Thermistor And Thermistor Current Selection; Introduction; Thermistor Range; Table 5 - Tec Link Conditions - Newport 6000 Operation And Maintenance Manual

60 Chapter 5
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
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
100 µA
Tl Lim
R Lim
Open
-20°C
25°C
50°C
10 µA
Temperature Controller Operation
TEC has exceeded low temperature limit
TEC has exceeded R limits (high or low)
TEC module is open

Table 5 - TEC Link Conditions

Sensitivity
5600 ohms/°C
439 ohms/°C
137 ohms/°C
Temperature Range
-51 to 40°C
-10 to 100°C