HP 435B Operating And Service Manual page 96

SERV
IC
E
S
HEE
T 4
PR
I
N
CI
PLE
S O
F
O
PER
ATIO
N
General
P
ower so urces
f
or
t
h e
P
o w e r M ete r ar e lin e ( Ma ins) po w er or
t
h e
rechargea ble
b
attery . If
t h e batte ry is being
u
sed as α
p
o wer
source,
i
t will rece ive α ch a r gin g c urre nt any
t
im e th e lin e
v
o ltage
i
s coupled to t he in st rum e n t a nd
t
he LINE switch is set to ON .
W
hen
t
he lin e
v
o ltage
i
s di sco nn ected, th e b attery autom atically
becomes t he p ower source.
CA
U
TIO
N
Α voltmeter o r osc illoscop e w hich
i
s u se d to
m
ea-
s ur e t he 24V ou tpu t ac ross th e +12 V termina ls
must have α
f
loat ing ground inp u t.
Th e 12 V S hunt
R
egulato r esta blishes α
r
efer ence gro und at th e
h a lf voltage poin t of
t
h e 24V Seri es
R
egulator output a nd
t
hu s
establishes t he +12 a nd -12 Vdc out puts with res p ect to ground.
Over Voltage
P
rotectio n
Circuit
Th e O v er Vo ltage Protecti o n Circui t co n s ists of ca p acitor C39,
thyristo r Q20, r es istors
R
81 a nd R 82, and z en er dio d e VR6 . The
function of
t
his c ir cuit
i
s to prev e nt compo n ent d am age in th e
p
ower supply due to p owe r lin e tr a n si en ts, wr on g
v
oltages b ein g
applied to the
P
ow er Module (Α6) or
t
h e sh ort in g of Q 13's collector
to ground .
- 24 V
Series Regulator
ΝΟΤΕ
The exp la n atio n of t he 24V Series Regulato r i s
based on th e assump t io n
t
h at ΤΡ9
i
s the refe ren ce
gro und a nd
t
he
r
egulato r
o
u tput
i
s 24 Vdc at
ΤΡΙΟ .
Α
r
eferen ce
v
ol tage o f -12 Vdc
i
s esta bli s hed on t h e b ase o f Q1 1 by
VR4.
B ecause Q10 and Q11 are α diff e r entia l ampli fier p air α
difference in v o l tage b
etween
t h e b ase o f Q11 and
t
h e
b
ase o f Q10,
half
t
he 24V ou tput (re f e r to t h e
n
ote a b ove), pro duces an err or
output on t h e co ll ecto r o fQ 11 . Thi s e rro r vo l tage
i
s coupl e d to Q 16,
the
r egulato r driver, a nd from t h e r e to Q13, th e series regul ato r. If,
f or exa mpl e, t h e o u tpu t
v
ol tage s uddenly decreased to -23
v
o lts,
the curren t t hr o ug h Q11 w ould in c r ease a nd
t
he co llector v o l tage
would becom e
l
ess n egativ e . C urr e n t flow
t
hr ough Q16 incr eases
and
t
he coll ector vo l tage goes m o r e n egat ive. Th e e mitter v oltage
of Q13
f
ollo ws the co ll ecto r vo l tage o f Q16 and approac hes -24 V.
A
s the o ut put
v ol tage beco m es mor e
n
egativ e, th e Q10 base vol-
tage also b eco mes m or e n egative until it equ a ls th e b ase
v
oltage
o f Q11 .
A
t t hi s instant, t he output
v
o ltage
i
s -24 Vd c a nd
t
h e
c ir c uit acti on (v oltage ch an ge) ceases .