60 Chapter 5 Temperature Controller Operation
Tl Lim
TEC has exceeded low temperature limit
R Lim
TEC has exceeded R limits (high or low)
Open
TEC module is open
Table 5 - TEC Link Conditions
See the section in the main Model 6000 manual of linking for a complete description
of the linking process.
5.4.4.1 Thermistor and Thermistor Current Selection
5.4.4.1.1 Introduction
Choosing the right sensing current depends on the range of temperature you want to
measure and the resolution you require at the highest measured temperature. To
correctly set the thermistor current you must understand how the thermistor and the
6000 interact and how temperature range and resolution values are inherent in the
nature of thermistors.
5.4.4.1.2 Thermistor Range
Thermistors can span a wide temperature range, but their practical range is limited by
their non-linear resistance properties. As the sensed temperature increases, the
resistance of the thermistor decreases significantly and the thermistor resistance
changes less for an equivalent temperature change. Consider the temperature and
sensitivity figures below.
Temperature
Sensitivity
-20°C 5600 ohms/°C
25°C 439 ohms/°C
50°C 137 ohms/°C
In the 6000 the practical upper temperature limit is the temperature at which the
thermistor becomes insensitive to temperature changes. The lower end of the
temperature range is limited by the maximum A/D input voltage of the 6000.
Thermistor resistance and voltage are related through Ohm's Law (V = I x R). The
6000 supplies current to the thermistor, either 10 µA or 100 µA , and as the
resistance changes a changing voltage signal is available to the thermistor inputs of
the 6000. The 6000 will over-range when the input voltage exceeds about 5 Volts.
Figure 31 graphically shows the lower temperature and upper voltage limits for a
typical 10 k Ohm thermistor. The practical temperature ranges for a typical 10 K
thermistor (a 10 K thermistor has a resistance of 10 k Ohms at 25°C) are given in the
table below.
Sensing Current
Temperature Range
10 µA -51 to 40°C
100 µA -10 to 100°C
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