Sinking and Sourcing PLC Outputs Explained
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- čas přidán 26. 06. 2024
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▶ Check out the full blog post over at
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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:
6ES7322-1BP50-0AA0 bit.ly/3XCf6hC
6ES7322-1BP00-0AA0 bit.ly/44vvP8x
Unless we dig deeper, we can’t know for sure what type of switching device is used.
The Siemens 6ES7322-1BF01-0AA0 (bit.ly/3JKhWeR) is an 8 output Transistor Output Module.
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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To learn more, you might want to review our other articles:
PLC Digital Output Types | Solid State vs Relay realpars.com/plc-output/
PLC Hardware Explained realpars.com/plc-hardware/
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Thank you for answering my questions from years ago. No elsewhere I can find better and clearer explaination. I appreciate it.
That's amazing to hear! Happy learning.
@@realpars 1.can we use capacitive touch pad switch (with 5v /24 v dc relay )...... in place of start/stop/selector switch ? in a 24 v dc PLC?
2. As we are using retentive timers......how we can design ladder logic for PLC.. so that process start ITSELF after power supply resumes ? (i.e ..not starting with the help of start switch)
Thank you RealPars for your clear and wonderful videos, please keep it up.
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Love realpars, quick and easy for keeping sharp on the fundamentals
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@@realpars no problem it's the truth, been a fan for many years
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.
It was another awesome video! Quick and Simple ! Congratulations!
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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.
Hi Dave. Thanks for your input and feedback. Switching power or switching ground is certainly a contentious issue yet vendors continue to market output modules allowing both. It's certainly buyer beware!
Thanks Dave. Sink your inputs. Source your outputs. Design your electrical control cabinet accordingly and enjoy a standard methodology. Short & Sweet Notes.
Best Practice in Electrical point of view.
@@realpars Hello Dave, thanks for the nice and simple explanation. I have six pieces of modulating valves (Samson type) connected to DO modules. If I connect all the valves, three will come up and the other three will not, I thought I was experiencing a short circuit from the cables. I have rewired the cables, still the same. pls, what can possibly be the cause? thanks.
Nice. I was able to understand it now.
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VERY NICE AND USEFUL EXPLANATION - THANK YOUY شرح مفيد و مبسط - شكرا لكم
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Would be nice to have a video on isolated inputs/outputs as well. Relays, conditioners and multiplexers.
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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.
Hi Ryarios. Thanks for your input. As you say, sinking input modules are more prevalent. This means that sourcing input field devices are required in order for everything to work. That's not a big deal if the input devices are passive, but critical if the input devices are active. We've got a video in the works for PLC input modules. Switching power or switching ground is certainly a contentious issue yet vendors continue to market output modules allowing both.
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@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
Hi Voytechj ! Thanks for your eagle-eyed feedback. You are correct. @5:00 both modules have NPN opto-isolators. @5:30 the animation should also show current flowing into and out of the driver. The opto-coupler electrically isolates the load and when turned on, will, in turn, activate the output driver completing the load circuit. My bad for missing this!
Nice explanation!
But what's the practical meaning when designing schematics to choose between sinking and sourcing output types?
PLC programmers and engineers need to know what I/O field devices are being used and how they will be wired. For example, if all the loads are connected to a common power supply return or ground, the output module needs to be a sourcing type. If the input I/O sensors are 3-wire PNP type, the input module needs to be a sinking type.
@@realpars Thank you for response! Just in my field of work I'm facing all variants of outputs (just depending on distributor), so I've just wondered - is there any particular reason for such variety besides voltage supply organization?
@@viktormakarov5232 This is related to the working principles of an NPN and PNP transistors. NPN is great for connecting and disconnecting load from the ground (sinking). One of his three pins (Emitter) is connected straight to the ground and other pin (Collector) to the output, then output to the load (relay, bulb, etc.) and finally to the power. In this way it works the best as a switch with very low resistance. If you try connect NPN straight to the power and try connect/disconnect power from the load, you will get very poor switch with high resistance or it wont work at all.
PNP do an opposite, it is very good at disconnecting power from the output (sourcing). Some devices need that property of a switch and you have no choice and use PNP(sourcing) as an output.
Hi sir, if module is sinking type, 24v DC connection to the I/O module is "COM", if sourcing type, 24v DC is to I/O module "+" right
Hi @kahwai choong. Thanks for your question. If the I/O module is a sinking typle, the COM terminal will be connected to the negative "-" terminal of the power supply. . If the I/O module is a sourcing type, the COM terminal will be connected to the positive "+" terminal of the power supply.
I have your courses they are wonderful but I didn't find pulse width modulation or PWM technic in D.O modules i need to understand it thank you
Hi Salouh, Pulse width Modulation is a topic on its own. PWM is a method of producing an analog type of control by digitizing the duty cycle of a digital output. PWM does really not fit in the DO module category where the output device is either fully on or fully off.
So what is the reason why you would choose 1 over the other? I understand why you would choose transistor or relay output but not sure why you would choose sinking or sourcing output
Hi L CT, That’s a very good question. There’s really no simple answer other than the module chosen is often determined by the load. What do we mean by that? It’s possible that your load is wired in such a way that it is a sinking device. If that’s the case, you’ve got no choice but to go with a sourcing output module.
Hi , i just feel a bit confused about an output module being an sinking type since the output always flow the current to the load and the input module current flow is from the load??? thank you.
Hi @Amadou Bailo. It can be confusing for sure. First off, the current does not always flow from the output module output terminal to the load. The current sometimes flows into the output terminal. An input module doesn’t actually have a load, but some type of a switching device like a push-button switch or a proximity switch. And, believe it or not, sometimes the current flows out of the input module input terminal to this switching device. We explain PLC input module sinking and sourcing in one of our videos. You can find the video here. czcams.com/video/B65detMhnoc/video.html
It boils down to determining current flow between 2 devices. We can call them Device #1 and Device #2. If the current flows from Device #1 to Device #2, then Device #1 is the sourcing device and Device #2 is the sinking device. Don’t hesitate to reach out if you have more questions.
❤
So what is the COM terminal on PLCs? And why do we use it? What is the difference between COM dc or ac?
COM is short for COMMON, the terminal to which all loads in a circuit to which the high side are referenced. So when a power supply puts out 24VDC, it is 24VDC as referenced to COM. In AC circuits, we use the term Neutral for the common low voltage wire.
Thx, so if the digital output are relays, both sinking and sourcing are not valid?
If I understand your question correctly, they are still valid.
If the PLC is sourcing the output to the relay, then the ground will be common and the output from the PLC will turn on the relay.
If however, PLC is sinking, then the +24V will be common and the ground output from the PLC output will turn on the relay.
Yes it is for outputs. If the relay is switching the ground then it’s sinking, if the relay is switching the plus side then it’s sourcing. I do not like sinking outputs. The load can operate without the output commanding it to, if the negative side of the load is grounded due to failure or accident. On a sourcing output the system either works when commanded or blows the circuit protection. (Unless the relay sticks shut)
Hi Chikenz, The concept of sinking and sourcing is still valid. The module will either sink or source the load current. The big difference is that with dry relay output modules, the current can be either sinked (.....if that's a word...) or sourced by the same module, or in many cases, both! Dry relay modules are happy with AC current too.
Sir I have a doubt, you are talking about NPN and PNP, npn means sinking and pnp means sourcing and both devices basic concept is flow of current, now kindly tell me how BJT can allow to flow current in both directions as it is an unidirectional device, its flow of current will be in one direction(emitter to collector).
Hello Akash. You are correct. A BJT is a uni-directional device, and current flows between the emitter and the collector. I'm referring to conventional current flow in my discussion. In the NPN transistor, current flows from collector to emitter. In the PNP transistor, current flows from emitter to collector.
Rules to live by:
1) If a Sensor is connected directly to the Positive(+) Power supply terminal, the sensor is PNP/sourced(with Positive power)
Rule of thumb 2 P's: means the sensor receives and sends a positive electric charge. (Input card will be the opposite = Sinking)
2) If a Sensor is is connected directly to Negative(-) Power supply terminal, the sensor is NPN/Sinking. (Receive and Send negative power)
To put it into an image:
1) Sourcing is a Faucet (Positive) (+)
2) Sinking is a Drain (Negative) (-)
Thank you for sharing that!
👍🏻
Sir, how are you telling if it is NPN it is sinking module but if it is sinking module it can either be NPN or PNP ?
Hi there. Thanks for your question. A sinking module cannot be a PNP type. A PNP module is a sourcing module. An NPN module is a sinking module.
@@realpars Thank you sir
When to select source output and when to select sinking output??
Hi vikrant....... Checkout our responses to Ryarios and Dave Bennet below
DOES PNP OUTPUT HAVE ANY SPECIFIC VOLTAGE?
Hi @Shahroz Khan. Thanks for your question. Every module will have its own specifications. for example, the AB 1756OB8 is an 8-output Sourcing module and can operate anywhere between 10V and 30V DC. Its nominal operating voltage is 24V DC. The current specification is more important to be aware of as excessive current can damage the internal transistor very quickly. The AB 1756OB8 is rated for a maximum of 2A per output point. In most plant applications, each output will be fused to prevent damage.
will you translate the videos into Italian in the future?
Hi Cristiano,
We currently do not have any plans on adding translations in the near future. However, I will happily pass this on to our team.
We do have English subtitles to almost all our video courses if that would be useful.
Thanks for your feedback and happy learning!
@@realpars thank you very much
Strange. You have used a ground symbol to complete the circuit. Why not just a line connecting the battery to PLC common? Very important thing in PLC wiring is that current loops through the ground to be avoided. The video explanation does not require a ground symbol R egards.
Hello there. Thanks for your feedback. Old habits die-hard is the only response I have for your comment. I've used that ground symbol for years to indicate common, or the negative return of a power supply. I hesitate to draw lines connecting all commons as drawings tend to become confusing. You are correct. The IEC60417 5017 symbol for earth ground is shown. Perhaps a better choice would be the IEC60417 5020.
Why not just stick to the old terms SUPPLY & LOAD?
Hi there @D.G. Sinking and sourcing are the terms adopted by the industry and vendors, so we also use that.
Thank you very much!
You're welcome!