Tec Control Modes - Newport 8000 User Manual

66 Chapter 4
bipolar TEC output. The following sections discuss these functions of the Bipolar
Output Stage.
4.6.8.1 Current Limiting
The output of the proportional amplifier and integrator together form the control
signal. Output current limiting is effected by bounding the control signal so that it is
always less than the limit current. The limit current is set with the front panel
controls or through the GPIB. The bipolar current limit levels are established by the
output of the current Limit DAC.
4.6.8.2 Current Limit Condition Sensing
Comparators sense the output to determine when output current limiting is occurring.
When this condition occurs, the I Limit signal is sent to the microprocessor.
4.6.8.3 Voltage Controlled Current Source
The bounded output control signal is applied to an amplifier. This amplifier and the
current sensing amplifier form the output voltage controlled current source. The
output of this stage directly drives the externally connected TE cooler module.
4.6.8.4 Voltage Limit Condition Sensing
Comparators sense the output to determine when the TEC output compliance voltage
limiting is occurring. This condition occurs whenever the TEC output is open or
connected to a high resistance. If this condition occurs, the V Limit error signal is
passed to the microprocessor.
4.6.9

TEC Control Modes

The 8000 provides three control modes for operation, constant T (temperature),
constant R (resistance, voltage, or current), and constant ITE (current) modes. Each
of these modes is discussed in the following sections.
4.6.9.1 T Mode
In constant T mode the TEC is driven to the set point temperature. This temperature
is monitored by the sensor in the TEC. In the case of a thermistor sensor, the
thermistor's resistance is used to determine TEC's temperature by using the Steinhart-
Hart conversion equation. The resistance is determined by measuring the voltage
across the thermistor (with a known current of 10µA or 100µA). The ITE current is
also measured and saved. The TEC's output current is sensed across a resistor and
the voltage is converted to an ITE current value.
When an LM335 sensor is used, a two-point conversion equation is used to determine
the temperature. Its voltage is measured as well as the ITE current.
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Temperature Controller Module Operation