Tec Controller Operation; Thermistor And Thermistor Current Selection - Newport 6100 User Manual

Principles of Operation
6.3

TEC Controller Operation

6.3.1

Thermistor and Thermistor Current Selection

6.3.1.1 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 6100 interact.
6.3.1.2 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
-20C
25C
50C
In the 6100 the practical upper temperature limit is the temperature at which
the thermistor becomes insensitive to temperature changes. The maximum
ADC input voltage of the 6100 limits the lower end of the temperature range.
Thermistor resistance and voltage are related through Ohm's Law (V = I x R).
The 6100 supplies bias current to the thermistor, and as the resistance
changes, a changing voltage signal is available to the thermistor inputs of the
6100. The 6100 is capable of supplying the following bias currents to the
thermistor: 10 mA (100 Ohm), 1 mA (1 kOhm), 100 A (10 kOhm), 10 A
(100 kOhm), 1 A (1 MOhm). The 6100 will over-range when the input
voltage exceeds about 2.5 Volts. Figure 34 graphically shows the lower
temperature and upper voltage limits for a typical 10 kOhm 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
10 A
100 A
Sensitivity
5600 ohms/C
439 ohms/C
137 ohms/C
Temperature Range
-35 to 40C
+8 to 80C
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