Wiring And Connecting Thermocouples With External Compensation - Siemens SIMATIC Manual

Principles of analog value processing
4.6 Wiring and connecting thermocouples
4.6.2

Wiring and connecting thermocouples with external compensation

Function principle of external compensation with compensating box
With external compensation, the temperature at the reference junction of the thermocouples
is evaluated using a compensating box.
The compensating box contains a bridge circuit which is calibrated to a defined reference
junction temperature /calibrating temperature.) The reference junction is formed by the
connecting ends of the thermocouple's equalizing conductor.
The resistance of the temperature-sensitive bridge changes as a function of the difference
between the actual reference temperature and calibrating temperature. This difference
induces a positive or negative compensating voltage, which is added to the thermoelectrical
voltage.
Wiring and connecting the compensating box
Terminate the compensating box at the COMP terminals of the module; the compensating
box must be installed at the reference junction of the thermocouples. The compensating box
be supplied with an electrically isolated voltage. The power supply module must provide
adequate noise filtering, for example, by means of grounded cable shielding.
The terminals for the connection of the thermocouple on the compensating box are not
required and must therefore be jumpered (for example, see figure
thermocouples to reference junction)
Restrictions:
● The channel group parameters always apply to the all its channels (for example, input
voltage, integrating time etc.)
● External compensation with connection of the compensating box to the COMP
connections of the module can only be made for one thermocouple type. That is, all
channels that work with external compensation must use the same type.
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Connection of
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ