LDT-5500B Series
47
A P P E N D I X
B
S
ENSING
C
URRENT
AND
T
HERMISTOR
S
ELECTION
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-5500B Series 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
In the LDT-5500B Series 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-5500B Series Temperature Controller.
Thermistor resistance and voltage are related through Ohms Law (V = I x R). The
LDT-5500B Series Temperature Controller supplies current to the thermistor,
either 10
A or 100
A. As the thermistor resistance changes, a changing voltage
TEMPERATURE
SENSITIVITY
-20
o
C
5600 ohms /
o
C
25
o
C
439 ohms /
o
C
50
o
C
137 ohms /
o
C
