HP 435B - Service Sheet 4

S ERV IC E S HEE T 4
PR I N CI PLE S O F O PER ATIO N
General
P owe r sour ces
f
o r t
h
e P o
w
e
r M
ete
r ar e lin e ( M a in s) p o w e r or t h e
r ec h a r gea bl e b atte ry . If t
h
e b atte
ry i s b ein g u se d as α p o w e r
so ur ce, i t will r ece iv e α ch a
r g in g
c
u rr e n t any t im e t h e lin e v o l tage
i s coupl e d to t h e in st
rum
e
n
t a
nd
t
h e LINE swi tc h i s set to ON .
W h e n t h e lin e v o l tage
i
s di
sco nn ecte d , t h e b atte ry au to m at i ca lly
beco m es t h e p owe r so ur ce .
CA U TIO N
Α voltmeter o
r
osc
ill osco p e w hic h i s u se d to m ea-
s ur e t h e 24V
ou tp u t ac r oss t h e +12 V te rmina l s
mu st
have
α f l oat in gg r o und in p u t .
Th
e 12
V S
hun t R eg ul ato r esta bli s h es α r e f e r e n ce g r o und at t h e
h
a
lf vo l tage p o in t o f t h e 24 V Se ri es R eg ul ato r ou t pu t a nd t hu s
esta bli s h es t h e +12 a nd -12 Vd c ou t pu ts wit h r es p ect to g r o und .
Over V oltage
P
rotectio
n Circuit
Th
e
O
ve r V o l tage Pr otect i o n Circ ui t co n s i sts of cap ac i to r C39,
t hyri sto r Q20, r es i sto r s R 81 a nd R 82, a nd z e n e r di od e VR 6 . Th e
fun ct i o n of t his c ir c ui t i s to pre v e n t co mp o n e n t d a m age in t h e
p o w er supply du e to po w e r lin e tr a n s i e n ts, w r o ngv o l tages b e ing
a ppli e d to t h e P o w er M o dul e ( Α 6) or t h e s h ort in g of Q 13's co ll ecto r
to g ro und .
-

24
V Series R egulator
ΝΟΤΕ
T he e xp l a n at i o n of t h e 24V Se ri es R eg ul ato r i s
base d on t h e ass ump t i o n t h at ΤΡ 9 i s t h e r efe r e n ce
g ro undand t h e r eg ul ato r ou tp u t i s 24 V d c at
ΤΡΙΟ .
Α re f e r e n ce vo l tage o f -12 Vd c i s esta
bli
s
h e d
on
t h e
b
aseo f Q1 1 by
VR 4 . B eca u se Q10 a ndQ11a r e α
diff
e
r e n t i a l ampl i fi e r p a ir α
diff e r e n ce
in v
o
l
tage b etwee n t
h
eb ase
o f
Q11
and t h e b aseo f Q10,
h a lf t h e 24 V
ou
t pu t (r e
f
e
r
to t
h
e
n ote abo v e), pro du ces an er r or
ou t pu t on t
h
eco
ll
ecto
r
o
f Q
11 .
Thi
s e
rr o r v o l tage i s coupl e dto Q 16,
t h e r eg ul ato
r drive r,
a
nd fro m
t
h
e
r
e to
Q13, t h e se ri es r eg ul ato r . If,
f o r exa
mpl
e,
t h
e
ou t pu t vo l
tage s
udd e nly d ec r ease d to -23 v o l ts,
t h ec u r r e n t t
hr
oug
h
Q11
w
ould
in
c
r
ease
a nd t h e co ll ecto r vo l tage
wo uld b eco m e l ess
n
egati v e . C
urr
e
n
t
flo w t hro ugh Q16 inc r eases
a nd t h eco ll ecto r v o
l
tage goes
m
o
r e n
egat
iv e . Th e e mi tte r vo l tage
of Q13 f o ll o w s t h e co
ll
ecto
r vo l
tage
o f Q16 and appr oac h es -24 V .
A s t h e ou t pu t vo l tage beco
m
es
m o r e n egat iv e, t h e Q10 b ase vol-
tage also beco m es m
or
e
n egat ive un t il i t e qu a l s t h e b ase v o l tage
o
f
Q11 .
A
t t
hi
s
in sta n t, t h e o u t pu t v o l tage i s -24 Vd c a nd t h e
c
ir
c
ui t act i o n ( vo l tage c han ge) ceases .