A P P E N D I X B
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 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 | SENSITIVITY |
| |
-20oC | 5600 ohms / oC |
25oC | 439 ohms / oC |
50oC | 137 ohms / oC |
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
LDT-5525 33
