Figure 2.4 - Example Thermistor Resistance Vs. Temperature - Newport LDC-3706 Series User Manual

The LDC-3706 instrument has the ability to select the temperature sensor current range based
on the resistance of the measurement and will automatically switch between the 10 µA and 100
µA modes. Thermistor auto range can be selected under the SENSOR submenu within the
TEC parameters menu.
To ensure proper thermistor current and thermistor selection, certain principles must be
considered:
• To ensure measurement accuracy, the voltage across the thermistor must not exceed
4.5 V.
• To improve control responsiveness and accuracy, the thermistor voltage variations that
result when the load temperature deviates from the setpoint must be as large as
possible. The importance of maximizing voltage variation is shown in Figure 2.4, which
shows resistance as a function of temperature for a thermistor. The values shown were
selected for simplicity in this example, and may not reflect real thermistor values.
In the example shown in Figure 2.4, the thermistor resistance is 25 kΩ at
one degree at 20 °C cause a resistance variation of about 2 kΩ. If using the 10 µA setting,
there is 20 mV of feedback to the control circuit. Using the 100 µA setting provides 200 mV of
feedback. The larger feedback signal means that the temperature is more precisely controlled.
Notice also that the lower slope of the curve at the higher temperatures results in a smaller
feedback signal. If higher temperatures are being controlled, it may be necessary to use a
thermistor with a different curve.

Figure 2.4 - Example Thermistor Resistance vs. Temperature

December 2019 32
20°C. Deviations of
LDC-3706 Series Laser Controller