HP Lab Series User Manual

4-32
CONSTA?fT
CURRENT
COMPARATOR
CIRCUIT
4-33
This
circuit
is
similar
in
appearance
and
operation
to
the
constant
voltage
comparat~'r
circuit.
It
consists
of
the
coarse
and fine
curr@nt
program-
ming
resistors
(Rl6.A
and Rl6B), and a
differential
amplifier
stage
(Q2
and
associated
components).
Like
transistor
QI
in
the
voltage input
circuit,
Q2
consists
of two
transistors,
having
matched
char•
acteristics,
that
are
housed
in
a
single
package.
4_.34
The
constant
current
comparator
circuit
con-
tinuously
compares a fixed reference
voltage
with
the
voltage
drop
across
the
current sampling
resis-
tor.,
If
a
difference
exists,
.thed:lfferential
amplUJ.er
produces
an
"er:rix"
voltage
which
is
proporti~nal
to
this
difference.
The remaining components
in
the
feedback
loop
(amplifiers
and
series.regulator)
func-
tion
to
maintain
the
drop
across
the.
currenUampling
resistor,
and
consequently
the
outP\it
current,
at
a
constant
value.
4-3
5
Stage
Q2B
is
connected
to
+S
through
imped-
ance
equalizing
resistor
R26-
ResistqrsR25
and:R28
are
used
to
zero
bias
the
input
stage,
offae~ing
minor
base
to
emitter
voltage
differenees
in
Q2.
Instantaneous
changes
ilioutpµt·current
on
the
pcJsi;..
tive
line
are
felt
at
the
current
summing
point
(ter-
minal
AS)
a~d.
hence,
the
base
of
Q2A.
Stage
Q2A
varies
its
conduction
in
accordance
with
the
polar-
ity
of
the
change
at
the
summing
point. The
change
in
Q2A'
s
conduction
also
varies
the
conduction
of
Q2B
due
to
the
coupling
effects
of
the
co~on
emit-
ter
resistor,
R22. The error voltage
is
taken
from
the
collector
Q2B
and
ultimately
varies
the
conduc-
tion
of
the
series
regulator.
4-36
Resistor
R20,
in
conjunction
with
R21
and
C3.
helps
stabilize
the
feedba-ck
loop.
Diode
CRS.
limits·
voltage
excilrsions
on
the
base
of Q2A.
Resistor
Rl9,
shunting
the
pullout
resistor,
serves
as
a
trimming
adjustment
for
the
programming
current
flowing
through
Rl6A and Rl6B.
4-37
VOLTAGE
CIAMP
CIRCUIT
4-38
The
voltage
clamp
circuit
keeps
the
constant
voltage
programming current
relatively
constant
when
the
power
supply
is
operating
in
the
constant
current
mode. This
is
accomplished
by
clamping
terminal
AS,
the
voltage
summing point,
to
a
fixed
bias
voltage.
During
constant
current
operation
the
constant
voltage
programming
resistor
is
a
shunt
load
across
the
output
terminals
of
the
power supply.
When
the
output
voltage
changes,
the
cmrent
through
this
resistor
also
tends
to
change,
Since
this
programming cUITent
flows through
the
current
sampling
resi~or,
it
is
erroneously
interpreted
as
a
load
change
by
the
current
comparator
circuit.
The clamp
circuit
eliminates
this
undesirable
effect
4-5
by maintaining
the
constant
voltage programming
current
constant.
.
4~39
The voltage
divider,
RSL
R52,
and
VRS,
back
biases
CR30
and
QlO
during constant
voltage
opera-
tion.
When
the
power supply goes
into
constant
cur-
rent
operation,
CR30
becomes forward
biased
by
the
collector
voltage of QlA
•.
This
results
in
conduction
of.
QlO
and
the
clamping
of
the summing
point
at
a
potential only
slightly
more negative
than
the
nor-
mal
constant
Voltage
potential. Clamping
this
volt-
age
at
approximately the same
potential
that
exists
in
constant
voltage operation,
results
in
a
constant
voltage
across,
and consequently a
constant
current
through,
the
current pullout
resister
(Rl2).
4-40
MIXER
AND
DRIVER
AMPLIFIERS
4-41
The
mixer
and
driver amplifiers
amplify
the
error
signal
from
the
constant voltage
or
constant
currmt
input
circuit
to
a level
sufficient
to
drive
the
series
regulator
transistor.
Transistor
Q3
re-
ceives
the
error voltage input
from
either
the
con-
stant
voltage
or
constant
current
circuit
via
the
OR-
gate
diode
(CR3
or
CR4)
that
is
conducting
at
the
time.
Diode
CR3
is
forward
biased,
and
CR4
re-
versed
biased,
during
constant
voltage
operation.
The
reverse
is
true
during
constant
current
operation.
4-42
The
RC
network, composed of
CS
and
R30,
is
an
equalizing
network which provides
for
high
fre-
quency
roll
off
in
the
loop gain
response
in
order
to
stabilize
the
feedback loop. Amplifier Q4
serves
as
the
driver element for the
series
regulator.
4-43
REFERENCE
CIRCUIT
4-44
The reference
circuit
is
a
feedback
power
sup-
ply
similar
to
the
main supply.
It
provides
stable
reference
Voltages
which
are
used throughout
the
unit.
The
reference
voltages
are
all
der~ved
from
smoothed
de
obtained
from
the
full
wave
rectifier
(CR22
and
CR23)
and
filter
capacitor
ClO.
The +6.2
and
-6.2
voltages,
which
are
used
in
the
constant
voltage
and
current input
circuits
for
comparison
purposes,
are
developed
across·ternperature
com-
pensated
Zener
diodes
VRl
and
VR2.
Resistor
R43
limits
the
current through
the
Zener
diodes
to
estab-
lish
an
optim~
bias
level.
4-45
The regulating circuit
consists
of
series
regu-
lating
transistor
Q9
and
error
amplifier
QS.
Output
voltage
changes
are
detected
by
QS
whose
base
is
connected
to
the
junction
of
a voltage
divider
(R41,
R42)
connected
directly
across
the
supply.
Any
error
signals
are
amplified and inverted
by
08
and
applied
to
the
base
of
series
transistor
Q9.
The
series
ele-
ment
then
alters
its
conduction
in
the
direction
and
by
the· amount
necessary
to
maintain
the
voltage
across
the
supply
constant.
Resistor R46,
the
emit-
•
•
•

HP Lab Series Specifications

General

Brand	HP
Model	Lab Series
Category	Power Supply
Language	English