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11
1.
3.2.6.
Diode meauserements
The measurement
of diodes
and semiconductor
junctions
is
performed
in
the
same
way
as
for resistance
measurements
in
the
2kfZ
range.
The
value displayed
is
the equivalent of the
voltage
measured
in
the
forward
or reverse direction across the
diode junction
in
the
2V
range;
i.e.
the constant current multiplied
by
the
diode
resistance.
For diode
measurements
the constant current derived
from
the
OQ0063
is
1mA
(see
previous
section).
1.
3.2.7.
Temperature measurements
For temperature measurements,
a
constant current
from
the
A401
current source flows
through the
resistance
element of
the
temperature probe
to
produce
a voltage
drop
across
it.
This
resistance
is
connected
as
one
of
the
ratio
arms
of
a
Thomson
bridge.
The
voltage
drop
is
an
indication of
the
temperature
of the
Pt-100 probe
(—
50°C
...
+
200°C) and
is
applied to the
ADC
for
measurement.
circuit
element
principles:
Thomson
bridge:
The
temperature probe
(resistance
thermometer)
is
included
in
one
arm
of
a
balanced
resistive
4-wire
bridge.
The
balancing
potentiometer
is
R708;
the
slider
connects the
output
of the bridge
directly to
the
ADC.
In this
configuration, the small
resistances
of the
connecting
leads
are counter-
acted;
they
are either
in
series
with the current source or
in
series
with
the
much
higher
resistance
arms
of the
bridge,
so
their effect
is
negligible.
3
1
BU4
C
.
AV'
=SMALL
RESISTANCES OF
PROBE LEADS
ST3364
Fig.
8.
Thomson
bridge
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