Sensing Current And Thermistor Selection; Thermistor Range; Selecting The Sensing Current - ILX Lightwave LDT-5412 User Manual

2
C H A P T E R

Sensing Current and Thermistor Selection

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 current switch you must understand how the thermistor and
the LDT-5412 interact and how temperature range and resolution values are
inherent in the nature of thermistors.

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 -the
thermistor becomes less sensitive. Consider the temperature and sensitivity
figures in Table 2.3.
Table 2.3 Thermistor Sensitivity Values
-20C
25 C
50 C
In the LDT-5412 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 input voltage of the 5412.
Thermistor resistance and voltage are related through Ohms Law (V = I x R). The
LDT-5412 supplies current to the thermistor, either 10 uA or 100 uA, and as the
resistance changes a changing voltage signal is available to the thermistor inputs
of the 5412. The LDT-5412 will over-range when the input voltage exceeds about
2.0 Volts. Figure 2.5 graphically shows the lower temperature and upper voltage
limits for a typical 10 Kohm thermistor. The maximum 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.
Table 2.4 Temperature Ranges (10 K Thermistor)
10 A
100 A

Selecting the Sensing Current

To select the current setting for a typical 10 K thermistor, determine the lowest
temperature you will need to sample and set the switch according to the range
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 LDT-5412
O P E R A T I O N
Sensing Current and Thermistor Selection
Temperature
Sensing Current
-37 to over +60 C
+8 to over +100 C
Sensitivity
5600 ohms/C
439 ohms/C
137 ohms/C
Temperature Range