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38
Protection for the current source
is
afforded
by
the
PTC
resistor
R401
and
zener diodes
V450,
V451
,
V452
and diode-connected
transistor
V403.
In
the event of
a
high voltage/current
on
the input terminals
R401
goes high
resistance.
To
prevent
part of
IRx
leaking
through
the protection diodes, the emitter of
V403
and
the
cathode
of zener diode
V450
are
connected
to
Vx
and
the electrodes
are
routed
back
to
the -Vin input of the
compensation
amplifier.
The
leakage current
through
the diodes
is
therefore balanced
out
in
the
same
way
as
compensation
is
achieved
for
the
ADC
input
current.
1.
4. 2. 6.
Diode
measu remen
ts
Diode measurements, and measurement
of
semiconddctor
junctions
are
performed
in
the
same
way
as
for
resistance
measurements
in
the
2kf2
range.
The
value displayed
is
the
voltage
in
forward
or
reverse direction
across
the
diode
in
the
2V
range.
In
the diode measuring
range,
the constant current derived
from
the
OQ0063
is
1mA
(see
previous
section).
1.
4.2.7.
Temperature measurements (°C)
When
the
°C
function
is
selected,
the constant current
IRx
is
routed
from
pin
2
of the
probe connector
BU4,
which
is
connected
via
the
probe
lead to
one end
of the
PT100
resistance
thermometer.
The
output of
the
Thomson
bridge
is
applied to the
ADC
via
function switch contact
°C2.
The
earth return to the
ADC
is
routed
to the
common
sources of
V607
and V607'.
This point
is
now
raised
above
earth
by
the
potential
drop
across the bridge
caused
by
the
measuring
current source;
i.e.
lOOmV
for
0°C.
The
Data Hold
switch
output
is
derived
from
pin
5
of
BU4.
TEMPERATURE
BRIDGE
Fig.
27.
Temperature
bridge
ST3379
81
1210
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