Wahl 2500MX, Instruction Manual

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6.4 Cold Junction Compensation
The thermocouple output is a function of the difference in temperature between its
probe and end copper connection junction (cold junction). In case of Type K material,
the output is 40 microvolts per degree C. Since the microprocessor’s temperature con-
version tables contain reference thermocouple values to ice point and the actual con-
nection is at ambient temperature, the temperature difference must be accounted for
by a compensation circuit. The output of the thermocouple must be summed with the
cold junction temperature compensation in order to display the actual temperature to
the probe end of the thermocouple. This is accomplished by sensing the temperature
of the connector with a temperature sensitive diode, CR1, and summing its negative
temperature coefficient (-2.40 mVPC) at the analog-to-digHal convertor non-inverting
input. The initial ambient temperature (25°C) compensation voltage offset is deter-
mined by R12, R13, and U2. This posHive voltage of approximately 60 millivolts can
be adjusted by R9 to conform to the range of the actual ambient temperature encoun-
tered in final test. R11 will determine the diode temperature characteristic curve and is
set to bias the diode at 35 microamps which gives the desired coefficient.
6.5 Dual Slope A/D Converter
The A/D converter, U5, is a commercially available type. The theory of operation of
these devices will not be covered here.