Sinking and Sourcing PLC Outputs Explained

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⌚Timestamps:
00:00 - Intro
00:56 - Sinking and sourcing
02:17 - PLC digital output modules
02:32 - 1) Sinking output modules
02:49 - 2) Sourcing output modules
03:20 - PNP and NPN modules
04:45 - MOSFET modules
05:05 - Siemens 8 DO transistor module
05:38 - A quick way to identify sinking or sourcing

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

Digital output modules can be Solid State or dry Relay.

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.

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.
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.

- 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.

1) Sinking output modules
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.

2) Sourcing output modules
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.

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.

Sometimes adding confusion to already murky concepts, some vendors refer to Sinking and Sourcing outputs as NPN and PNP outputs.
- An NPN Module is a Sinking module
- A PNP Module is a Sourcing 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.

In many output modules the actual switching 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.

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

For example, we have the following Sinking and Sourcing modules:
Unless we dig deeper, we can’t know for sure what type of switching device is used.

This output module is a Sourcing module as the current flows out of the module and into the loads.
If this module is incorrectly wired as a Sinking module, it will not operate.

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

- 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.

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[tieliu]

Thank you for answering my questions from years ago. No elsewhere I can find better and clearer explaination. I appreciate it.

[gybx]

A few brands have selectable banks on the same card. They allow the same card to sink or source on the selected channel banks. It's very important to know the amperage requirements of the external circuit being sinked/sourced. Most PLC's don't have channel protection and input limits can be as low as 20mA.

[Hamza_Algmaty_]

Thank you RealPars for your clear and wonderful videos, please keep it up.

[johanburger]

Love realpars, quick and easy for keeping sharp on the fundamentals

[hafizahmad]

Hey, send my regards to the creator team. They did a very good work.

[davebennett]

You're either breaking the hot or the neutral (negative). Break the hot: sourcing. Break the neutral (or negative): sinking.

Breaking the neutral of a load with a switch or output is STUPID because now you have a hot load out there looking for a path to ground, which might be found via a rubbed wire or some other mechanical / electrical defect instead of your PLC output.

Sinking VS Sourcing for inputs - that is a more worthwhile distinction because you're not controlling "loads" as in moving equipment, you're controlling "loads" as in a sensor. You can program in some safety to prevent unintended behavior due to a single malfunctioning switch or an input going high at an unanticipated time.

Sink your inputs. Source your outputs. Design your electrical control cabinet accordingly and enjoy a standard methodology.

[engineeringwire]

Would be nice to have a video on isolated inputs/outputs as well. Relays, conditioners and multiplexers.

[georgehosny]

VERY NICE AND USEFUL EXPLANATION - THANK YOUY شرح مفيد و مبسط - شكرا لكم

[jc-shbw]

I wish university lecture is same like this... thnks realpars🤗😍

[alikhammas]

that is, in clear way ...thank you Realpars

[vigneshb]

Your videos are always useful. Keep it

[souza_canada]

It was another awesome video! Quick and Simple ! Congratulations!

[duonguckhai]

Thank you so much for sharing your knowledge. Awesome video

[Ryarios]

Personally, I prefer sourcing I/O from its reliability standpoint. Sinking discrete (digital) inputs are the standard for PLCs in the US at least. Power is sent to the field through the field device and back to the input, then to ground. This means a failure on the field side of the input card has a real chance of bringing down the power supply or blowing a fuse. On a sourcing input, this won’t happen since the actual load is the PLC input and it’s inside the panel. Sourcing outputs may not be desirable for failsafe inputs since they can go active due to failures in the field and that’s not failsafe.

[bisdakmillennial]

Nice. I was able to understand it now.

[viktormakarov]

Nice explanation!
But what's the practical meaning when designing schematics to choose between sinking and sourcing output types?

[christoperantony]

Great Video. Explain was very good. Creation team did grate job. Just try to explain about S/S wiring connection In Mitsubishi PLC. thanks a lot.

[TechSouls-wnjc]

I am compelled o appreciate your work thanks

[voytechj]

@5:00 They are all NPN transistors and are an irrelevant for an output type (sinking/sourcing). They are inside an optoisolator for a galvanic separation with max current ~50mA. The real switch is in a rectangle box with a triangle symbol. This is a driver with bigger current rating ~2A and the type of transistor inside determines whether output is sinking or sourcing.
@5:30 I think animation would look more correct if a current was flowing from 24V into a driver (box with a triangle) to the output, load and to the ground. There wont be flowing current higher than 50mA through this optotransistor.

IMHO putting an NPN optotransistor in schematic and hiding the transistor type inside a driver only makes people more confuse. I would do the opposite

[IngDzib]

Thank you, regards from Cancun Bonfil