Fluke 78 - Continuity; Temperature Function

78
Service Manual
2-6
2-9. Continuity
The continuity function is the 400 J range of the ohms function with no active filtering
of the input signal. A comparator is used to turn on the beeper when the input drops
below about 50 J.
2-10. Temperature Function
The thermocouple voltage is measured by the a/d converter through R2, S1-2 and S1-3,
and the SENS input of U1-93. The voltage is the result of any temperature difference
between the thermocouple and the reference junction (at the meter). U3 produces an
output voltage of 10 mV per degree C, proportional to the meter temperature (reference
junction), which is divided by R15 and R16, and measured by the a/d converter. The
thermocouple and reference junction measurements are combined, resulting in the
temperature of the thermocouple.
At the beginning of each measurement cycle, an open thermocouple detect voltage
(OTD, U1-89) is applied to the U1 SENS input through resistor R12. If the thermocouple
wire is broken (open) or not installed, the SENS is driven high enough to be measured as
an overload. The display then flashes the meter (reference junction) temperature.
OHMS
VOLTAGE
SOURCE
A/D
INTEGRATE
REFERENCE
LOW
HIGH
R8
+
–
S1
R1+RT1
ACTIVE
FILTER
A/D
INTEGRATE
UNKNOWN
R
HIGH
LOW
I
X
VR
X
VR
X
VR
REF
=
I
X •R X
I X •R REF
R
REF
R
X
=
+
–
R2
R
X
UNKNOWN
RESISTOR
VR
REF
6
7
Z1
1.0001k
REFERENCE
RESISTOR
S1
Z1
9.996M
2
1
5
6
9
6
zr03f.eps
Figure 2-3. 400 Ohm Range Simplified Schematic