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General Guidelines for Sensor Selection and Safety Limits
This section presents some guidelines to assist in selecting the optimal settings for your
application.
Sensor Options
The LDT-5412B Thermoelectric Temperature Controller can measure resistance of a thermistor
and display the output either in resistance or a calculated temperature. When the rear panel
switch is set to 100 µA or 10 µA the LDT-5412B will display resistance. When the rear panel
switch is set to °C LINEARIZED FOR 10k the LDT-5412B will display a calculated
temperature.
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, therefore the thermistor becomes less sensitive. Consider the temperature
and sensitivity figures in Table 2.2.
In the LDT-5412B 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-5412B. Thermistor resistance and
voltage are related through Ohms Law (V = I x R). The LDT-5412B supplies current to the
thermistor, either 10 µA or 100 µA, and as the resistance changes, a changing voltage signal is
available to the thermistor inputs of the LDT-5412B. The LDT-5412B will over-range when the
input voltage exceeds about 2.0 Volts. The maximum temperature ranges for a typical 10 kΩ
thermistor (a 10 kΩ thermistor has a resistance of 10 kΩ ohms at 25 °C) are given in Table 2.3
below.
Selecting and Using Thermistors
The type of thermistor you choose will depend primarily on the operating temperature range.
From Figure 2.3 you can also see that 10 kΩ thermistors are generally a good choice for most
laser diode applications where high stability is required at near room temperatures. Similarly, 5
kΩ thermistors are often a good choice for detector cooling applications where you want to
operate at temperatures from -30 °C to room temperature. Much higher or lower temperature
ranges can be controlled through the use of higher or lower resistance thermistors. For more
March 2013
Table 2.2 Thermistor Sensitivity Values
Temperature
-20 °C
25 °C
50 °C
Table 2.3 Temperature Ranges (10 kΩ Thermistor)
Temperature
10 µA
100 µA
10
Sensitivity
5600 Ω/°C
439 Ω/°C
137 Ω/°C
Sensitivity
-37 °C to over +60 °C
+8 °C to over +100 °C
LDT-5412B
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