Connection of Resistance Thermometers in 3-wire Configuration to the \ Analog Inputs

When resistance thermometers (Pt100, Pt1000, Ni1000) are used, a constant current must flow through them to build the necessary voltage drop for the evaluation. For this, the module CI521-MODTCP provides a constant current source which is multiplexed over the max. 4 analog input channels.

The following figure shows the connection of resistance thermometers in 3-wire configuration to the analog inputs AI0 and AI1. Proceed with the analog inputs AI2 and AI3 in the same way.

Connection of resistance thermometers in 3-wire configuration to the analog inputs

With 3-wire configuration, 2 adjacent analog channels belong together (e. g. the channels 0 and 1). In this case, both channels are configured according to the desired operating mode. The lower address must be the even address (channel 0), the next higher address must be the odd address (channel 1).

The constant current of one channel flows through the resistance thermometer. The constant current of the other channel flows through one of the cores. The module calculates the measured value from the two voltage drops and stores it under the input with the higher channel number (e. g. I1).

In order to keep measuring errors as small as possible, it is necessary to have all the involved conductors in the same cable. All the conductors must have the same cross section.

The following measuring ranges can be configured CI521-MODTCP and CI521-MODTCP:

Pt100	-50 °C…+70 °C	3-wire configuration, 2 channels used
Pt100	-50 °C…+400 °C	3-wire configuration, 2 channels used
Pt1000	-50 °C…+400 °C	3-wire configuration, 2 channels used
Ni1000	-50 °C…+150 °C	3-wire configuration, 2 channels used

The function of the LEDs is described under Diagnosis and displays / Displays CI521-MODTCP.

The module CI521-MODTCP performs a linearization of the resistance characteristic.

To avoid error messages from unused analog input channels, configure them as “unused”.