Sink vs Source in PLCs: What is the Difference?

Sinking vs Sourcing Overview 

The concept of sinking vs sourcing describes a current flow relationship between input and output devices in a control system and their power supply. The two terminologies apply only to DC (Direct Current) logic circuits. A logic circuit is an electric I/O circuit whose output is a function of the input. It can include a single binary (ON/OFF) output and one or more binary inputs. Such circuits may include any type of binary electronic or electric devices such as switches, transistors, solid-state diodes, and relays.

Sink and source DC logic circuits are mainly associated with digital PLC Input/Output signals. However, the concept of sourcing and sinking in PLCs might be confusing to most PLC users. But it’s a fairly simple concept, whose basis is the basic electrical theory. This theory states that to complete a DC circuit, DC current must flow from the +ve (DC+) terminal through a load to the –ve (DC-) terminal. Therefore, sourcing and sinking depend on the circuit polarity defining the direction of current flow between two devices. Let’s consider the following illustrations:

In the illustration above, the current is flowing from Device (A) to Device (B). Therefore, Device (A) can be said to be sourcing the current, while Device (B) is sinking the current.

In this second illustration, the current is flowing from Device (B) to Device (A). Thus, Device (B) is sourcing the DC current, while Device (A) is sinking that current.

In every situation where DC current is flowing between two digital devices: the device transmitting current out of its control terminal to another device is said to be the current sourcing device. While the current-sinking device is the device that receives that current into its control terminal.

Note: Active Sinking or Sourcing digital input and output devices allow current flow in only one direction. Therefore, when selecting the type of digital input or output modules for your PLC system, it is very important that you have a solid understanding of the sourcing vs sinking concept in real-world devices. In this article, we’re going to discuss Sourcing vs Sinking in digital PLC input modules and how they connect to digital field input devices.

Sinking vs Sourcing in PLC Digital Input Modules 

In a PLC system, the input and output field devices connect with the processor via the I/O modules. These modules are either digital or analog. Digital input modules use input signals that have only two possible values, such as High or Low. Voltage and current signals are the most commonly used PLC input signals. The discrete (digital) input modules provide an interface between the CPU of a PLC and digital field input devices. They do so by detecting one of the two possible conditions of an input field device and transmitting the detected input as a voltage or current signal to the PLC processor. Examples of digital field input devices include switches, proximity sensors, pressure sensors, pushbuttons, transmitters, and encoders.

Thus, for a field input device to interface with a digital PLC input module, it must be providing a digital output. For example, the input signal to a PLC input module may read HIGH if the output voltage of the connected field input device is at 24 VDC. If the output voltage of such a device is less than 24 VDC or is 0 Volts, then the input signal transmitted to the PLC input module will be read as LOW. The common output conditions of digital field devices detected by digital PLC input modules include:

• General Status: 0/1, High/Low, True/False  
• Load Condition: ON/OFF 
• Switching Mechanism: Activated/Deactivated  
• Switching Contact Status: Close/Open.  

A) Sourcing PLC Digital Input Modules

As previously discussed, the concept of sinking and sourcing in logic DC circuits is all about the direction of flow of conventional current between two devices. Based on the connection polarity between a digital PLC input module and a digital field input device; the sourcing module has current flowing out of it into the field input device. In addition, sourcing modules have their input card internally connected to a positive voltage like a power supply. So, they provide the positive voltage required to complete a logic circuit.

B) Sinking PLC Digital Input Modules  

According to the connection polarity between a discrete field input device and a digital PLC input module, the sinking module has current flowing into its control terminal. Such modules have their input cards internally connected to ground. For this reason, the internal circuit of a sinking PLC input module provides the necessary ground connection for an external circuit.

Detecting the Output Conditions of PLC Input Devices 

As we discussed earlier, a digital PLC input module will detect one of the two possible conditions of a digital field input device, either: High or Low; Closed or Open; ON or OFF. And depending on the type of input module used and its connection polarity with the field input device, the DC current can flow in or out of the module. This is where the terms sourcing and sinking come into the picture.

To better understand the concept of sinking versus sourcing in PLCs, let’s look at how digital input modules detect the output conditions of two types of digital field input devices- Pushbuttons and Proximity sensors.

A) Pushbuttons

Pushbuttons are mechanical switching devices that make or break an electrical connection by closing or opening momentary electrical contacts when actuated by mechanical or human interaction. You’ll usually come across Start or Stop pushbuttons in a PLC control panel, which are either categorized as Normally Closed (NC) or Normally Open (NO).

Connecting a NO pushbutton to a PLC input module creates an OPEN circuit with no electrical continuity, hence current doesn’t flow through the pushbutton. But when an NC pushbutton is connected, it shorts its terminals thereby creating electrical continuity that allows the flow of current. So, how does the concept of sinking vs sourcing come about? When NC pushbuttons are used as digital field input devices in PLC systems.

Consider the diagram to the right, in which a Normally Closed (NC) pushbutton (labeled as Device #2) is connected to a digital PLC input module (labeled as Device #1).

As you can see from Figure 1, the PLC input module (Device #1) is connected to a positive voltage presumably a power supply. Hence, it’s a sourcing PLC input module that has current flowing from its control terminal to the input field device (Device #2). The direction of the current flow is as shown by the blue arrow. The Normally Closed Stop Pushbutton in this case is a sinking input field device that accepts current from the PLC input module. In other words, Device #1 sources current to Device #2, which in turn transmits that current to the ground.

For the sinking scenario, let’s consider the illustration to the left:

In Figure 2, Device #1 is a digital PLC input module whose input card is internally connected to the ground thereby providing a ground connection for the circuit. While Device #2 is a Normally Closed (NC) Stop pushbutton that is connected to a power supply. So, Device #2 is sourcing current to the PLC input module as indicated by the direction of current flow (blue arrow). The PLC input module in this case is a sinking type, as it accepts current from the field input device (NC Stop pushbutton) into its terminals. Simply put, the NC Stop pushbutton sources current to the PLC input module which in turn sinks that current to the ground.

B) PNP and NPN Sensors

PNP and NPN proximity sensors are digital field input devices, which provide discrete inputs to digital PLC input modules. They detect the presence or absence of metal objects without any physical contact. Instead, they emit electromagnetic fields and then determine the changes in the emitted fields due to the presence or absence of an object. PNP stands for Positive-Negative-Positive, it’s usually a three-wire sensor. Similarly, an NPN device is a three-wire sensor with the term NPN standing for Negative-Positive-Negative.

Note: Proximity sensors have internal switching mechanisms that classify them as either PNP sensors or NPN sensors. Also, sometimes proximity sensors are referred to as proximity switches because they give binary outputs- High or Low. That means when an object is detected it can provide a High or Low voltage output, and in the absence of an object, they output either a Low or High voltage signal.

For example, PNP sensors give a High voltage signal when an object is detected and a Low voltage when no object is detected. On the other hand, NPN sensors are naturally ‘ON’. So when an NPN sensor detects a metal object it goes off and gives a low voltage signal. But when the metal object is removed from its range, the NPN sensor will go ON and give a High voltage output signal.

The big question now is: How will the PLC detect the state of the output voltage signal from either the PNP or NPN sensor? Well, this is done by connecting either of the two proximity sensors in a closed circuit with a digital PLC input module, as we’ll discuss next.

First, let’s consider a simple logic circuit consisting of a 3-wire PNP sensor, PNP transistor, a power supply, and a digital PLC input module, as shown to the left.

In the diagram above, the field input device is the 3-wire PNP sensor consisting of three wires. In which, the brown wire is connected to the power supply (+24 VDC), the black wire is the signal output wire that is connected to the input of the digital PLC input module, and the blue wire is connected to the ground. A PNP transistor is installed between the positive voltage (brown wire) and the signal output wire, and it is used to positively switch the PNP sensor. The digital PLC input module is acting as the controlled load of the circuit.

Since the PNP sensor is connected to the power supply, it supplies a positive voltage to the PLC input module that is providing a ground connection for the circuit. In simple terms, the PNP sensor sources current to the PLC input module which in turn transmits that current to the ground. The PNP sensor is thus the sourcing device while the PLC input module is the sinking device. In general, PNP sensors are known to be Sourcing PLC inputs whose input is usually +24V, and they must be connected to Sinking digital PLC input modules that are internally connected to the ground (negative voltage).

Next, let’s consider a DC logic circuit consisting of a 3-wire NPN sensor, an NPN transistor, a Power supply (+24 VDC), and a digital PLC input module (controlled load), as illustrated to the right:

As you can see from the second diagram, the NPN transistor is used for negative switching and it’s connected between the ground (blue wire) and the signal output wire (black wire). So the NPN sensor is providing the ground connection (negative voltage) for the circuit, and to complete the circuit a positive voltage is required. This positive voltage is being provided by the PLC input module that is internally connected to a power supply.

So, the PLC input module sources current to the NPN sensor and that makes it the sourcing device in this case. And since the NPN sensor is accepting the current and transmitting it to the ground, then it’s the sinking device. NPN sensors are known as Sinking PLC inputs that have -24 VDC input, and for that reason, they must be connected to Sourcing digital PLC input modules that can give them positive voltage(+24 VDC).

Note: PNP and NPN sensors are active digital field input devices, which are polarity-dependent. Hence, for them to work their connection polarity must be done correctly during wiring or else they won’t function at all. That’s why you’ll find a PNP sensor labeled as a Sourcing device, while an NPN sensor is labeled as a Sinking device. Unlike passive field input devices like Start/Stop Pushbuttons that are not polarity-dependent. Meaning, they allow current to flow in either direction of connection polarity.

Let’s review what we have discussed in this article: 

• In general, the concept of Sinking vs Sourcing is associated with DC (Direct Current) circuits only. In PLCs, this concept describes a current flow relationship between digital field I/O devices and digital PLC I/O modules. In this article, we based our discussion on digital input field devices and digital PLC input modules. 
• A digital PLC Input module detects only two possible conditions of an input field device, either a no-voltage (Low) or voltage (High) condition; open or closed conditions of a switch. 
• A device sending current out of its control terminal to another device is said to be a current sourcing device. 
• A device accepting current into its control terminal from another device is said to be a current-sinking device. 
• A Sinking PLC input module is internally connected to the ground, and it accepts current from a Sourcing field input device. 
• A Sourcing PLC input module is internally connected to a positive voltage, and it sends out current to a Sinking field input device. 
• A PNP sensor is a Sourcing device that has current flowing out of it. 
• An NPN sensor is a Sinking device that has current flowing into it. 

This entry was posted on March 21st, 2022 and is filed under PLC, Uncategorized. Both comments and pings are currently closed.