The XTR103 is a temperature-to-current (T-1) converter that contains a linearization circuit for PT100 sensors. As usual in industrial applications, the measuring current is made available via the supply lines. The value of resistors R2-R4 ensures that the measuring current, Io, rises from 4 mA to 20 mA when the temperature at the PT100 rises from 0 °C to 200 °C.
The resistance value, Rp, of PT100, is calculated from
Rp = 100(1+3.90802. 10-3.T-
At 200 °C, Rs = 175.8 Ω. When this value is connected to the input of the PT100 via a potentiometer, the maximum measuring current (+,- 20 mA) is easily checked. In this way, the current can also be determined by others values of the PT100 resistance (read: temperature).
According to the manufacturer’s data published that Rs should not be higher than 400 Ω with a supply voltage of 15 V.
As already mentioned, the values of resistors R2-R4 specified are right for temperature measurements in the range 0-200 °C.
These values should be altered to 100 Ω, 385 Ω, for 0-400 °C, and to 19 °C, and to 19 Ω, 184 Ω, and 838 Ω for -200 °C.