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.
2. Causes of 24V Voltage Instability
2.1 Insufficient Power Supply Capacity
An undersized SMPS cannot handle sudden load changes.
2.2 Excessive Cable Length or Thin Wires
Results in:
- Voltage drop
- Heat generation
- Unstable end-point voltage
2.3 Loose or Poor Connections
Common in terminal blocks after long operation.
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.
2.5 Load Variation (Especially Solenoids & Coils)
Electric valves and relays draw high inrush current → causes momentary dips.
3. How to Measure and Diagnose 24V Instability
3.1 Multimeter Measurement
Measure:
- No-load voltage
- Loaded voltage
- Drops during operation
3.2 Oscilloscope Ripple Check
Ripple should be:
- < 200 mV for quality SMPS
Higher ripple indicates power supply degradation.
3.3 Ground Potential Difference
Improper grounding leads to reference drift → sensor errors.
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.
5. Real Engineering Cases
Case 1 — Cylinder Actuation Causes PLC Voltage Drop
High inrush solenoid coil → instant voltage dip → PLC resets.
Case 2 — Multiple Solenoids Triggered Together
Momentary overload → SMPS current limit → system instability.
6. Solutions to Improve Voltage Stability
✔ Increase SMPS Capacity (1.5× Total Load)
Prevents overload dips.
✔ Separate Power for Servo Control Circuits
Servo enable circuits should not share weak supplies.
✔ Add Buffer Capacitors
Absorb inrush current from solenoid valves.
✔ Use Thicker Wires or Reduce Cable Length
Minimizes voltage drop.
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.
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