Industrial Control Power Conditioning: Introduction to the Buffer Module

Automation and Control Industrial Automation and Control
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A buffer module provides short-term ride-through capability for a 24 VDC control bus ranging from 100 ms to a few seconds, thereby eliminating nuisance disruptions. For PLC-based systems, the buffer module signals an impending power loss. At the system level, this provides the opposite of the PLC’s first scan; instead, the PLC can perform the last scan as part of an orderly shutdown.

This engineering brief provides a workbench view of the buffer module (Figure 1). It includes an oscilloscope screen capture showing the changing output voltage and PLC signaling lines. It supports the classification of the buffer module as a capacitor-based energy storage device with regulated DC output.

Key Takeaways

  • The buffer module is not a UPS.
  • The buffer module is installed in parallel with the primary power supply.
  • The 24 VDC supply must be sized to power both the load and the buffer module’s charging current.
  • The buffer module contains a DC to DC converter to provide a constant DC output voltage. This is desirable as it prevents the voltage sag associated with directly connected capacitors or batteries. The steady 22 VDC plateau provides clean power to the PLC and other downstream equipment.
  • The featured buffer module stores energy in high voltage capacitors, not batteries or supercapacitors.

This article is part of the DigiKey Field Guide for Industrial Automation

Location: Understand It → 24 VDC Control Power
Difficulty: :gear: Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 07 Apr 2026

Figure 1: A representative three-phase power supply (left) and a corresponding buffer module (right).
Figure 1: A representative three-phase power supply (left) and a corresponding buffer module (right).1215×911 220 KB

Figure 1: A representative three-phase power supply (left) and a corresponding buffer module (right).

Clarification

There are three classes of backup power:

  • Buffer module: milliseconds to seconds
  • UPS: minutes to hours
  • Backup generator: hours to days

The buffer module is not a UPS. While they share similar attributes, we distinguish the difference in terms of:

  • Runtime: The buffer module has a runtime measured in terms of seconds. UPS runtime is generally measured in minutes.

  • Expandability: The buffer module is self-contained. While some UPS units are also self-contained, many have provision for external batteries or capacitor banks.

In practical terms, the buffer module provides enough time to perform an orderly shutdown. By contrast, a UPS will keep the system, or parts of the system, running until power is restored. Both are like insurance for your control panel. The UPS leans toward long-term preservation. On a related note, critical installations demand a full backup generator for runtime measured in hours to days.

This runtime distinction is in the bones of the hardware. The typical UPS will include heat sinks for long-term runtime. By contrast, the buffer module is designed for short runtime. Little to no heatsink capability is required as the power semiconductors do not run long enough to overheat.

Signaling an Impending Shutdown

Most buffer modules include an output that is asserted when the buffer is active. In my opinion, this control line should be handled as a high priority interrupt.

For the system integrator, this is a signal for the PLC to prepare for a shutdown. After all, the primary 24 VDC supply is missing and the system has only moments before all power is lost.

Instead of the PLC’s “first scan,” we should think in terms of the “last scan.”

Tech Tip: You may not have as much time as you think. Remember that most PLCs have an input filter set, by default, to prevent switch debouncing errors. This safe port setting delays the buffer module’s signal by 10 to 20 ms. See this article for more information about PLC response times.

Workbench Demonstration

The buffer module is best understood by examining the waveform as the 24 VDC system transitions from the normal running state to the buffering state.

Safety Disclaimer

\color{red}\Huge \fbox{WARNING}

Use a bench power supply to power the buffer module.

If you use a line-powered industrial supply, you open yourself to the dangers of high current and high voltage systems. There are serious safety considerations as a mistake could cost money, damage equipment, cause a fire, or even hurt a person.

As a starting point, ensure:

  • You are qualified to perform the work, or you work under the close supervision of a knowledgeable mentor.
  • Appropriate enclosures and construction practices are used to prevent electric shock.
  • An appropriate circuit breaker or fuse is in place to prevent fire.
  • PPE, such as safety glasses, are worn, as malfunctioning high-current systems present an explosion hazard.
  • You, along with your supervisor and other essential personnel, perform a risk assessment and safety analysis prior to performing the work.
  • Proper Lock Out Tag Out (LOTO) processes are in place.
  • Follow all applicable employer, state, and federal guidelines.

Remember, safety is your responsibility. Protect yourself and those around you. For more information, refer to DigiKey’s Terms of Use and Conditions of Order

Results

The results are shown in Figure 2. Observe:

  • At time zero, the power from the 24 VDC supply is turned off.

  • There is an initial drop from 24 VDC to 22 VDC (orange trace).

    • This is a significant event as the load is now being powered by the buffer module.
    • The 22 VDC output is the nominal output voltage for the operating window of the buffer.
    • The PLC and other downstream equipment do not see a sagging voltage. Instead, they see a 22 VDC plateau.
    • The buffer module’s control line (“B”) is asserted (blue trace) when the buffer is active.

  • The buffer module continues to power the load with its 22 VDC output (plateau) for about 10 seconds. At this point the energy is depleted and the buffer’s output is turned off.

Tech Tip: This 22 VDC buffered voltage is well within the voltage tolerance required for most equipment. However, we are dealing with a nominal voltage. Recall that the typical industrial supply is adjustable. Many can provide up to 29 VDC to compensate for voltage drop across long wire runs. We could encounter problems with corner-case loads when we have large currents at the end of the wire run.

With restrained caution, I’ll suggest installing a buffer module at the end of the wire run, thereby providing a local source of power. However, there is no guarantee that this will work for your equipment. In fact it may add latent gremlins such as oscillations on system startup.

Figure 2: Waveforms showing the buffer module in operation.
Figure 2: Waveforms showing the buffer module in operation.1094×901 39.2 KB

Figure 2: Waveforms showing the buffer module in operation.

Comparison to Datasheet Values

The datasheet included a graph (Figure 3) showing the buffer module’s runtime as a function of the load current. The experiment supports the datasheet specification with an anticipated runtime of about 10 seconds. For clarity, the experiment was conducted with a 22 VDC buffer output driving a 35 Ω 50 W resistor.

Figure 2: Waveforms showing the buffer module in operation.
Figure 2: Waveforms showing the buffer module in operation.882×953 34.6 KB

Figure 3: Datasheet-specified buffer module runtime as a function of load current.

Components

The following components were used to demonstrate the operation of the buffer module:

  • External 24 VDC power supply capable of providing at least 5 A. Remember that the power supply must gracefully handle the load as well as the recharge current of the buffer module.
  • Buffer module such as the Mean Well DBUF20-24
  • Power resistor: The value isn’t critical, anything in the 20 to 40 Ω range is acceptable. However, the resistor must be in the 50 W range. For this experiment, an Ohmite RJS35RE set to full range was used.
  • Storage oscilloscope: A Digilent ADP2230 was used to provide the Figure 2 screenshot.

Tech Tip: The 24 VDC supply must be sized to power the load and the buffer module’s charging current. For reference, the featured buffer module has a 0.9 A charging current.

Failure to properly size the 24 VDC supply could result in an oscillating startup voltage as the power supply comes in and out of an overcurrent condition. This oscillation could cause unpredictable behavior in the PLC or other 24 VDC control panel devices.

Experiment Setup

The setup mirrors the familiar 24 VDC PLC wiring:

  1. The buffer module was connected directly to the output of the 24 VDC supply.

  2. A 24 VDC bench-type power supply was connected to the V_S terminal of the buffer module.

  3. The power resistor was connected to the buffer module.

  4. Finally the oscilloscope was connected with channel 1 measuring the 24 VDC supply and channel 2 monitoring the buffer active (“B”) output of the buffer module.

Precautions

The chosen 35 Ω resistor is dissipating approximately 16.5 W. This is well within the resistor’s rated power, but it will be hot.

Procedure

The procedure requires a few steps:

  1. Activate the 24 VDC supply.

  2. Observe the front panel of the buffer module. The LED will blink as the unit is charging. The LED will be steady when the unit is fully charged.

  3. Configure the oscilloscope for a one-shot display, triggered on the rising edge of the B signal.

  4. Turn off power and allow the oscilloscope to record the data.

Buffer Module Misconceptions

Originally, I had assumed the buffer module operated by placing supercapacitors directly across the 24 VDC line. This is not the case. Electrically, we know this is wrong as the Figure 2 waveform shows a straight line voltage. This implies an active power supply element as a pure capacitor solution would droop according to the textbook RC curve.

As one of our friends says, don’t turn it on, take it apart! Inside there are several 2200 µF, 200 VDC rated capacitors (Figure 4). This further supports the idea that a capacitor charger and a power converter are built into the buffer module.

Note that there are no large heatsinks in the buffer module. We can speculate that they are not required given the short runtime. By contrast, a UPS would have included large heatsinks. The semiconductors in the buffer module are just starting to heat up when the system is shut down.

Figure 4: Picture of the capacitors used in the Mean Well buffer module.
Figure 4: Picture of the capacitors used in the Mean Well buffer module.1263×947 113 KB

Figure 4: Picture of the capacitors used in the Mean Well buffer module.

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About This Author

Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.

Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.

Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.