In this video,
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 videos,
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 video
is on solid state output modules operating DC loads.
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.
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.
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 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.
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.
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.
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 video
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
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 or 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 here.
If you want to order these PLC modules to use in your projects,
you can find the links in the video description.
The Siemens 6ES7322-1BF01-0AA0 is an 8 output Transistor Output Module.
You can find the link to order this PLC module in the video description as well.
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.
If this module is incorrectly wired as a Sinking module, it will not operate.
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 or ground,
you have a Sourcing module.
If you want to learn more,
you might want to review 2 of our other videos
“PLC Digital Output Types | Solid State vs Relay”
and “PLC Hardware Explained”.
You can find the links to these videos in the description.
OK…let's review what we’ve covered in this video:
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 or ground,
you have a Sourcing module.
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