TL;DR
Unexpected CompactLogix shutdowns almost always trace to the 24V DC power supply going out of regulation or an undervoltage event. Measure the 24V supply under load with an oscilloscope or data logger to catch the transient.
What you might see
- controller faults out and reboots without a programmed command
- controller fault log shows power loss or unexpected powerdown
- 24V DC supply voltage measured below 21.6V during fault event
- other 24V devices on the same supply also reset at the same time
Likely causes
24V DC power supply output voltage drooping below 21.6V (CompactLogix minimum) under peak load
Aging power supply with degraded capacitors or switching regulator components
Shared 24V bus with large inductive loads that cause voltage dips on coil release
Wiring resistance on a long 24V run causing excessive voltage drop at the controller
Required tools
- Digital multimeter (DC volts)
- Oscilloscope or data-logging voltmeter
- DC load resistor for supply load test
- Replacement 24V DC power supply (if needed)
Safety first
- Replacing a 24V power supply on a live panel requires working near other live voltages. Follow LOTO for the AC input to the supply, but note that other AC circuits in the panel may remain live.
- Do not restart the controller after an unexpected shutdown without reading the fault log. Restarting into the same fault condition can cause process damage.
Procedure
- 1
Read the controller fault log in Studio 5000 (Controller Properties, Fault Log tab). Look for a fault code indicating power loss or major fault on power-up.[1]
- 2
Measure the 24V DC supply at the controller's input terminals with a digital multimeter under normal operating conditions. It should be between 21.6V and 26.4V.[1]
- 3
Connect an oscilloscope or data-logging voltmeter to the 24V supply and monitor for 15 minutes while the process runs. Look for voltage dips below 21.6V that correlate with load events.
- 4
If dips are found, identify what event triggers them: solenoid valve energization, relay coil activation, or motor starter coil pick-up. Add a snubber or a separate 24V supply for the inductive load causing the dip.
- 5
Load-test the power supply: measure output voltage while applying a resistive load equal to 80% of the supply's rated amperage. Output should stay above 22V.[1]
- 6
If the power supply sags under load, replace it with a unit of equal or higher current rating. Size for at least 125% of the actual measured load.
- 7
Check the wiring from the power supply to the controller for conductor gauge and length. More than 0.5V drop on a long run means the wire is undersized.
Sources
Allen-Bradley CompactLogix 5380 Controllers User Manual (Pub 5069-UM001), Rockwell Automation (Allen-Bradley)
Allen-Bradley CompactLogix 5380 Controllers User Manual, power supply specifications and fault log interpretation (general)
View source
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