A PLC that kicks out of RUN on AC power is usually seeing a voltage outside its printed input range for a moment too short for a panel meter to catch. Measure at the controller's supply terminals during the event, compare the lowest reading to the printed minimum, and fix the specific disturbance the recording shows before adding filters or converting to 24 VDC.
A PLC that kicks out of RUN for no visible reason is usually seeing an input voltage outside its printed range for a fraction of a second, and a panel meter will never show it. The measurement that settles the question is taken at the controller's own AC supply terminals, during the event that leads up to the dropout, and compared with the minimum printed on the controller's label. The controller is designed to run inside that range, so an input below the floor is one it is not specified to run on. A second controller with a wider input range in the same cabinet rides through a sag that takes the smaller unit below its floor, which is why a healthy neighbor tells you the sag is real and says something about its depth. Capture the disturbance first, then fix the specific problem the recording shows.
What each symptom points to
Match the pattern in front of you to a cause before opening the panel.
- Drops out when large loads start. A press, compressor or drive starting across the line pulls the shared feed down for tens to hundreds of milliseconds. That is a sag, and it is the most common cause behind a dropout that filters never cured. Log the feed through several working days and line the timestamps up with the starts.
- Drops out at random, with no pattern against any load. Points at a grounding problem, a loose supply or ground termination, or noise coming in on a communications port. Pull and re-terminate the supply and ground connections first, then isolate comms and I/O one path at a time.
- A wider-range controller in the same cabinet never drops. Expected when the sag lands below the compact controller's printed floor but inside the larger unit's envelope. Useful too: it confirms the disturbance is real and repeatable, and it brackets the depth.
- The HMI on the same feed misbehaves in the same window. Backlight flicker, a reset or a blank panel alongside the controller's dropout points at the shared supply rather than a controller fault. The supply checks in HMI backlight flickers when polling: check the power supply cover the panel side, and a panel that comes back without its project after a power event is the same story one step later; HMI panel loses its project after a power cycle walks that one.
- A filter went in and the dropout stayed. The filter was selected for a disturbance nobody has measured yet. Capture the waveform before spending again.
Measure at the supply terminals, not the panel meter
Measure line to neutral across the controller's AC supply terminals with the machine idle, then again during the load start that precedes the dropout, and record the lowest and highest readings. A panel meter or a multimeter spot-check updates over seconds; a sag that lasts 100 to 200 ms falls between its updates, so the meter shows a healthy reading while the controller is watching one below its floor.
Working inside a live panel is qualified-person work. Match the meter's measurement category to the point you are probing and follow your plant's rules; the meter-safety questions that apply in an MCC are the same ones here, and measuring voltage safely inside live equipment covers them.
The printed input range is the acceptance test. One compact AC-powered PLC in the class that fills small panels prints 95 to 240 VAC, 40 VA maximum on its input; whatever your unit prints, the measurement has to stay inside the floor and ceiling for the whole event, not only at idle. Some labels print a nominal with a tolerance instead: a 115 VAC input specified +10%/-15% accepts 126.5 VAC at the top (115 x 1.10) and 97.75 VAC at the bottom (115 x 0.85). Convert your label the same way before comparing.
While you are there, note the input's VA rating. These controllers draw a few tenths of an amp, which is why they end up fed from whatever circuit is handy, including ones shared with motors.
Capture the disturbance before buying another filter
Connect a power-quality analyzer to the feed serving the control panel, configured by a qualified person for that installation. Let it run through several working days, cold starts included, because an intermittent sag will not show itself during a one-hour visit. Set the trigger to catch anything a few volts below the printed floor and a few volts above the ceiling, then correlate every recorded sag, spike or flat-topped waveform with the timestamps of the dropouts and with the starts of large loads.
While it logs, work the wiring paths. Poor-quality power, a grounding fault and induced noise on a communications port are the three causes behind nearly every controller that leaves RUN or lights a fault indication with its input in range. Re-terminate the supply and ground connections, and reroute comms cables that run in a trough with power conductors. Where switching coils live in the same panel, suppress them at the coil and check the bonding first, as laid out in PLC restarts when solenoid valves switch: suppress and ground first.
If the controller reboots after a sag and its comms then time out, that is the same event seen one step later; the supply and boot-order checks in PLC COM timeout after power-up: check supply and boot order apply to the restart.
Why the wider-range controller stays in RUN
The range printed on the label traces back to IEC 61131-2, the standard that specifies the functional and electromagnetic compatibility requirements and the related verification tests for industrial control equipment, including the supply-voltage conditions and immunity tests a controller design is checked against. A controller is qualified for the range its manual prints, and two controllers in one cabinet can carry different printed ranges. A larger controller family typically accepts a wider incoming voltage and frequency range than a compact one, so the same sag can sit inside one envelope and below the other's floor.
That is diagnostic information. The neighbor staying in RUN while the compact unit drops brackets the sag: the true minimum lands between the compact controller's printed floor and the wider range's. Use both labels to estimate how deep the disturbance is before choosing hardware.
If the log shows in-range power through a dropout, run the isolation test: with the machine left in a known safe state, disconnect I/O and communications one path at a time and watch for a change. A dropout that stops when one path is disconnected points at that path. A dropout that continues with everything isolated and the input in range goes to the controller's technical support with the logs and the fault indication, not to another filter.
Example: a sag the panel meter never catches
Scenario: a packaging line runs a compact AC-powered PLC whose printed input is 95 to 240 VAC, 40 VA maximum, fed at nominal 120 VAC from a control circuit shared with a hydraulic power unit. A spot check at the terminals reads 118 VAC, and the controller drops out once or twice a shift when the pump starts.
The analyzer records the pump start pulling the terminals down to 91 VAC for about 150 ms, about 9 cycles on a 60 Hz feed. The printed floor is 95 VAC, so the input sits 4 VAC below the minimum the unit is designed to run on, about 4.2% under. The 40 VA draw is only about 0.34 A at 118 VAC (40 / 118), so the controller's own branch is not producing the dip; the pump's inrush across the shared feeder is.
The fix that matches: move the controller feed to a circuit without the pump start, or bridge it with a true-sine UPS sized above the 40 VA load. In this scenario the feed moves to the control transformer serving the panel's other light loads, and the next logged pump start bottoms out at 104 VAC, 9 VAC above the floor. The controller stays in RUN, and the analyzer confirms the input held inside the printed range through every recorded event.
The fix that matches the recording
| The analyzer records | The fix | Confirm by |
|---|---|---|
| Sag below the printed floor on large starts | Separate feed, or a true-sine UPS sized above the controller's VA | Re-log; event minimum above the floor |
| Spikes or ringing on coil switching | Suppression at the coil, grounding checked first | Re-log; events gone |
| In-range power through a dropout | Grounding, comms routing, I/O isolation, one path at a time | Symptom follows the disconnected path |
| Nothing captured over several days | Longer log covering every shift pattern; trigger set a few volts under the floor | The event lands in the recording |
A UPS on the control supply bridges sags and short interruptions; it does not clean spikes or harmonics, and its runtime is what the battery and the VA sizing give. Field practice favors a true-sine output, because square-wave output has caused compatibility problems with some electronics. Define which loads must stay up and what the machine does when the battery is spent.
Converting to a 24 VDC-powered controller is a practical option where the AC side cannot be cleaned. It works only if the DC supply's own printed input range covers the worst sag logged, so check that before ordering, and size the supply for the connected load with inrush margin; the same budget work in Sizing a separate 24 VDC supply for HMI inrush current applies. A downstream DC supply does not remove an upstream brownout: if the incoming AC collapses, its output collapses with it. The tradeoffs between 120 VAC and 24 VDC control power are laid out in choosing between 120 VAC and 24 VDC control power.
The manuals for this class of controller recommend mains filtering, and that recommendation is sound once the disturbance is known. A filter chosen blind is a repeated expense with no measurement behind it: pick the type the recording points at, install it, then log the feed again and confirm the event is gone.
Recommendation
Recommendation: Log the feed with a power-quality analyzer for several working days and read the lowest sag against the controller's printed minimum before you buy a second filter or swap hardware. If a large start pulls the supply terminals below the floor, fix the sag with a separate feed or a true-sine UPS sized above the controller's VA. If the input stays inside the printed range through every logged dropout, stop changing power hardware and work grounding, comms routing and I/O isolation one path at a time. Convert to a 24 VDC controller when the AC side cannot be cleaned, sizing the DC supply for the connected load and checking its own input range against the worst recorded sag.
Questions
Should I swap the compact controller for the same model the neighbor uses?
A controller with a wider input range riding through the same sag solves one symptom. The sag stays in the feeder and will surface somewhere else: contactor chatter, lamp flicker, nuisance faults on other equipment, another compact controller next year. Fixing the sag at the source protects the whole panel, so treat the wider-range neighbor as a measurement clue and spend the budget on the feed.
Does converting to a 24 VDC controller end the problem?
It ends the problem only when the DC supply is specified for the same dirty AC. Check that the supply's printed input range covers the worst sag logged, size it for the connected load with inrush margin, and read its hold-up figure: industrial switch-mode supplies commonly hold the output for roughly 20 ms at full load after the input is lost, which bridges short sags only.
What if the analyzer shows the power in range through the dropout?
First check the instrument. Set the trigger a few volts under the printed minimum, use a sample rate fast enough to catch a 100 ms event, and log through several working days including cold starts. If a proper log still shows the input inside the printed limits through a dropout, the supply path is cleared and the cause is grounding, induced noise on a comms port, or I/O.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.