General Guidelines For Sensor Selection And Safety Limits; Sensor Options - ILX Lightwave LDT-5900 Series User Manual

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C H A P T E R

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-5900 Series Temperature Controllers can measure temperature through
a variety of sensor options; thermistors, IC sensors (IC-I, IC-V) or RTDs.
Thermistor - When a thermistor sensor is selected, the LDT-5900 measures
temperature based on using a negative temperature coefficient (NTC) thermistor.
An NTC thermistor is a device whose resistance decreases as its temperature
increases. The controller provides a sense current (100 A or 10 A) through the
thermistor which results in a voltage across the thermistor. This voltage is used as
a feedback signal by the LDT-5900 digital control loop to maintain a constant
temperature. The thermistor should be connected across the RT+ and RT- pins,
as well as across the RT+_SENSE and RT-SENSE pins.
In constant temperature mode, the quantity that is maintained constant by the
module is the sensor resistance. In constant temperature mode (T), the LDT-5900
converts the temperature setpoint to a thermistor resistance setpoint using user
defined constants.
The Steinhart-Hart equation is used to convert a temperature to a resistance for
thermistor sensors. The equation describes the nonlinear resistance versus
temperature characteristics of typical thermistors. Calibrating a thermistor
consists of measuring its resistance at various temperatures, and fitting these
measured data to the Steinhart-Hart equation. The resulting coefficients C1, C2,
and C3 effectively describe the thermistor. More information about the Steinhart-
Hart equation is contained in ILX Application Note #4. Contact ILX Customer
Service (see page xii for contact information) or go to the ILX website
(www.ilxlightwave.com).
To measure the precise temperature of a load, you must use a calibrated sensor.
For example, when using a thermistor, enter its Steinhart-Hart coefficients C1, C2,
and C3 in the TEC page 3 menu (see Basic TEC Operation Instructions). If the
exact temperature is not crucial (within ±1.5
thermistor, use the default constants provided by the LDT-5900. However, the
LDT-5900 temperature accuracy specifications apply only to a calibrated
thermistor. For more information about calibrating your thermistor, go to
www.ilxlightwave.com/support/index.html and click Technical Solutions.
Thermistor resistance changes with temperature. The LDT-5900 supplies
constant current, either 10 A or 100 A, through the thermistor so that a
temperature change results in a voltage change across the thermistor. This
voltage change is sensed by the instrument and fed back to the control loop. The
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