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Sinking and Sourcing PLC Outputs Explained

Learn about Sinking and Sourcing PLC digital output modules and how they connect to field devices.

In this article, we’re going to talk about Sinking and Sourcing PLC digital output modules and how they connect to field devices.

As we’ve discussed in our other articles, the two types of PLC output modules are Digital and Analog.

Drilling down even further, Digital output modules can be solid state or dry relay. The focus of our article is on solid state output modules operating DC loads.

PLC Digital Output Modules Types

The purpose of a PLC Digital Output module is to operate or control a DC voltage physical device based on field device conditions connected to an input module and decisions made by the PLC program.

Purpose of a PLC Digital Output module.

Sinking and sourcing

If you’ve been in the instrumentation and automation field for any length of time you have quite likely heard the terms Sinking and Sourcing.

Let’s spend some time clearing up any misconceptions you might have about these terms.

Here’s a tip for you… It’s all about conventional current flow!

When we talk about sinking and sourcing, it’s all about defining the direction of conventional current flow between 2 devices.

Think of it this way… Current is flowing between Device #1 and Device #2.

Ask yourself: Which direction is the current flowing?

The current is flowing from Device #1 to Device #2. Therefore we say Device #1 is Sourcing the current, and Device #2 is Sinking the current.

Sinking and Sourcing

OK… what if the current was flowing from Device #2 to Device #1?

In this case, we say Device #2 is Sourcing the current, and Device #1 is Sinking the current.

In every situation where you have a current flow between 2 devices, one of the devices will be Sourcing and the other will be Sinking.

OK… now that we’ve got that cleared up, let’s move on to real world devices where we aren’t representing current flow using a single-line drawing.

PLC digital output modules

1) Sinking output modules

PLC vendors manufacture both Sinking and Sourcing Digital Output Modules. Let’s explore what they are and how they differ.

Keep in mind what we discussed earlier… it’s all about the direction of conventional current flow.

Let’s have a look at 2 devices with current flowing between them. One device is a PLC Output Module, and the 2nd device is a relay.

Based on the connection polarity, the Sinking module will have the current flowing into it from the load.

Sinking Output Modules

2) Sourcing output modules

OK… let’s do that again, but this time one of our devices will be a Sourcing module. Based on the connection polarity, the sourcing module will have the current flowing out of it and into the load.

Sourcing Output Modules

If you recall, earlier we said that when there are 2 devices with current flowing between them, one device will be sourcing and one will be sinking. In our example, the output module is the sourcing device and the relay is the sinking device.

PNP and NPN PLC outputs

Sometimes adding confusion to already murky concepts, some vendors refer to Sinking and Sourcing outputs as NPN and PNP outputs.

You will hear more about NPN and PNP in a later article as we dig into PLC digital Input modules and discuss how input field devices are connected to them.

We’ll explain the reason for the different names, but first here’s something to remember about output modules:

– An NPN Module is a Sinking module

– A PNP Module is a Sourcing module

PNP and NPN Module

Why are the names PNP and NPN used?

Whether it’s sinking or sourcing, an output module must act as a closed switch to complete the circuit.

The actual switching in many output modules is performed by Bipolar Junction Transistors (BJTs):

– In a Sinking module, the transistor is an NPN

– In a Sourcing module, the transistor is a PNP

The current flows into a sinking module and out of a sourcing module.

If a PLC output module is listed as an NPN type, you can be sure that it is a Sinking type. But, if a PLC module is listed as a Sinking type, it may or may not be an NPN type.

NPN Module and sinking module

MOSFET modules

Many vendors use other solid state devices such as MOSFETs to perform the switching.

BJT and MOSFET modules
S7-300 digital output SM 322 - 1BP

Unless we dig deeper, we can’t know for sure what type of switching device is used. 

S7_300 digital output SM 322 - 1BP - NPN and PNP

Siemens 8 DO transistor module

The Siemens 6ES7322-1BF01-0AA0 is an 8 output Transistor Output Module.

Let’s connect 2 of the 8 outputs to loads. This output module is a Sourcing module as the current flows out of the module and into the loads.

Siemens 8 output Transistor Output Module

If this module is incorrectly wired as a Sinking module, it will not operate.

Siemens 6ES7322-1BF01-0AA0

A quick way to identify sinking or sourcing

You might ask “When looking at a wiring diagram, is there a quick way to identify if a module is sinking or sourcing?”

Here’s something that might help you… Each module output used will connect directly to one terminal of a load. The other terminals of each load will be wired together.

If the wired-together load terminals connect to the plus terminal of the power supply, you have a Sinking module.

If the wired-together load terminals connect to the negative terminal of the power supply (ground), you have a Sourcing module.

Is there a quick way to identify if a module is sinking or sourcing

Summary

OK… let’s review what we’ve covered in this article:

– There are 2 types of PLC Digital Output Modules – Solid State and Relay.

– Sinking and Sourcing boil down to defining the direction of conventional current flow between 2 devices.

– A Sinking module will have the current flowing into it from the load.

– A Sourcing module will have the current flowing into the load from the module.

– An NPN Module is a Sinking module.

– A PNP Module is a Sourcing module.

– If the wired-together load terminals connect to the plus terminal of the power supply, you have a Sinking module.

– If the wired-together load terminals connect to the negative terminal of the power supply (ground), you have a Sourcing module.

We recommend checking the following related articles, if you haven’t already, to have a better understanding of PLC Outputs:

If you have any questions about the Sinking and sourcing PLC outputs or about PLC modules 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.

The RealPars Team

By Ted Mortenson

By Ted Mortenson

Automation Engineer

Posted on Feb 22nd, 2021

Ted Mortenson

By Ted Mortenson

Automation Engineer

Posted on Feb 22nd, 2021

SIMATIC S7-300, Digital output SM 322, isolated, 8 DO, 24 V DC, 2A, 1x 20-pole

Digital output SM 322 – 1BF01

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