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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