Troubleshooting

Random PLC Resets and Faults: Finding 24 V Supply Dips

Faults appear with no pattern in the program, but always near a big load switching. The 24 V supply is the suspect — here is how to catch it.

· 8 min read · By TryPLC Team

  • Power Supply
  • 24 VDC
  • Control Cabinet
  • Troubleshooting

The symptom

The PLC occasionally restarts, a remote IO station drops out, sensors flicker, or the HMI reboots. The program and network look fine. The faults seem random.

Many "random" faults are a 24 VDC supply that dips for a few milliseconds — too short for a multimeter to show.

Step 1 — Add up the load

List everything on the supply: CPU, IO modules, sensors, valve coils, HMI, relays, signal lamps, switches. Use datasheet currents. Then add inrush: HMIs, valve islands and capacitive loads draw several times their running current at switch-on.

A 5 A supply carrying 4.6 A continuous has no headroom when four valves fire together.

Step 2 — Measure correctly

A multimeter averages and misses short dips. Use:

  • A multimeter with MIN/MAX recording on DC volts, left connected for a shift.
  • An oscilloscope or a power-quality logger triggered on a fall below, for example, 21 V.

    • Measure at the load end (the PLC terminals), not only at the supply output.

    Step 3 — Voltage drop in wiring

    Long, thin cables drop voltage under load. 1.5 A through 50 m of 0.75 mm² cable (100 m round trip) drops roughly 3.5 V. A field device at 20.5 V may work until something else draws current.

    Step 4 — The primary side

    If the 24 V supply itself dips when a large motor starts direct-on-line, the mains supply to the power supply sags. Check that the supply's input range covers the sag, or feed it from a stiffer source.

    Fixes

  • Split loads: a separate supply (or electronic circuit breaker channel) for actuators such as valves and relays, and a clean one for the CPU and IO electronics.
  • Size with margin: typically 20–30% headroom over the calculated peak.
  • Use supplies with a power boost or buffer modules (capacitor or battery) to ride through short dips.
  • Electronic circuit breakers per branch, so a short on one sensor cable does not take the whole supply down.
  • Correct wire cross-section for long runs, or distributed supplies in the field.

    • Confirm the fix

    Re-run the MIN/MAX measurement over the same production period. The minimum voltage should stay well above the lowest input voltage the devices accept. Read the hardware chapter on signals for input thresholds.

    Frequently asked questions

    Can a 24 V power supply cause random PLC faults?

    Yes. Short voltage dips under inrush or overload can reset CPUs, drop IO stations and make inputs flicker without any fault in the program.

    How do I find short 24 V dips?

    Use a multimeter with MIN/MAX recording or an oscilloscope triggered on low voltage, measured at the load terminals over a full production period.