How to make connection between thermocouple and PLC

Connection between thermocouple and PLC is an important thing to consider. The cost & efficiency both factors are needed to take in to consideration. Also the environment (temperature) of the sensor location, distance to the PLC is a factor.

Thermocouple connection in a system comprises with connection channel cable and cold end temperature generator usually pt-100 temperature sensor. As pt-100 temperature sensor has this option to directly making interface with PLC plus it is cheap, it is more popular to use thus.

Extension is normal copper cable

Thermocouple connection -

1.      Extension cable is normal Copper-cable

2.      Cold end is generated into connection box

3.      Low price and easy to get, by using standard instrumentation cables

4.      Cold end compensation have to do in connection box for example by using Pt-100 sensor

                                                    Fig: Normal copper wire is used as extension cable

Extension is thermocouple metal

Thermocouple connection-

1.      Extension cable is compensation cable (same material)

2.      Cold end is generated into PLC-terminals

3.      Cold end compensation is done inside PLC by internal temperature measurement (Pt-100)

4.      Relatively expensive set-up

5.      Compensation cable is difficult to get in some countries

6.      Not so flexible to handle and do connection than Copper

                          Fig: Thermocouple material metal is used as extension cable material

Amplifier used in Extension channel

Thermocouple connection-

1.      Amplifier takes care about “cold end”- compensation

2.      We can use analog input card for receiving mA-signal

3.      Useful for marine installations where limited space

4.      Testing can be done by shorting thermocouple input, so amplifier should send internal temperature signal forward to PLC.

5.      More costs, because amplifier is needed in every measuring channel

                                    Fig: Amplifier is used in Thermocouple extension channel

CAN bus used as extension channel

Thermocouple connection-

1.      Data transfer with digital CAN-bus, better EMC behavior

2.      Cold end is generated into WECS/UNIC terminals

3.      Cold end compensation is done inside WECS/UNIS by internal temperature measurement (Pt-100)

                   Fig: CAN bus or controller area network cables is used as extension channel

Why CAN bus as extension channel with thermocouple is better

This is a better efficient and cheap connection. Only one Pt-100 temperature sensor used to generate cold end temperature. Any number of incoming thermocouple can be used for this one. Thus a common single junction box with single pt-100 and multiple incoming thermocouple.

Moreover, the CAN bus or controller area network bus is fast, efficient comparing to its pricing.