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Weir 413 - Page 6

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NOTE:
Before
connecting
the
power
supply
to
the
load
circuit,
refer
to
Safety
Regulations
64.3
(a)
6
(a),
64.3
(e)
6
(b)
and
64.4
(a)
7.
For
output
voltage
in
excess
of
42V,
available
from
the
423,
it is
recommended
that
the
supply
is
switched
off
before
connecting
to
the
load
(see
Safety
Regulations
64.5
(b)
(b)).
The
power
supplies
may
be
used
in
series
for
voltages
up
to
300V
d.c.
which
are
dangerous.
Care
must
be
taken
to
shield
the
output
terminals
(see
Safety
Regulations
64.4
(b)
(b).
All
supplies
in
the
range
rely
on
adequate
cooling
of
the
heatsink
and
internal
components
for
correct
operation.
DO
NOT
obstruct
the
airflow
by
placing
other
equipment
or
papers
on
top
of
the
units
when
they
are
operating.
TWIN
PACK
BENCH
VARIABLE
POWER
SU
PPLIES
Any
combination
of
units
from
the
following
list
of
Mini
400
Series
Power
Supplies
may
be
specified
in
Twin
Pack
dual
power
supply
configurations:-
413,
423,
413D, 423D,
400,
430
and
460.
Any
unit
may
have
the
'T'
option,
i.e.,
10-turn
variable
voltage
potentiometer
for
fine
resettable
linear
control.
3.0
TECHNICAL
DESCRIPTION
This
description
should
be
read
in
conjunction
with
Fig.
2
(The
Power
Supply
Circuit
Diagram).
The
a.c.
input
is
fused
by
FS1,
double-pole
switched
by
S7A
and
S7B
and
applied
to
the
transformer
primary
via
the
voltage
selector.
This
connects
the
primaries
in
series
or
parallel
and
adjusts
the
taps
to
suit
the
input
voltage.
The
secondaries
are
connected
in
series
or
parallel
by
S3A,
S4A,
S5A
and
S6A
to
provide
the
correct
unregulated
voltage
for
the
selected
range.
With
S6
or
S5
depressed,
secondary
3
is
connected
in
parallel
with
the
series
combination
of
secondaries
1
and
2
to
give
the
lowest
voltage,
highest
current
output
to
the
rectifier.
If
either
S3
or
S4
is
depressed
at
the
same
time,
the
combination
of
secondaries
1
and
2
is
disconnected;
as
this
action
automatically
lowers
the
current
limit,
secondary
3
can
supply
the
load.
With
S4
depressed,
secondaries
1
and
3
are
connected
in
series
for
the
middle
output
range;
depressing
S3
simultaneously
with
S4
has
no
effect
as
S4
has
disconnected
one
end
of
the
secondary
2.
With
S3
depressed,
secondaries
1,
2
and
3
are
connected
in
series
to
supply
the
highest
voltage
range.
The
a.c.
output
from
the
transformer
is
rectified
by
MR2,
MR3,
MR4
and
MRS
(BR1
on
413),
with
C2
acting
as
the
reservoir
capacitor.
VT10
and
VT11
are
the
series
regulator
transistors
with
R29
providing
current
sensing.
R29
and
R30
also
act
to
ensure
current
sharing
between
VT10
and
VT11.
The
output
is
decoupled
by
C8
(C13
on
centre
tapped
units)
with
MR13
acting
as
protection
against
a
reverse
voltage
being
applied
to
the
output
terminals.
Secondary
4
of
the
transformer
is
a
low
current
secondary
supplying
power
to
the
control
and
driver
circuits.
Its
output
is
rectified
by
MR1
and
filtered
by
C1.
Note
that
the
negative
rail
of
the
control
supply
is
common
to
the
positive
rail
of
the
output.
R21,
R22,
VT8
and
MR12
act
as
a
constant
current
source
supplying
current
to
the
main
reference
rail.
This
rail
is
stabilised
by
the
amplifier
VT6
and
VT7,
with
MR11
acting
as
feedback
round
this
amplifier.
The
regulated
rail
is,
therefore,
5-6V
plus
the
base
emitter
forward
voltage
of
VT6
and
appears
at
the
emitter
of
VT7.
R19
sets
the
standing
current
through
MR11.
The
excess
current
from
the
collector
of
V
T7
is
used
as
the
base
drive
for
the
drive
transistor
V
T9.
R2
and
RV2
provide
a
preset
adjustable
voltage
for
the
voltage
amplifier
on
the
variable
ranges.
R5,
R6
and
R7
convert
this
into
a
reference
current.
Depressing
S5
connects
R5,
R6
and
R7
in
series
to
provide
the
lowest
reference
current
and,
hence,
the
lowest
output
voltage
range;
as
S5B
has
disconnected
S4B
and
S3B,
depressing
these
simultaneously
does
not
affect
the
output
voltage.
Depressing
S4
connects
R5
and
R6
in
series
to
give
the
middle
range;
again,
S4B
disconnects
S3B
so
that
depressing
S3B
does
not
affect
the
output
voltage.
With
S3
depressed,
R5
provides
the
reference
current.
This
switching
arrangement
ensures
that
the
output
is
always
the
lowest
of
the
selected
voltages.
The
reference
circuit
is
balanced
against
the
feedback
current
drawn
by
RV6
through
MR7
and,
as
the
base
current
of
V
Tl
is
negligible,
these
are
equal.
Note
that,
as
the
bases
of
VT1
and
VT6
are
both
one
forward-biased
diode
voltage
drop
above
the
positive
output,
their
emitter
base
voltage
drops
cancel
and
MR11
determines
the
voltage
across
R5,
R6
and
R7.
Similarly,
the
forward
voltage
drop
of
MR"
cancels
the
forward
voltage
drop
of
V
T1
base,
so
that
the
voltage
across
RV6
is
equal
to
the
output
voltage.
Temperature
variations
in
V
Tl,
VT6
and
MR7,
therefore,
cancel.