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Analog Modules
Analog input modules for temperature acquisition
Temperature values can be acquired with two different sensor technologies: Pt100 resistance
thermometers and thermocouples.
b Pt100 resistance thermometers
These devices measure the resistance of a platinum element, which increases with tem-
perature. At 0°C the element has a resistance of 100gC (thus the name Pt100). The resis-
tance sensors are connected in a three-wire configuration, which helps to ensure that the
resistance of the connecting cables does not influence the measurement result.
The maximum measurement range of Pt100 resistance thermometers is from -200°C to
+600°C but in practice it also depends on the capabilities of the temperature acquisition
module being used.
b Thermocouples
These temperature measurement devices take advantage of the fact that a voltage is gen-
erated when heat is applied to an element made of two different metals. This method thus
measures a temperature with the help of a voltage signal.
There are different kinds of thermocouples. They differ in their thermal electromotive force
(thermal e.m.f.) and the temperature ranges they can measure. The material combinations
used are standardized and are identified by a type code. Types J and K are both commonly
used. Type J thermocouples use a combination of iron (Fe) and a copper/nickel alloy
(CuNi), type K thermocouples use a NiCr and Ni combination. In addition to their basic con-
struction thermocouples also differ in the temperature range they can measure.
Thermocouples can be used to measure temperatures from -200°C to +1,200°C.
Example of temperature measurement:
Peripheral
equipment
Analog output modules
Analog output modules convert a digital value from the PLC base unit into an analog voltage or
current signal that can be used to control an external device (digital/analog conversion or D/A
conversion).
The analog output signals generated by the MELSEC FX family use the standard industrial
ranges of 0–10V and 4–20mA.
The example on the next page shows an analog signal being used as a setpoint value for a fre-
quency inverter drive. In this application the current or voltage signal from the PLC adjusts the
speed of the motor connected to the frequency inverter.
7 – 2
Temperature probe
Temperature
acquisition
module
Temperature
Analog/Digital
conversion
e. g. 47 gC
Processing Analog Values
Base unit of
FX family
Digital value
e. g. 470
MITSUBISHI ELECTRIC
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