Dynamic power supply response is just as important as rated voltage, power, and current. Activation of a large load can cause the supply to momentarily enter a current-limited mode. The PLC may ride through the transient, but the inputs may be misread. Failure to address dynamic system-level response leads to latent and intermittent bugs.
To demonstrate the hazards, a 120 VAC to 24 VDC power supply was used to power a PLC, interposing relay, and a DC motor. The disturbing results are included in the oscilloscope screen capture (Figure 1). Here we see the transient response as the DC motor is activated:
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Orange: PLC output signal (PNP transistor in a sourcing configuration) used to drive the relay
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Blue: 24 VDC supply voltage as measured at the output of the 24 VDC power supply
The devices used in this experiment shall remain unnamed as I do not want to cast shade on quality name brand products. Identification is not necessary as the results are universal.
Figure 1: Waveforms for a power supply with a 40 ms disturbance.
Description of the Transient Voltage Disturbance
The events start at time zero when the PLC activates the interposing relay.
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The interposing relay mechanically closes at 15 ms, thereby connecting the DC motor to the bus.
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Motor inrush current causes the supply voltage to dip from 24 VDC to 5 VDC. This is normal behavior for a current-limited power supply.
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The power supply voltage recovers as the motor accelerates.
The total transient time was approximately 40 ms.
Problems Associated with Voltage Transients
A nominal 24 VDC supply pulled down to 5 VDC may cause problems in an industrial control environment.
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Inadvertent release of a latch. Smaller (faster) relays will be impacted first.
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Logic true inputs read as false by the PLC. This article demonstrates that the logic low / high distinction is about 9 VDC. Figure 2 is included here for convenience.
We can envision a PLC reading an input at the same time as the voltage drops to 5 VDC. Yes, there are debounce filters on the PLC inputs, but they are typically in the 10 ms range.
Consider an example where a PLC reads the Boolean value from a mechanical switch once per scan cycle. A large load causes a voltage dip. The PLC will read the switch and classify the 5 VDC signal as logic low causing the PLC to make an incorrect decision or enter a fault state.
Assume that every 24 VDC input is lying to you during the voltage transient.
Figure 2: Diagram showing the PLC’s logic thresholds where 11 VDC and above is a logic-1 while 9 VDC and below is a logic-0.
PLC Voltage Domains
The PLC power supply domains shown in Figure 3 are useful to differentiate a PLC reset from a condition where the I/O is untrustworthy.
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Microcontroller: The microcontroller that lives within the PLC is at the center of the diagram. Most modern 32-bit microcontrollers have a well-filtered 3.3 VDC supply. Filter capacitors provide a long time constant thereby allowing the microcontroller to ride-through short transient power supply outages.
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Output Domain: The next level in the diagram includes the PLC’s semiconductor type outputs. Physically we expect a polarity protection and a degree of capacitor filtering for EMI. This configuration is implied in Figure 1 where we see the PLC output degrade from 24 VDC to 16 VDC over a 15 ms period. It also recovers with the same characteristics as the 24 VDC power supply recovers. Note the PLC output is always less than the supply voltage by about one diode drop.
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Power supply domain: The PLC and field devices live within the 24 VDC power supply domain. This is a harsh noisy environment exemplified by the blue oscilloscope trace in Figure 1.
Figure 3: Voltage domains associated with the PLC.
Mitigation Techniques to Protect Against Voltage Transients
Armed with the Figure 3 mental model we can explore mitigation strategies to protect against transients:
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Simple but wrong: Isolate the PLC supply rail. While this will prevent transients from reaching the PLC (emphasis 3.3 VDC core) it does nothing for transient inputs that could cause the PLC to misread input-side field devices as previously described. The PLC is electrically active (inner layer) but blindly making decisions on inputs that are unstable.
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Better: Isolate the PLC and all field devices that connect to PLC inputs. This will ensure that the PLC rides though transients and that the inputs will be read correctly. To be clear, this requires an independent bus for the PLC and everything that feeds the input side of the PLC including all mechanical switches, relays, and electronic sensors.
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Best: Shift large loads to independent power supplies and physically segregate sections to reduce EMI. This reduces the interaction between the PLC’s input and output section. It also minimizes wire length and the potential for antenna-like interactions between sections as described here. This becomes increasingly important with analog signals or high speed signals in which the PLC’s input filter time has been reduced. As a crude example, we don’t want the step drive to advance when an unrelated motor starter is activated.
Resist the temptation to “just” isolate the PLC. It’s a well-intentioned idea but it will guarantee the intermittent bugs survive.
Isolation Techniques with Buffers
In the previous sections we assumed isolation via independent power supplies; one for the large loads and one for the PLC and associated inputs.
An alternative is to add buffers or a UPS to the control panel. One example is the Mean Well DBUF40-24. This type of buffer is placed between a 24 VDC power supply and a PLC along with its sensitive inputs. The buffer’s supercapacitors provide ride-through capacity for short voltage transients exemplified in Figure 1.
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Note that the buffer provides control signals that may be read by the PLC. This signaling allows the PLC to perform an orderly shutdown. For example, within seconds, the PLC can log the error, enter a user determined safe state, and write startup conditions into retentive memory.
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A full UPS may be used if a longer run time is required. While the buffer is good for about a second, a UPS may operate for tens of minutes with run time determined by the size of the UPS battery pack.
Classification of the Buffer and UPS
The buffer provides the energy to ride-through a transient or provide time for a controlled PLC shutdown. Conversely, a UPS can keep the PLC and associated equipment such as an HMI or a network connection active.
One overlooked benefit of the UPS is the elimination of the reboot time. Here, the PLC is active for the duration of the power outage within the constraints of the UPS battery capacity.
Figure 4: Image of a Mean Well buffer based on supercapacitor technology.
Parting Thoughts
If you remember nothing else, remember that power supply transients can impact the inputs of the PLC. Something as simple as the stop pushbutton (normally closed contact) can be misread during supply transients. This will appear as a random bug as the PLC releases the start / stop latch.
Finally, if you ever see flicker in a panel lamp, assume the PLC is compromised. Stop immediately and use an oscilloscope to search for transient voltage events.
Sincerely,
Aaron
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About This 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, completing a decades-long journey that began as a search for capacitors. Read his story here.