Farnell E30 - Operating Instructions; Circuit Description

Installation
The units are normally supplied set for use with a.c. mains supplies
of 24OV nominal. On reguest at the time of ordering, units can be
set for 220v or llOV noninal operation. Units are set to ll0, 220
or 240V input by neans of a simple tap change on the trans former,
which is labelled to indicate the appropri ate conne ctions.
Check that the unit supplied is set to the correct noninal input.
Unless a label on the unit states 220 or l10v it may be assumed that
the unit is set for 240V a.c. input.
The mains lead is wired as follows :
Brown
Blue
Green/yellow
Operating instructions
OPERATING INSTRUCTION
Depress the button
will illuminate.
Observing the correct colour coing and supply requirements connect
the mains lead to the supply .
Mains live
Mains neutral
Earth
Select the required VA range using the centre push button marked
RAN GE'
Depress (in) for 0-30v and release (out) for 0-15v.
Overlbad protection
INPUT ON/OFF. The mains neon on the front panel
Select 'vOLTS' on the 'METER' push button and adjust the coarse and
fine controls until the required output is shown on the upper scale
(volts) of the meter.
Connections to the load are taken from the front panel screw term
inals which will accept 4mm plugs, spade terminals or the bared ends
of the connecting wires.
By depressing the 'METER' button it is possible to read the amount
of current being supplied to the load.
Fuses
The current limiting facility provides automatic protection in the
event of accidental short circuits or overloads. This occurs at
approxinately ll0% of the maximum current on each range. The circuit
limits the current that may be drawn from the unit.
With
progressive
overload the voltage at the output falls while the current remains
On removal of overload the output will reset automatically to its
original voltage.
The mains input and regulated output circuits are each protected by
fuses accessible on the back panel of the unit.
Before removing either fuse ensure the unit is dis connected
No te :
from the mains supp ly
General
The
output
from
the
twO
main
secondaries
of
the
mains
transformer
MT1
are
switched
by
Sw2
a
and
sw2b in
series
or
parallel for
0-30 or
O-15
volt
operation
respective
ly.
This
a.c.
voltage
is
rectified
by
a
bridge and
smoothed
by
a
reservoir
capacitor
to
provide
an
un
stabilised d.c.
supply. The
positive
of
the
unstabilised
supply is
fed
to
the
collector of
VT6
(and
vT7)
through
Rl6
(and
Rl8) to
the
positive output terminal. The
negative
of the
unstabilised
supply
is
fed
through F2
to
the
negative output.
An
auxiliary
supply for
Stabilisation circuit
CIRCUIT DESCRIPTION
Current
is
taken
from the
auxiliary supply
across
C1 and
fed
through
R1 to zl which is the first zener iode and provides rough stabili
sation.
The voltage across Zl is further stabilised by ~2, D3 and
Z3 which
are
fed
by R2.
These provide the stabilised voltages for
the amplifier, namely +10v (nominal ) -0.7V (nominal) and -5.8v
(nominal) , which are referred to the positive output terminal via
the
feedback
connection.
Current from the +l0V line is passed thro
ugh R3 to z4 which provides a stable reference voltage.
The reference voltage passes current down the potential divider
formed by R4 and Tl and Pl and P2. (R7 and T2 shunt Pl and P2
through SW2c only in the O-15V mode) The other end of the potential
divider is connected via the negative feedback lead to the negative
output. The differential amplifier formed by vTl and vT3 senses any
Thus, assuming the voltage on the output tries to rise, the base of
VTl will become negative and VTl will tend to turn off. This reduces
the current through R5 and the base/emitter junction of VT2 decreas
ing the current through VT2 collector This decreases the current
drive into VT5 base through R8, and hence the current drive into vT6
base. Therefore the current into the output load and the voltage
across it decreases. Alternatively if the voltage on the output
tends to go down, the reverse process increases the output current
and therefore the output voltage.
The sum effect is to keep the voltage at a value fixed by the value
of Pl and P2 against all load variations. 24 provides a stable ref
erence against mains supply variations.
Overload protection
The current through Rl6 gives a voltage proportional to output cur
rent. This is sensed by R9, and R10. RlO provides a negative
bias on the base of VT4 which is turned off in normal use. As the
output
current
rises
the
voltage
at the
base
of VT4
rises
until
it
is
positive
enough
with
respect
to VT4
emitter, which
is
set at
-0.7V,
to turn
vT4
on. Drive
current
to the
base
of VT5
is
then
shunted through VT4 collector.
This
limits
the
maximum
current
available at the output.
The
value
of
current
available
is
preset
by
T3.
On
removal
of
the
overload
VT4
is
switched
off
and
the
out
Rll is switched to
5
approxinately constant.
the
stabilising
circuit is
obtained from
the
other
secondary winding,
which
is
rectified by
Dl
and D2
and
smoothed by
Cl.
voltage difference between the junction of Tl and Pl, and the posi
tive output.
put voltage returns to its previous level.