Chapter 5 Temperature Controller Operation 57
are primarily intended for users who know a sensor set point in “sensor” units, not in
º
C. I
TE
current is also displayed in these modes.
5.4.3.2.3 Constant Current Mode (Const I
TE
)
Unlike the modes above, the Const I
TE
mode allows the operator to explicitly set the
amount and direction of current flow through the TEC, using “
Is=
” and “
Ite=
”
variables. If a sensor has been selected, the TEC temperature will be displayed.
Although temperature is not a factor in the amount or direction or current flow, the
high and low temperature limits are observed, and will shutdown the output if
exceeded, in Const I
TE
mode if a sensor is selected. For no temperature limits, set the
sensor type to “None.” Use caution when limits are not active, as the temperature
may exceed your TEC or Laser’s thermal limits.
5.4.3.2.4 Effects of Calibration on TEC modes
On startup, the TEC performs an auto-calibration to eliminate most of the error in
ADC and DAC values. After this auto-calibration, each sensor type supported by the
module has an offset calibration, while the I
TE
set point and read back has a two
point calibration. These calibration constants are then used to calibration a set point
or read back value. This includes “cross-mode” values, such as displaying actual
current while in constant temperature mode. While the current set point calibration
has no effect in Const T mode, the read back calibration is used to more accurately
display the actual current.
5.4.3.3 Gain
The Gain function controls two parameters of the hybrid PI control loop;
proportional gain and integration time.
When the actual temperature and the set point are different, an error voltage is
generated. This error voltage is directly related to the difference in the actual and set
point temperatures. The error voltage is then amplified by the proportional gain.
This amplified error voltage controls the amount of current driven through the TEC.
The higher the gain, the more current will be driven for any given temperature
difference, with the maximum current being determined by the current limit.
The error voltage also drives an integrator. The integrator’s output also controls the
amount of current being driven through the TEC. The integrator is an amplifier
whose gain is proportional to time. The longer a given error voltage is present, the
more current will be driven through the TEC, with the maximum current being
determined by the current limit. The speed at which the integrator’s output increases
is the integration time, which can be “Slow” or “Fast”. Some TEC modules do not
support the Slow setting, and therefore omit the Fast designator in the range of
settings.
The allowed Gain values are: 0.2 Slow, 0.6 Slow, 1 Slow, 1 Fast, 2 Slow, 3 Fast, 5
Fast, 6 Slow, 10 Slow, 10 Fast, 20 Slow, 30 Fast, 50 Fast, 60 Slow, 100 Fast or 300
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