OPERATIONS
Operating the TE Controller from the Front Panel
7/01 LDC-3700 Series 25
CHAPTER 2
Temperature Controller Sensor Selection
Because we have selected a 10 KΩ thermistor as our temperature sensing element and we have
chosen 35 °C as our set point temperature, we need to set the SENSOR SELECT switch to THERM
and the THERM RANGE switch to 100 µA.
Your LDC−3700B’s Temperature Controller operates in a closed loop fashion with a temperature
sensor being the feedback element. For the controller to operate correctly, the type of sensor needs
to be selected on the rear panel of the instrument. The SENSOR SELECT switch allows you to
select a temperature sensor type and, in the case of a thermistor sensor, the source current level.
For thermistors, LM335 and AD590 temperature sensors, use the lower switch to select the
appropriate sensor type. When the sensor switch is changed, a sensor position code will be
indicated on the TEC display.
Note: Note, for RTD sensors, the RTD Temperature Sensor Converter option is required,
ILX Model 599.
Table 2.2 shows the SENSOR SELECT positions and corresponding position code.
Thermistor resistance changes with temperature. A constant current source integrated into the
controller supplies current through the thermistor so that a temperature change results in a voltage
change across the thermistor. This voltage change is sensed by the controller and fed back to the
control loop. If you have the thermistor sensor selected, then select an appropriate current range
with the upper switch. The 10 µA and 100 µA designations are current levels supplied by the
controller. The selected supply current depends on the thermistor operating temperature range and
the required temperature resolution. A general rule of thumb for a 10 KΩ thermistor is to use the 10
µA range for temperatures between −30 °C and +30 °C , and the 100 µA range for temperatures
between +10 to +70 °C For a more complete description of thermistor selection see our Application
Note #2 titled, "Selecting and Using Thermistors for Temperature Control".
Setting Temperature Sensor Constants
After the sensor type is selected, it will be necessary to enter the sensor calibration constants into
the controller for accurate conversion to actual temperature. These constants are used in a
temperature conversion equation to calibrate the TEC and are supplied with your thermistor’s data
sheet. The Steinhart−Hart equation is used to derive temperature from the non−linear resistance of
an NTC (Negative Temperature Coefficient) thermistor. Linear equations are used to convert sensor
voltage or current signals to actual temperature when an LM335 or AD590 sensor is used. (See the
TEC Specifications in Chapter 1 for these equations). When CONST is selected in the TEC
Table 2.2 SENSOR SELECT Switch Positions
Switch Position Code
100 µA−−01
10 µA−−02
LM335 −−03
AD590 −−04
THERM RANGE
SENSOR SELECT
THERM
N / A
LM335
100 A
µ
10 A
µ
AD590
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