of t h is sup p ly . T he 0 .75 Ω resistor in t h
e emitter
circ u
it of t he series pass t r ansistor is for sensing
an
overloa d cu r re n t . Wh
en e n oug hcur r en t flows t h roug h t h is
resistor to tur n
on
Q1621,
base drive to t he series pass
transistor is reduce d . Th is sets an up p er limit
to t he
amou n t of c u rre n t t h at
can be d rawn from t h is su pp ly . If
t he +15 V s u pply is raise d
beyon d abo
u
t +19 .5 V, for any
reason, an scr crowbar circuit
on
t h
e floati n g interface
board
s
h
uts d ow n all supplies exce p t t he +5 V .
-15 V Su p ply . T h
e -15 V supply is refere nced to t he
+15 V su pp ly . Tra n sistors
Q1710 an d Q1715 form α
differe n tial am p lifier for t h is
series regulating su pp ly . Any
c han ge of out p ut voltage level, dueto
α s
h
ift i n load, isfe d
bac
k
to Q1710 . Tran sistor Q1710 drives t h
e series p ass
tra n
sistor t hroug h Q1720 a nd Q1722 so t h at t he su pp ly
remai ns at -15
V over its range of cu rre n t h an d ling
capability . T he curren t sensi
ng resistor for t h is su pp ly is
R
1726 . Wh en t h e voltage across t h
is resistor b ecomes
h ig h
enoug h to tur n on Q1721, base drive
to
t h
e series
pass tra n
sistor in t he p ower mo d ule is re d uce d and the
sup ply becomes
curren t limite d . S h utdow n of t h e su pp ly
d ue to overvoltage,
occurs at -18 .7 V . T he p ower mo d ule
su pp lies -33 .5 V do t h roug h co n tacts 8 Α and
8Β fo r t h is
supply .
T h eo r y of Operation- F
G 504 (S NΒ 040000 & UP)
+25 V Sup p ly . Th e two 25 V ac windings and
t
h
e 17 .5 V
ac wi n di n g
are eac h conn ected to full wave
b ri d ge
rectifiers . T h e rectifier out
pu ts are connecte d in series to
provide a pp roximately p lus a n
d mi nus 40 V . T h is d o
outp
ut powers t he +25 V and -25 V
series regulator
circuitry
.
Tra n sistors Q1640 a nd Q1642
ma ke up t he
d iffere
n tial am p lifier for t h e +25 V
sup p
ly .
T he +15 V
reference
is ap p lie d to t he base of Q1640 . T h
e +25 V
fee d bac k for voltage
se
n
si ng is con necte d t hr ou g hR 1644
to t he base of Q1642 . T he series
pass tra n sistor is d rive n
by Q1642 t h roug h Q1650 an d Q1660 . T h e
curren t limit
sensi n g
resistor is R 1663 . Wh en t he voltage
across t h is
resistor is h ig h en
oug
h
to turn on Q1652, base drive tot h
e
series p ass tra n sistor is
reduce d and current limiting
occ u rs . If t he output voltage
drops a pp roximately 27 V
below t he raw su pp ly, VR 1652 tur n
s οη Q1652, limiting t he
curre n t . T h
is is α fol d bac k ove r load safety
actio n . If t he
sup p ly ou tput voltage
moves i n α p ositive d irectio n , t h
e
crowbar circuit Q1677, activates at
+31 .5 V disabli ng all
p ositive su pp lies
except t h e +5 V .
-25 V Supp ly .
Tra n sistors Q1742 and Q1740
form α
d iffere n tial am p lifier
for t he -25 V su p p ly . T he -25 V
is
sense d t h roug hR 1744 a nd
refere n ced to the +15 V supply
by R 1742 . T he bases of t h ediffere n tial
amplifier o p erateat
α zero
voltage level . C han ges i n t he
out pu t level are
amplifie d to d rive
01750 a nd Q1760 w h ic h control t h
e
power module series p ass
transistor . T h is p rovi d es -25 V
s up p ly
regulatio n . Curre n t limiti ng
is ac h ieve d by t he
voltage drop across R 1763 . As t he cu r r
e
n t
i ncreases
beyond
safe limits, Q1752 tur ns
on,
re d
ucing d rive to t he
series pass
transistor . If t he output voltage
rises to
approximately 27 V
above t h e raw s up p ly, VR 1752 turns
on Q1752, limiti ng t h e curren t .
T h is provides fol d bac k
overloa d protectio n . Over voltage cond itions
occu r at
approximately
-31 .5 V w h en actuatio n of t h
e crowba r
circuit disables all n egative
su pp lies .
+5 V Sup p ly . The +5 V do is sup p lie d
from +11 .5 Vd o
from t h
e p ower mo d ule t hrough reg u
lator U1690 . T h is
regulator also p
rovides sh ort circuit limit at a pp
roximately
750 mA .