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How to Wire a Thermocouple to a PLC

Learn about thermocouple cold junction compensation and challenges when working with extension wires.

In this article, we’re going to show you how to wire a thermocouple to a PLC analog input module. We’re also going to discuss thermocouple Cold Junction Compensation and challenges when working with Extension wires.

Ok… Let’s kick things off with a quick review of what a Thermocouple is.

Junctions and voltages

A thermocouple is made up of two dissimilar metal wires. These two dissimilar metals are joined together at one end and open at the other end. The joined end is called the Measurement Junction.

Thermocouple Junction

Heat applied to the Measurement Junction will produce a voltage across the open end. As the applied heat increases, so does the open-ended voltage.

Thermocouple manufacturers have purposely chosen metal wire pairs that produce 0 volts at 0 °C.

Thermocouple Voltage

Thermocouple types

Certain wire pairs have become popular as industry standards and assigned letters to indicate their Type. For example, a Type K thermocouple is made from alloys Chromel and Alumel.

Thermocouple types

Thermocouple Tables have been produced listing the resulting voltage at specific temperature values.

Well, if we want to make a temperature reading it seems easy enough!

Thermocouple Table

All we need to do is measure the open-end voltage. Unfortunately, it isn’t that easy.

First of all, the voltages produced are very small and the actual change in voltage per degree Celsius is minuscule.

Thermocouple small Voltage

Secondly, as soon as you connect a measuring device to the open end, you create more junctions that create unwanted voltages!

Thermocouple unwanted Voltages

Connect a thermocouple to a PLC

Alright… let’s connect a thermocouple to a PLC input module. Most PLC vendors offer Thermocouple modules. These specially designed analog modules provide what’s referred to as Cold Junction Compensation that eliminates the effect of any new junctions created.

Connect a thermocouple to a PLC

Let’s take a closer look at a typical Thermocouple module. The Siemens 6ES7 531-7PF00-0AB0 is an Analog Input module capable of connecting eight thermocouples.

Siemens 7PF00 Analog Input module

This particular module offers several options for providing cold junction compensation. Why are there different options?

Well, for one reason, the cold junction can be at different locations depending upon the application and the location of the thermocouple.

Keep in mind that anytime a thermocouple wire is connected to a different material, a new junction is created. We’ll talk more about that later.

The Siemens thermocouple module manual shows all the possible compensation options available.

Cold junction compensation options

1) Thermocouple connected directly to PLC

Let’s look at an application where the thermocouple is connected directly to one of the module inputs.

In this example, we’re wiring a Type K thermocouple to terminals 3 and 4 of the module. The positive yellow-insulated wire is connected to Terminal 3. The negative red-insulated wire is connected to Terminal 4.

As a rule of thumb, thermocouple wires with red insulation are negative.

Thermocouple connected directly to PLC

Here are examples of three typical thermocouples. Note that all of the negative wires are insulated red.

Typical thermocouples negative wire

In this wiring scenario, the Cold Junction is right at the terminals of the analog module. The module has an internal sensor that can compensate for the cold junction.

Cold junction compensation - internal sensor

The Thermocouple Type and the source of cold junction compensation are configured in the Siemens STEP 7 software program.

Cold junction compensation software configuration

2) Thermocouple located at a distance from the PLC

Let’s move on to a different scenario. What if the thermocouple was located at a distance from the PLC?

In this scenario, you can use the thermocouple extension wire because using standard copper wire creates new junctions and unwanted voltages.

A thermocouple extension wire is made of the same material as the thermocouple thereby eliminating any possible new junctions. Using thermocouple termination blocks will ensure that new junctions are not created.

With thermocouple extension wires, the Cold Junction is at the terminals of the analog module and therefore the module can be configured as in our first scenario.

With thermocouple extension wires, the Cold Junction is at the terminals of the analog module and therefore the module can be configured as in our first scenario.

We’ve just described two possible wiring scenarios out of many. If you aren’t sure if your wiring option is possible, your best resource is the manual for your module.

As we said earlier, all compensation options and wiring diagrams are specified in the module’s manual.

Summary

Ok… let’s summarize what we’ve discussed:

– A thermocouple is made up of two dissimilar metal wires joined together at one end and open at the other end.

– The joined end is called the Measurement Junction.

– Heat applied to the Measurement Junction will produce a voltage across the open end.

– Thermocouple Tables have been produced listing the resulting voltage at specific temperature values.

– Connecting a measuring device to the open end creates more junctions and unwanted voltages.

– Most PLC vendors offer Thermocouple modules providing several options for Cold Junction Compensation.

If you have any questions about thermocouple wiring or about thermocouples in general, add them in the comments below and we will get back to you in less than 24 hours.

Got a friend, client, or colleague who could use some of this information? Please share this article.

Ted Mortenson

Ted Mortenson

Automation Engineer

Posted on Feb 28th, 2022

Ted Mortenson

Ted Mortenson

Automation Engineer

Posted on Feb 28th, 2022

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