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Farnell TOPS 3D - Page 13

Farnell TOPS 3D
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Display section
The
voltmeter and current meter are based
on
digital voltmeters
each
having
an
f.s.d.
of
399.9mV.
In
the case
of
current readings
a
voltage
proportional
to
the
current
flowing
is
detected
across
a
series
resistor.
Since
the
voltmeter
and
current
meter
are
identical
only
the
volt
meter
will
be
described.
Rectified
ac
is
regulated
by
IC1
to
provide
a
nominal
5V
supply
for
the
display
and
driver.
Separate
analogue
and
digital
star
grounds
are
provided
to
prevent
current
loops
affecting
the
reading.
The
chip
oscillator
controls
the
sampling
and
multiplex
frequency.
The
frequency
of
the
oscillator
is
determined
by
R24
and
C4.
The
decimal
point
is
also
multiplexed
and
in
order
to
synchronise
this
with
the
appropriate
digits
the
base
of
VT5
is
fed
from
the
collector
of
the
corresponding
drive
transistor
VT1-VT4.
(In
the
case
of
the
voltmeter,
point
'E'
is
linked to
'A',
and
for
the
current
meter
point
'D'
to
point
A).
The
analogue
reference
voltage
at
pin
18
of
IC2
is
nominally
2
volts.
This
is
derived
by
potting
down
the
voltage
of
Z1
by
R19,
P1
and
R12.
Pin
14
is
the
feedback
sensing
function
of
1C2
and
the
feedback
voltage
from
pin
17
and
16
is
divided
by
5
using
R16
and
R15.
C3
provides
smoothing
of
the switched waveform which
is
then compared with the input voltage
on
pin
11.Zero
offset
current
is
provided from
the
reference
by
R20,
P2
and
R18.
In the case
of
the
current meter,
in
order
to
minimise digit flicker
ing
an
increased
time
constant
(R28
and
C8)
can
be
switched
in
by
the damping push button.
9

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