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Analog modules

5.13 Connecting thermocouples

S7-400 Automation System Module Data

Reference Manual, Edition 09/2009, A5E00850736-06

233

Principle of operation of internal compensation

Internal compensation allows you to form the reference point at the terminals of the analog

input module. In this case, route the compensating lines directly to the analog module. The

internal temperature sensor measures the module's temperature and returns a

compensation voltage.

Note: internal compensation is not as accurate as external compensation.

Principle of operation of external compensation with compensating box

If you employ external compensation, the temperature of the reference junction of the

thermocouples is taken into account via a compensating box, for example.

The compensating box contains a bridge circuit calibrated for a definite reference junction

temperature. The reference junction is formed by the connecting ends of the thermocouple's

compensating lines.

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 thermal voltage.

Note

The figures below do not show the connecting lines required to connect the electrical

potentials of the analog input module and transducers. In other words, you must continue to

read and follow the instructions provided by the generally applicable information on

connecting sensors.

Connection of thermocouples without compensation or using the reference temperature value

Wire the thermocouples either directly to the inputs of the module, or indirectly via

compensating lines. Each channel can use a thermocouple type supported by the analog

module independently of the other channels.

M+

M-

M+

M-

: Measuring line (positive)

: Measuring line (negative)

(1) Compensating leads (same material as thermocouple)

Figure 5-16 Connection of thermocouples without compensation or using the reference temperature

value to an isolated AI

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