Random PLC Resets and Faults: Finding 24 V Supply Dips

The symptom

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

Many of these "random" faults come from a 24 VDC supply that dips for a few milliseconds, which is 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 and switches. Use the datasheet currents. Then add inrush, because 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 left when four valves fire together.

Step 2: measure the right way

A multimeter averages its reading and misses short dips. Use one of these instead:

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

Step 3: voltage drop in the 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 that sees 20.5 V may keep working until something else on the same circuit draws current.

Step 4: check the primary side

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

Fixes

Split the loads. Give actuators such as valves and relays their own supply (or their own electronic circuit breaker channel), and keep a clean one for the CPU and IO electronics.

Size with margin, typically 20–30% headroom over the calculated peak. Supplies with a power boost, or buffer modules (capacitor or battery), ride through short dips.

Fit electronic circuit breakers per branch, so a short on one sensor cable does not take the whole supply down. On long runs, use the correct wire cross-section or put distributed supplies out 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. The hardware chapter on signals lists the input thresholds.