Farnell L30A - Page 5

SECTION
II
OPERATING
INSTRUCTIONS
Care should be taken to
ensure
that the input
selector
on the
rear of
the unit is
set to
match
the
mains supply from
which the
supply is to be operated.
The
mains lead is
wired
as
follows
:
—
BROWN Mains
Line.
BLUE
Mains
Neutral.
GREEN/YELLOW
Earth.
NORMAL
CONSTANT
VOLTAGE
OPERATION
Before
connection to the
mains supply is made,
the
output
"ON/OFF"
switches,
should be set to
"OFF". The link on the
bottom
of
the unit should be in
the
"CV" position.
With
the mains supply
connected, the mains
"ON/OFF" switch
is set to "ON"
and the
mains
indicator
lamp should light.
With
the meter switch set to
"VOLTS", the
required
output
voltage is selected by
adjusting the
"COARSE"
and "FINE"
voltage
controls
on the front panel
and the current limit
control
set to
maximum' (fully
clockwise). Unless use
of
the current
limiting is to be
made,
the load is
now
connected and the output
"ON/OFF"
switch
set to "ON".
Output
current can
he monitored
by
setting
the meter
switch to
"CURRENT".
CURRENT
LIMIT
SETTING
If
a
certain maximum
current must not be exceeded
or a
roughly
constant
current is
required,
use
may
he made of the
current
limiting facility,
maximum current being set by the
front
panel
control marked
"CURRENT LIMIT".
(1)
APPROXIMATE
CONSTANT
CURRENT
OPERATION
With the
mains
supply
connected
to
the unit, mains and output
switches
set
to "ON", the
meter
switch set to "CURRENT" and
the
output voltage controls set
to
maximum,
the
positive output
terminal is
linked to the negative
output terminal and the current
limit
control set to give the
required
current.
On
removal of
the
link and
connection to the load,
the unit
will give
a roughly
constant
current over a
voltage
range
from
zero to the maximum
specified output
voltage
depending
on
the bad
resistance.
(2)
CURRENT
CEILING
OPERATION
If a
certain
maximum
current is not
to
be
exceeded
then
the
setting up
procedure
is as
follows
:
—
With
the mains supply
connected
to the unit,
mains and
output
switches to
"ON",
the
meter
switch set
to
"CURRENT"
and the
voltage
controls set to
the
required value,
a
variable
load
is
connected to
the unit and is
set to
give the
required
maximum
current.
The current
limit
control is then
adjusted
until the
current
just
starts to fall. The
unit
will now
prevent
more
current
than
this
value
being
drawn
from it. It
is
advised
that
this
current
"ceiling"
is
set to
approximately
10%
in excess of the
expected
maximum to
prevent
any
modification
to the
voltage
regulation
characteristic. The
variable
load is now
disconnected
and
the unit is ready
for
use.
ACCURATE
CONSTANT
CURRENT
OPERATION
(See Fig.
1}
If
a fixed resistor Rs
is
connected
between
the
positive
output
terminal and
the
"I const"
or
"—
F/B"
terminal, the
link on the
bottom
of the unit set to
CI,
and the
load Rl
between the "I
const"
or
"—F/B"
terminal and the
"O/P
-
ve"
terminal,
then
the unit
will keep
the voltage
across
Rs
constant.
Thus a
constant current
will flow in Rs.
Since Rl is
in series
with
Rs, the same
current
will flow in
Kt..
The value of
Rs
is given by
:—
See
Errata
and
V
Addenda
-
Page 15
Rs
=
Rl max.
I out
Where V is the
maximum
specified output
voltage
of the unit.
Rs must
have a
power
rating
well
in excess of
the power
it
dissipates
in order
that
the change
in resistance
due to self heating
is minimised.
The
power dissipated
in Rs is
given
by
:
—
P(Rs)
=
I out-
Rs.
Rs is then
connected
between
the
"0/P
+
"
and "I const"
terminals.
The "I const"
or
"—F/B"
terminal is shorted to
the
"O/P—"
terminal and
the
"VOLTAGE
ADJUST'
controls set
until
the
meter reads the
required
output
current. The "short"
is
then
removed
and the load
connected in its place.
The
output
current
will be constant
with the same
specification as the constant
voltage
specification
with the
load
varying from zero to
Rl
max.
note i
—
The front
panel
"CURRENT
LIMIT" control
should
be set
to
maximum
for this
mode of operation.
3