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Careful thermistor value selection therefore enables control over a wide
temperature range.
The temperature resolution and thermistor sensitivity must also be taken
into account. Thermistors achieve their highest sensitivity at the lower
end of their temperature range. Therefore, the lower the absolute tempera-
ture to be maintained, the lower the resistance value the thermistor should
be. For example, to operate at 0°C a 10k ohm thermistor would be the best
selection while at 100°C a 1k ohm thermistor would be a better selection.
The rule of thumb is to operate the thermistor near the lower end of its
temperature range and use the 100µA current bias. Please contact
Newport’s applications engineers if you have any questions regarding the
selection of the proper thermistor for your application.
3.3 Working With IC Sensors
Two IC temperature sensors may be used with the Model 300 Series Tem-
perature Controllers, the AD590 Series and the LM335 Series. Both come in
several different package types having various accuracy and stability
characteristics. These temperature sensors are easy to use and give a
temperature reading directly in °C on the Model 300 Series display.
3.3.1 AD590 SERIES IC SENSORS
The AD590 Series devices are two-terminal IC temperature sensing devices,
which produce an output current directly proportional to the absolute
temperature at 1µA/°K. The AD590 Series sensor is particularly useful in
remote sensing applications where long cable lengths are used, due to its
high impedance current output. A linear output is displayed in °C on the
Model 300 Series Temperature Controller, which makes it ideally suited for
most temperature sensing applications. The recommended upper operat-
ing limit for AD590 Series devices is 150°C.
3.3.2 LM335 SERIES IC SENSORS
The LM335 Series devices are similar to the AD590 in that they are two
terminal devices and their output is directly proportional to absolute
temperature. Operating like a two terminal zener diode, the LM335 Series
temperature sensors have a breakdown voltage directly proportional to
absolute temperature at +10mV/°K. When using the LM335 Series sensors,
voltage drops resulting from using long cables will introduce inaccuracies
in the temperature display measurement.
3.4 Using Thermo-Electric Modules
Thermo-electric (TE) modules are semiconductor devices that act as heat
pumps when a voltage is applied to them. This effect is called Peltier
cooling or heating. The direction of the current flow determines whether
the TE module is cooling or heating a device such as a laser diode or IR
detector. A TE module consists of a matrix of thermoelectric couples made
of p-type and n-type semiconductor material. A TE module can be fabri-
cated with as few as one couple or with as many as several hundred
couples sandwiched between two ceramic plates. The ceramic plates form
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