Why are the terms sourcing and sinking important?
The terms sourcing and sinking identify circuit configurations often found in industrial control and automation circuits. With a sourcing configuration, current is supplied by the controlling device while in the sinking circuit, the controlling device connects the load to ground. While this definition is sufficient, it fails to capture the foundational nature of this concept. We cannot overstate the importance as the concepts dominate procurement decisions ranging from simple devices such as the Banner Engineering touch button, as shown in Figure 1, all the way to the most complex Programmable Logic Controllers (PLC). The concepts also extend to sensors with a closely related PNP and NPN designation. There you will find the PNP is a synonym for sourcing and NPN for sinking.
This engineering brief explores a simple application example focused on the Banner Engineering K30 touch button. This device was chosen as it has “bipolar” outputs, which in this instance, implies a device with both sinking and sourcing outputs.
Figure 1: Picture of the Banner Engineering K30ABTRGHQ touch button and Mallory FL1P-22NA-1-Y24V panel lamps along with the B&K Precision 1550 DC power supply.
Why is there confusion when we consider sinking vs sourcing?
There are two likely reasons including historical and nomenclature. For a historical perspective we can go all the way back to Benjamin Franklin and J.J. Thomson. This encompasses the initial discoveries of polarity followed nearly two hundred years later by the Nobel Prize level discovery of the electron and the “correct” understanding of current flow.
The problem with history
Unfortunately, this historical difference has left us with some interesting problems. As a rule, engineers are taught to think of current flow in Franklin’s terms while technicians are generally taught in Thomson’s refined terms. In fact, there are pairs of textbooks that are written in both conventional flow (Franklin) and electron flow (Thomson). A side-by-side comparison will show that every current direction is flipped, and the handedness of magnetic fields is flipped right to left hand.
To make sense of the terms sourcing and sinking we need to resolve this disconnect between the engineer and the technician. For better or worse, we will say Franklin is correct. Current flows from positive to negative and magnetic fields direction are defined by the right-hand rule. It’s good enough for most industrial electronics work. Should you find yourself in a semiconductor theory class or if you explore vacuum tube technology, you can readdress this situation and claim Thomson is correct. Stated another way, the first to define the terms wins.
In a DC circuit, current flows from the positive source to the negative return.
The problem with nomenclature
When we explore the concept of sourcing and sinking, we need to carefully differentiate between the controller and the load:
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Controller: For a digital circuit, the controller is a switch or the thing that contains the switch contacts.
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Load: The load is the part that is being controlled by the controller. The load responds to the controller.
It’s no wonder that folks new to electronics have such confusion with the concept of sinking and sourcing. A single misunderstanding of the direction of current flow or misapplication of the terms controller or load and the entire concept is lost. Please consider these examples:
Example 1:
Suppose we have a circuit composed of a switch and a relay coil. In this situation, the switch is the controller, and the relay coil is the load.
Example 2:
Suppose we have a sensor and a PLC input terminal. In this situation, the sensor is the controller, and the PLC input circuit is the load. We must be very careful with our terminology and focus. With a big picture focus we are tempted to say that the PLC is a controller. However, in this focused situation, the PLC input circuit with its digital input serves as the load and the sensor is the controller. In this situation, the sensor in control of the PLC’s input. It helps to model the PLC with input resistance that serves as a load.
Sinking and Sourcing controller outputs with the Banner K30ABTRGHQ touch button
The Banner touch button featured in Figure 1 and Video 1 is an ideal device from which to explore sinking and sourcing as it contains one each of each output type. The wiring diagram from the device datasheet is included as Figure 2. This is a typical diagram for industrial controls. It assumes you are readily able to identify the sinking and sourcing loads. As will be shown, the load associated with the white wire (one leg to +24 VDC) is in a sinking configuration and the load associated with the black wire (one leg to the ground return) is in a sourcing configuration.
Figure 2: Wire diagram for the K30ABTRGHQ red / green touch button from the device datasheet.
Video 1: Operation of the Banner touch button. One panel lamp is sinking the other is sourcing.
For clarity, the circuit may be redrawn as shown in Figure 3. This is most helpful as the orientation of the diagram now associates sinking to the left and sourcing to the right. This is coupled with a left to right flow of current through the associated circuit.
Tech tip: Wire color is closely associated with the concepts of sourcing and sinking. Let’s assume that white with blue stripe wire is used for all connections that are hard wired to the return. Also, let blue wire be used for the 24 VDC source and any wire that could be at a 24 VDC potential. The results show that a load in a sinking configuration will have one blue and one white with blue stripe wire as one leg of the load is always solid at ground (return potential). A load in a sourcing configuration will always have two blue wires as both side of the load may measure at 24 VDC depending on the state of the controller.
Figure 3 Picture of the Banner touch button and a simplified representative wire diagram showing two panel lamps. Note that the button is the designated controller, and the panel lamps serve as the load. One circuit is sinking and the other is sourcing.
It’s important to recognize that the Banner touch button is designed with two semi-independent controllers. Here, we are using our previous definition of controller as the thing with the switch contacts that controls the load. In this case, the touch button is greatly simplified as a Double Pole Single Throw (DPST) switch with a magic internal X that will be red or green depending on the state of the button press.
What is a sinking configuration?
By definition, a sinking configuration has continuity when the controller’s contacts close thereby connecting the load to the ground return. In Figure 3 we see that Panel Lamp number 1 (PL1) is part of this sinking configuration. If we follow Franklin’s conventional current flow, the controller provides a “sink” (ground return) for the load’s current.
What is a sourcing configuration?
By definition, a sourcing configuration has continuity when the controller’s contacts close thereby connecting the load to the power supply. In Figure 3 we see that PL is part of this sourcing configuration. Here the controller serves as a source of current. The current then flows through the load and return via the ground return.
Tech Tip: We must be careful with the terms “ground” and “return”. They are often used interchangeable, but they do imply different circuit configurations. The term return is typically associated with a floating system while ground refers to a system where the negative terminal of the power supply is connected to the metal chassis. As a rule, 24 VDC control system float relative to ground. Consequently, it is more appropriate to use the term return. As an example, consider this Delta Electronics DRC-24V10W1AZ power supply. According to the datasheet is a double insulated Class 2 device and “No Earth connection is required”. This is further supported by analyzing the power supply block diagram which show galvanic isolation for the output stage.
Which circuit is better sinking or sourcing?
There is an English saying “horses for courses.” There is no one-size-fits-all solution. Instead, there are many different circuit configurations that require different product solutions. The best circuit is the one that gets the job done while simultaneously providing a clear and documented way for future assemblers, equipment integrators, and technicians to maintain and repair the system.
On a personal level, I prefer the sourcing configuration. There are a few reasons for this preference. First, I like to place the switches on the high-side of the load. Second, we tend to think in terms of positive logic with all things referenced to ground. For example, we reflexively place the meter’s reference probe (black) to ground. Third, this bias is reflected in the number of available products. For example, consider DigiKey’s proximity sensors if we select for 3 and 4 wire normally open NPN (sinking controllers) we find 1,788 results. That same search for PNP (sourcing controllers) reveals 2,713 results. This does support the bias. However, this also suggests that both sinking and sourcing circuit are popular. Consequently, all personnel involved with industrial controls must have a deep understanding of both configurations.
Tech Tip: Banner products such as this touch button are available in many different configurations. An important distinction is who or what is in control of the indicator lamps. The featured Banner K30 is on one end of the control spectrum where the user is in control of the color. At the other end of the spectrum, we find devices such as the Banner K50 where the color is under control of a PLC such as the Schneider Electric Modicon TM221CE24T PLC. Given this range, we must carefully select products for a given application.
Parting thoughts
This brief explores the concepts of sinking and sourcing circuit configuration through the lens so the Banner Engineering touch button. It includes a brief segue into related concepts such as PLC, relays, and sensors with PNP (sourcing) and NPN (sinking) outputs. Perhaps in the future we can further explore those applications. I believe that would be a good use of our time as the concept of sinking and sourcing is a fundamental building block upon which so much of our technology rests.
Please share your comments and critiques of this material.
Best Wishes,
APDahlen
Return to the Industrial Control and Automation Index.
About the author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.
Questions
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Define the terms “controller” and “load” as applied to sinking and sourcing circuits.
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Provide at least 3 examples of “controllers’ and three examples of “loads.”
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With regards to Figure 3, identify the color for each wire. Use white with blue stripe for all solid connection to the return and blue for everything else.
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T/F: The controller is the part of the circuit that initiates or controls the current though a circuit.
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Suppose we inadvertently swapped wires 2 and 4 in the Figure 3 wire diagram. Describe the results.
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Describe the term “sourcing” from a “flow of current perspective” for both controller and load.
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What is meant by “high side” switch? Is this associated with sinking or sourcing?
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Differentiate between the terms “ground” an “return.”
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You are using a multimeter to troubleshoot a circuit. Describe the meter’s reading when the black reference probe is connected to the metal chassis as compared to when it is connected to a terminal block associated with the floating return.
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Research PNP and NPN transistor configurations. For each type provide a schematic showing the power supply, transistor, and the load. Assume a fixed 24 VDC power supply. Also identify each schematic as sinking or sourcing.
Critical thinking questions
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Explain how a relay may be both controller and load in a relay-based control circuit.
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Describe how your troubleshooting steps differ in a sinking vs a sourcing circuit. Hint: Consider the measured voltages relative to the return when the controller’s contacts are open and when they are closed. Also, be sure to clarify where to place the meter’s reference probe?
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Assuming the circuit breakers in your home are controllers, is your breaker panel part of a sinking or a sourcing configuration? What are the safety considerations for the circuit? Hint: Assume you have inadvertently become part of the circuit hand on wire and a wet foot to earth ground.
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What are the pros and cons of grounding (earth reference the negative terminal) an industrial controller’s 24 VDC power supply.
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Returning back to the swapped wire mentioned in question 4. Without returning the wires to a normal configuration, how could the circuit be restored using two large resistors. Hint 1: Assume the resistors have a low resistance relative to the load. Hint 2: Be sure to read the article on PNP and NPN sensors especially the section of substitution a NPN for a PNP.
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Occasionally you will hear the absolute nonsense that “electricity follows the path of least resistance.” Explain how this thinking leads to a dangerous situation involving floating systems that have no apparent connection between the conductors and earth ground.