ILX Lightwave LDC-3700 Series User Manual

Appendix B
Sensing Current and Thermistor Selection
Introduction
Choosing the right sensing current depends on the range of temperature you want to measure and the resolution
you require at the highest measured temperature. To correctly set the
SENSOR SELECT switch you must
understand how the thermistor and the LDC-3700 Series Laser Diode Controller interact, and how temperature
range and resolution values are inherent in the nature
of thermistors.
Thermistor Range
Thermistors can span a wide temperature range, but their practical range is limited by their non-linear resistance
properties. As the sensed temperature increases, the resistance of the thermistor decreases ~ig~cantly and the
thermistor resistance changes less for an equivalent temperature change
-the thermistor becomes less sensitive.
Consider the temperature and sensitivity figures in Table
B. 1 below.
Thermistor Sensitivity at Various Temperatures
Tem~erature Sensitiviq
-20°C 5600 ohmdoc
25°C
439 oW°C
50°C 137 ohms/"C
Table
B.l Thermistor Sensitivity
In the LDC-3700 Series Laser Diode Controller, the practical upper temperature limit is the temperature at which
the thermistor becomes insensitive to temperature changes. The lower end of the temperature range is limited by
the maximum
AD input voltage of the LDC-3700 Series Laser Diode Controller. Thermistor resistance and
voltage are related through Ohms Law
(V = I x R). The LDC-3700 Series Laser Diode Controller supplies current
to the thermistor, either 10
pA or 100 pA, and as the resistance changes a changing voltage signal is available to
the thermistor inputs
of the LDC-3700 Series Laser Diode Controller's TEC VO connector. The LDC-3700 Series
Laser Diode Controller's
AID will over-range when the input voltage exceeds about 4.5 volts. Figure B.l
graphically shows the lower temperature and upper voltage limits for a typical 10
kR thennistor. The practical
temperature ranges for a typical 10 K thermistor (a 10
K thermistor has a resistance of 10 k(l at 25 "C) with the
LDC-3700 Series Laser Diode Controller are given in Table B.2, below. These temperature ranges may vary from
thermistor to thermistor, even though booth thermistors are nominally
10 K. This is due to manufacturing
tolerances in the thermistor, and is compensated for by determining C1, C2, and C3 (calibrating the thermistor).
The practical temperature ranges for a 10K thennistor are also shown
as solid bars at the bottom of Figure B. 1.
10K Thermistor Practical Temoerature Range
I
Sensing Current Temwrature Range
10
pA -30 to 30 "C
100
pA 10 to 70 OC
Table B.2 10K Thermistor Temperature Ranges
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ILX Lightwave LDC-3700 Series Specifications

General

Brand	ILX Lightwave
Model	LDC-3700 Series
Category	Controller
Language	English