Priniples of analog value processing
4.6 Wiring thermocouples
S7-300 Automation System Module data
4-16 Manual, 08/2006, A5E00105505-04
4.6.2 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 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 thermocouple terminals on the compensation box are not required, and should be short-
circuited (as an example, see the figure
Connecting thermocouples with reference junction
(order no. M72166-xxx00) to electrically isolated analog inputs.)
Restrictions:
● The channel group parameters always apply to the all its channels (for example, input
voltage, integrating time etc.)
● For external compensation with connection of the compensating box to the module's
COMP terminals, all thermocouples must be of the same type, and all channels operating
with external compensation must use the same type.
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