Appendix B Sensing Current And Thermistor Selection; Thermistor Range - ILX Lightwave Temperature Controller LDT-5525 User Manual

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 SENSOR SELECT switch you must understand how the
thermistor and the LDT-5525 Temperature Controller 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. At high temperatures, the
thermistor resistance changes less for an equivalent temperature change at lower
temperatures (the thermistor becomes less sensitive). Consider the temperature
and sensitivity figures in Table B.1 below for a 10 K thermistor.
Table B.1 Thermistor Sensitivity
TEMPERATURE
o
-20 C
o
25 C
o
50 C
In the LDT-5525 Temperature Controller, 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 LDT-5525 Temperature Controller. Thermistor resistance and
voltage are related through Ohms Law (V = I x R). The LDT-5525 Temperature
Controller supplies current to the thermistor, either 10 µA or 100 µA. As the
thermistor resistance changes, a changing voltage signal is available to the
S
ENSING
T
HERMISTOR
SENSITIVITY
o
5600 ohms / C
o
439 ohms / C
o
137 ohms / C
A P P E N D I X
C
URRENT AND
S
ELECTION
LDT-5525
B
33