How to Choose the Right SMPS for Industrial Control Panels

Selecting the right power supply is an important part of designing a reliable automation system. SMPS Selection for Control Panels is not simply about choosing a 24V DC unit with enough current rating. Instead, engineers must consider the complete electrical load, peak demand, operating temperature, voltage drop and future expansion.

An SMPS supplies critical automation components such as PLCs, HMIs, sensors, relays, communication modules and other field devices. Therefore, an incorrectly selected power supply can cause PLC resets, communication failures, unstable sensors and unexpected machine downtime.

A proper SMPS for Industrial Control Panels should provide stable power under actual operating conditions. This guide explains the key factors that help engineers and panel builders make a reliable selection.

Why Is SMPS Selection Important in Control Panels?

The SMPS is often considered a small component inside a control panel. However, it performs a critical function.

Most modern automation systems depend on a stable 24V DC control supply. A single SMPS may power:

PLC CPUs and I/O modules
Human Machine Interfaces (HMIs)
Proximity and photoelectric sensors
Relays and interposing relays
Solenoid valves
Industrial communication devices
Ethernet switches and gateways
Signal conditioners
Panel indication devices

If the power supply becomes overloaded or unstable, the entire automation system can behave unpredictably.

For this reason, selecting an SMPS only based on its rated current is not enough. The complete 24V DC power architecture should be evaluated before finalizing the power supply.

Industrial automation systems also have specific requirements for stable voltage, correct wiring, protection and environmental operating conditions.

1. Start with a Complete Load Calculation

The first step in SMPS Selection for Control Panels is calculating the total DC load accurately.

Do not calculate only the power consumption of the PLC. Instead, identify every device connected to the 24V DC power supply. Many control panels face problems because additional loads are ignored during the initial design stage.

Typical 24V DC loads include:

PLC CPU and I/O modules
HMI
Sensors
Relay modules
Solenoid valves
Communication devices
Ethernet switches
Signal conditioners
Panel indication lamps

Calculate the Total Current

Prepare a simple load calculation table before selecting the SMPS.

For example:

Device	Quantity	Current	Total
PLC and I/O	1	1.0A	1.0A
HMI	1	0.5A	0.5A
Sensors	10	0.05A	0.5A
Relay Modules	8	0.05A	0.4A
Solenoid Valves	4	0.3A	1.2A
Communication Device	1	0.3A	0.3A

Total Continuous Load = 3.9A

For a 24V DC system:

Power = Voltage × Current

Therefore:

24V × 3.9A = 93.6W

However, a 4A power supply may not always be the correct choice. The calculation should also consider peak current, temperature and future expansion.

2. Understand SMPS Sizing for Control Panels

Proper SMPS Sizing for Control Panels requires more than matching the calculated load with the nearest available current rating.

For example, if your system consumes 4A continuously, selecting a 4A SMPS leaves very little operating margin. As a result, the power supply may operate close to its maximum capacity during normal conditions.

Instead, consider the following factors:

Continuous load
Peak load
Startup current
Panel temperature
Load characteristics
Future expansion

A larger SMPS is not automatically better. However, the selected power supply should provide a practical operating margin based on the actual application.

Therefore, engineers should evaluate the worst-case operating condition rather than relying only on nominal current calculations.

3. Consider Continuous Load and Peak Load

Not every automation device consumes the same current continuously.

Some devices require higher current during startup, switching or simultaneous operation. For instance, solenoid valves, relays and certain communication devices can create temporary peak demand.

Continuous Load

Continuous load is the current required during normal system operation.

Peak Load

Peak load is the maximum short-duration current required during startup or simultaneous switching.

Worst-Case Load

The worst-case load represents the highest realistic combination of devices operating at the same time.

Therefore, 24V DC SMPS Selection should consider all three conditions. Average power consumption alone does not provide a complete picture of the system’s requirements.

Additionally, temporary overload capability should not be considered as extra continuous capacity. Always check the manufacturer’s specifications before relying on overload or power reserve features.

4. Check Temperature Derating

Temperature is one of the most overlooked factors during SMPS Selection for Control Panels.

The temperature inside an industrial control panel can become significantly higher than the surrounding environment. VFDs, servo drives, contactors, transformers and other electronic devices generate heat during operation.

As a result, an SMPS that performs well during testing may provide reduced output capacity inside a hot enclosure.

This reduction is known as temperature derating.

What Should You Check?

Before finalizing an SMPS, review:

Maximum operating temperature
Manufacturer’s derating curve
Internal panel temperature
Ventilation arrangement
Distance from heat-generating devices

Therefore, always consider the actual installation environment. Proper spacing and airflow can improve thermal performance and increase the reliability of the power supply.

5. Verify Voltage at the Actual Load

Many engineers check the output voltage only at the SMPS terminals. However, the voltage available at the PLC, HMI or field device may be lower because of cable and terminal voltage drop.

Voltage drop becomes more important when:

Cable runs are long
Current demand is high
Cable size is insufficient
Field devices are far from the panel
Multiple connections are present

For example, an SMPS may provide a stable 24V DC output, while the farthest sensor receives a lower voltage.

Consequently, the system may experience intermittent faults that are difficult to identify.

Therefore, measure voltage at important load points during commissioning, including:

SMPS output
PLC terminals
HMI terminals
Remote I/O
Farthest field devices

A reliable SMPS for Industrial Control Panels should support stable voltage throughout the complete DC distribution system.

6. Choose the Right PLC Power Supply Architecture

A PLC Power Supply is one of the most critical elements in an automation panel. PLC CPUs and I/O modules require stable DC voltage for reliable operation.

However, it is not always advisable to connect every load to a single unprotected 24V DC branch.

For example, a panel may contain PLCs, HMIs, sensors, relays and solenoid valves. High-current field loads can create sudden demand or faults that may affect critical control equipment.

Therefore, consider separating loads where the application requires it.

Critical Control Loads

Critical loads may include:

PLC CPU
PLC I/O
HMI
Communication modules

Field Loads

Field loads may include:

Sensors
Relays
Solenoid valves
Other DC actuators

This approach can improve fault isolation and simplify troubleshooting. Additionally, suitable branch protection helps prevent a single fault from affecting the complete automation system.

7. Check SMPS Protection Features

Current rating and output voltage are important, but protection features also matter.

A good industrial SMPS may include:

Short-circuit protection
Overload protection
Overvoltage protection
Overtemperature protection

However, different SMPS models respond differently during fault conditions.

Some models may use hiccup mode, while others provide constant current limiting or automatic restart. Therefore, engineers should understand the protection behavior before finalizing a model.

Do not select an SMPS based only on:

24V + Current Rating + Price

Instead, review the technical datasheet and check whether the protection characteristics match the application’s requirements.

8. Confirm Input Voltage Compatibility

Before starting 24V DC SMPS Selection, verify the available input power supply.

Industrial applications may use different input configurations, such as:

Single-phase AC
Three-phase AC
Wide-range AC input
Universal AC/DC input

The selected SMPS must match the available supply and operating conditions.

Additionally, sites with voltage fluctuations or unstable incoming power may require suitable protection or power conditioning.

Therefore, input compatibility should always be checked before purchasing the power supply.

9. Plan for Future Expansion

Control panels often change after installation.

Additional sensors, I/O modules, communication gateways or Ethernet switches may be added later. If the existing SMPS already operates close to its maximum capacity, future expansion becomes difficult.

Replacing an SMPS after commissioning can result in:

Panel modification
Additional wiring
Machine downtime
Updated documentation

Therefore, future requirements should be considered during SMPS Sizing for Control Panels.

However, avoid unnecessary oversizing. The objective is to provide practical expansion capacity based on realistic future requirements.

10. Consider Redundant Power Supplies for Critical Systems

A single correctly sized SMPS is sufficient for many standard automation panels.

However, critical applications may require higher availability.

Examples include:

Continuous process plants
Water treatment systems
Critical infrastructure
Utility operations
Data-critical automation systems

In such applications, engineers may use two power supplies with a redundancy module.

The purpose is to maintain the control supply if one power source fails. Consequently, redundancy can reduce the impact of a single power supply failure.

Nevertheless, redundancy should be selected according to application requirements rather than added to every panel.

11. Pay Attention to SMPS Installation

Correct SMPS Selection for Control Panels is only one part of reliable panel design. Installation location also affects performance.

Avoid placing the SMPS directly beside high-temperature equipment unless the manufacturer allows it.

Consider:

Air circulation
Mounting orientation
Clearance requirements
Nearby heat sources
Accessibility for maintenance

Additionally, follow the manufacturer’s installation instructions carefully.

Proper component placement improves thermal performance and makes maintenance easier.

12. Consider Efficiency and Heat Generation

SMPS efficiency affects the amount of heat generated inside a control panel.

A more efficient power supply converts more input energy into usable output power. Therefore, lower power losses generally result in less heat.

This becomes especially important in:

Compact panels
High-density automation systems
High-temperature environments
Panels with multiple electronic devices

However, efficiency should not be the only selection criterion. Engineers should evaluate efficiency together with reliability, capacity and environmental performance.

Why SMPS Selection Requires Practical Panel-Building Experience

SMPS selection is not only a theoretical calculation.

In real industrial projects, panel builders and automation engineers must consider how the system behaves after installation.

A power supply that appears correctly sized on paper may still face issues because of:

High panel temperature
Long field wiring
Simultaneous load switching
Poor DC power distribution
Incorrect branch protection
Unexpected future additions

This is why practical engineering experience matters.

At United Control Engineers India Pvt. Ltd., our approach to industrial automation and control panel engineering considers the complete application rather than selecting components individually.

From PLC and HMI systems to relays, terminal blocks, communication devices and control panel architecture, every component must work together as part of a reliable automation system.

With more than 25 years of experience in industrial automation, system integration and control panel solutions, our team supports customers in selecting suitable automation components based on actual application requirements.

Conclusion: Choose an SMPS Based on the Complete System, Not Just Amps

The right SMPS Selection for Control Panels depends on the complete system, not just the current rating.

A reliable selection process should include load calculation, peak current evaluation, temperature derating, voltage drop analysis, DC power distribution and future expansion planning.

Similarly, proper 24V DC SMPS Selection improves the reliability of PLCs, HMIs, sensors and other automation components.

When selecting an SMPS for Industrial Control Panels, engineers should focus on actual operating conditions rather than simply choosing the lowest-cost or highest-rated model.

Ultimately, proper SMPS Sizing for Control Panels helps reduce unexpected failures, improve system reliability and support long-term automation performance.

Need Help Selecting an SMPS for Your Control Panel?

United Control Engineers India Pvt. Ltd. provides industrial automation products, technical support, system integration and customized control panel solutions.

Our technical team can help evaluate your application requirements and support the selection of suitable power supply solutions for PLC panels, automation systems and industrial control applications.

Contact United Control Engineers for expert assistance with industrial automation and control panel solutions.

Frequently Asked Questions

How do I calculate the correct SMPS size for a control panel?

List all devices connected to the 24V DC supply and calculate their total continuous current. Then evaluate peak demand, temperature derating, simultaneous operation and future expansion before selecting the final SMPS rating.

What voltage SMPS is commonly used in industrial control panels?

24V DC is widely used for PLCs, HMIs, sensors, relays and industrial automation devices. However, always verify the voltage requirements of each connected device.

Should an SMPS operate at 100% capacity?

Continuous operation at maximum rated capacity leaves little margin for peak demand, temperature effects and future expansion. The required reserve should be determined according to the actual application and manufacturer specifications.

Why does an SMPS fail inside a control panel?

Common reasons include overheating, overload, poor ventilation, incorrect input supply, excessive load demand and operation beyond the manufacturer’s specified environmental conditions.

Is redundancy required for every industrial control panel?

No. Redundancy is generally considered for critical applications where the loss of control power can cause significant downtime or operational consequences.

How does temperature affect SMPS performance?

Higher ambient temperatures can reduce the continuous output capacity of an SMPS according to its derating curve. Therefore, the internal temperature of the control panel should always be considered during selection.