How to Choose an SMPS Power Supply: Complete Voltage and Current Selection Guide

1. Introduction — SMPS Is the Foundation of Control System Stability

In industrial automation, the SMPS (Switched Mode Power Supply) is the heart of the control system.

A weak or incorrectly chosen power supply causes:

• PLC random resets
• Sensor malfunction
• HMI communication loss
• Entire control system instability

Therefore, selecting the right SMPS is essential for reliable operation.

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2. Working Principle of SMPS

SMPS converts AC to DC using high-frequency switching.

2.1 AC to DC Rectification

The AC input is rectified into DC through:

• Bridge rectifier
• PFC circuits

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2.2 High-Frequency Switching Stage

The key process:

• DC is chopped into high-frequency pulses
• High-frequency transformer adjusts voltage
• High efficiency, small volume

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2.3 Regulated Output

After filtering:

• Stable DC 24V output
• Ripple minimized
• Overcurrent and overvoltage protection included

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3. Key Parameters You Must Know

3.1 Rated Output Current

The sustained load capability.

Example:
A 24V 5A power supply can provide 5A continuous.

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3.2 Peak Current

Short bursts used for:

• Relays
• Solenoids
• Servo activation

Some SMPS models support 150–200% surge.

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3.3 Ripple Voltage

Ripple causes:

• PLC resets
• Analog signal noise
• Communication instability

Lower ripple = better stability.

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3.4 Efficiency (%)

Higher efficiency → less heat → longer lifespan.

Industrial SMPS usually: >85%

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4. How to Select the Right SMPS

4.1 Load Total Current × 1.3 Rule

Example:

Total load = 3A
Recommended: 3A × 1.3 = 4A or higher

This prevents overload and heating.

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4.2 Independent Power vs Shared Power

Best practice:

• PLC, sensors, HMI → independent SMPS
• Motors, servos, VFD → separate supply

Never mix noisy loads with control loads.

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4.3 Choose Industrial-Grade SMPS

Benefits:

• Better EMI immunity
• Longer lifespan
• Wider temperature range

Avoid cheap consumer-grade power supplies.

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5. SMPS Wiring Guidelines

5.1 24V for PLC / HMI / Sensors

Typical distribution:

• PLC → 24V
• Sensors → 24V
• Relays → 24V
• HMI → 24V

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5.2 Single-Point Grounding

Prevent ground loop interference.

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5.3 Multi-Branch Output with Reverse Protection

Use:

• Fused distribution
• Reverse diode protection

Prevents backfeeding into the supply.

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6. Common Problems in SMPS Applications

6.1 Overheating

Causes:

• Ambient temperature too high
• Continuous overload
• Wrong placement in cabinet

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6.2 Excessive Ripple Causing PLC Reset

Often due to:

• Aging capacitors
• Cheap SMPS
• Overloading

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6.3 Voltage Drop Under Load

Indicates:

• Insufficient rated current
• Thin wires
• Long cable distance

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6.4 Wrong Series Connection

Incorrect wiring leads to unstable voltage or burnout.

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7. Best Practices

✔ Use Redundant Power (Dual PSU)

Ensures uptime in critical applications.

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✔ Separate Servo/VFD Power from Control Power

Reduces noise interference.

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✔ Add Soft-Start or Surge Suppression Module

Prevents inrush current damage.