Newport 350B, User Manual, Page: 50 / 66

Maintenance and Service 49
7
Principles of Operation
7.1 Introduction
Three factors must be taken into account when optimizing the operation of a Model 350B
Temperature Controller: selection of both the appropriate 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.
7.2 Thermistor
Model 350B Temperature Controller are designed to operate using a thermistor as the
temperature sensing device. 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 200.0 kΩ,
while for the 100µA source the maximum reading is 20.0 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 350B Temperature Controller. 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 kΩ
   
100°C

150°C

150°C
100 Ω
   
100°C

150°C

150°C