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Section 3
Principles of Operation
3.1 Introduction
Three factors must be taken into account when optimizing the operation of
a Model 300 Series Temperature Controller: selection of both the appropri-
ate temperature sensor and TE module heat sink, and the manner in which
they are mounted. Selecting the proper thermistor to cover a specific
temperature range of operation is a simple but important procedure. The
proper TE module must be selected to remove the heat dissipated by the
laser diode or other device. Finally, the arrangement of the TE module and
the heat sink, as well as the heat sink size, are crucial in maximizing the
heat transfer efficiency.
3.2 Thermistors
Model 300 Series Temperature Controllers are designed to operate using a
thermistor as one of the temperature sensing devices. Temperature is
displayed as a resistance (kΩ) corresponding to the temperature of the
thermistor. The actual temperature of the thermistor can be determined from
the manufacturers curves or tables listing temperature vs. resistance. Two
precision current sources, 10µA and 100µA, are used to generate a voltage
across the thermistor and it is this voltage that is read on the front display.
For the 10µA current source the maximum resistance that can be displayed
is 199.9 kΩ, while for the 100µA source the maximum reading is 19.99 kΩ.
The minimum resistance value that can accurately be read is 200 ohms
(100µA current source). These two current values allow a wide range of
temperatures to be controlled by the Model 300 Series Temperature
Controllers. Different ranges of temperatures can be adjusted for by selecting
the proper thermistor value. The following table illustrates this point:
Temperature vs. Thermistor Resistance
Thermistor value @ 25°C 1k 10k 100k
200 kΩ –75°C –37°C –11°C
100 kΩ –66°C –24°C 25°C
20 kΩ –40°C 8°C 65°C
10 kΩ –27°C 25°C 85°C
1 kΩ 25°C 93°C >150°C
100 Ω 100°C >150°C >150°C
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