24V Control System Faults: What Causes Voltage Instability and How to Fix It

1. Introduction — 24V Is the Lifeline of Industrial Automation

In modern industrial systems, 24V DC is the most widely used control voltage.
It powers:

• PLC logic circuits
• Sensors
• Relays
• HMI panels
• Communication modules
• Servo enable circuits

When 24V becomes unstable, the entire machine can malfunction:

• PLC resets
• Sensors mis-trigger
• HMI flickers
• Servo drive drops enable

Voltage stability is therefore critical.

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2. Causes of 24V Voltage Instability

2.1 Insufficient Power Supply Capacity

An undersized SMPS cannot handle sudden load changes.

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2.2 Excessive Cable Length or Thin Wires

Results in:

• Voltage drop
• Heat generation
• Unstable end-point voltage

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2.3 Loose or Poor Connections

Common in terminal blocks after long operation.

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2.4 High Ripple Voltage

Caused by:

• Aged SMPS
• Low-quality power supplies
• High load switching

Ripple can reset PLCs and cause noise in sensors.

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2.5 Load Variation (Especially Solenoids & Coils)

Electric valves and relays draw high inrush current → causes momentary dips.

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3. How to Measure and Diagnose 24V Instability

3.1 Multimeter Measurement

Measure:

• No-load voltage
• Loaded voltage
• Drops during operation

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3.2 Oscilloscope Ripple Check

Ripple should be:

• < 200 mV for quality SMPS
  Higher ripple indicates power supply degradation.

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3.3 Ground Potential Difference

Improper grounding leads to reference drift → sensor errors.

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4. How Voltage Instability Affects Equipment

PLC

Random restarts are the most common symptom.

Sensors

False triggering or dropout.

HMI

Screen flicker or reboot.

Servo Drives

Enable signal drops → alarms.

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5. Real Engineering Cases

Case 1 — Cylinder Actuation Causes PLC Voltage Drop

High inrush solenoid coil → instant voltage dip → PLC resets.

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Case 2 — Multiple Solenoids Triggered Together

Momentary overload → SMPS current limit → system instability.

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6. Solutions to Improve Voltage Stability

✔ Increase SMPS Capacity (1.5× Total Load)

Prevents overload dips.

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✔ Separate Power for Servo Control Circuits

Servo enable circuits should not share weak supplies.

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✔ Add Buffer Capacitors

Absorb inrush current from solenoid valves.

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✔ Use Thicker Wires or Reduce Cable Length

Minimizes voltage drop.

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7. Best Practices

✔ Separate Control Power from Power Devices (VFD/Servo)

Avoid noise and dips.

✔ Divide 24V into Zones (PLC / I/O / Actuators)

Localized faults won’t affect critical circuits.

✔ Provide Independent Power for VFD/Servo Signals

Ensures stable communication.