Industrial Power Supplies: The Unsung Hero of Automation Reliability - PLC ERA

Industrial Power Supplies: The Unsung Hero of Automation Reliability

Introduction: The Foundation of Every Control Panel

Behind every reliable PLC, every precise servo drive, every responsive HMI, and every accurate sensor lies an often-overlooked component: the industrial power supply. It converts raw AC line power into stable, clean DC voltage—typically 24V DC—that powers the brains and senses of the automation system.

When a power supply fails, the entire control system stops. No other component has such a single point of failure potential. Yet power supplies are frequently selected as an afterthought: the cheapest unit with sufficient current rating, installed in the corner of the panel and forgotten until it fails.

This guide changes that approach. We will examine the characteristics of industrial DIN rail power supplies: how to size them correctly (including inrush and derating), how to achieve redundancy for critical systems, when to add buffer or UPS modules, and how to protect against short circuits, overloads, and transients. We will also look at emerging trends: power supplies with IO-Link diagnostics, energy-efficient designs, and predictive failure alerts. By the end, you will be equipped to select power supplies that maximize uptime and simplify maintenance.

Part 1: Power Supply Basics for Automation Engineers

1.1 The Role of the Industrial Power Supply

An industrial power supply performs three essential functions:

  1. Voltage conversion: Transforms the incoming AC voltage (120V, 230V, 400V, 480V) to the required DC voltage (most commonly 24V DC, but also 12V, 48V, and 5V for special applications).

  2. Voltage stabilization: Maintains a constant output voltage despite fluctuations in input voltage or load current.

  3. Electrical isolation: Separates the output DC circuit from the input AC circuit, protecting sensitive electronics from line noise and transients.

In addition, modern industrial power supplies provide protection functions (overload, short circuit, overvoltage) and may offer diagnostic outputs, redundancy modules, and communication capabilities.

1.2 Why 24V DC Is the Industry Standard

The vast majority of industrial sensors, PLCs, HMIs, relays, and contactors operate on 24V DC. Reasons include:

  • Safety: 24V DC is considered low voltage, reducing shock hazards.

  • Standardization: Global adoption allows interchangeability of components.

  • Signal integrity: 24V DC provides sufficient energy for reliable switching over long cable runs (up to hundreds of meters).

  • Compatibility: Most I/O modules, sensors, and actuators are designed for 24V DC.

Other voltages: 48V DC is common for larger relays, contactors, and some motor brakes. 12V DC appears in older equipment or mobile applications. 5V DC is used for internal logic on some circuit boards but rarely distributed externally.

1.3 Primary vs. Secondary Switch-Mode Power Supplies

Virtually all modern industrial power supplies are switch-mode power supplies (SMPS) , which offer high efficiency (typically 85–95%), wide input voltage ranges, and compact size compared to older linear supplies.

Key characteristics to evaluate:

  • Efficiency: Percentage of input power delivered to output. Higher efficiency = less heat in the panel (reduces cooling requirements) and lower electricity cost.

  • Input voltage range: Wide-range units (85–264V AC or 88–370V DC) accept any global mains voltage without jumpers.

  • Hold-up time: How long the output remains within spec after input power is lost (typically 20–50 ms). Critical for PLCs to execute controlled shutdowns.

  • Ripple and noise: Residual AC component on the DC output (typically <50–100 mV peak-to-peak). Lower is better for sensitive analog circuits.

Part 2: Sizing Your Power Supply

2.1 Calculating Total Load Current

The first step is to sum the current consumption of all devices powered by the supply. Common 24V DC loads:



Device Type Typical Current Draw Notes
PLC CPU (e.g., Delta DVP) 0.2–1 A Depends on model and expansion modules
Digital input module (8 points) 0.05–0.1 A (module) + 0.01 A per active input Input current is small; field devices draw separate power
Digital output module (8 relay) 0.1–0.2 A (module coil current) Load current not drawn from power supply if external source used
Analog input module (4 channels) 0.1–0.2 A Minimal
HMI (7" touch) 0.5–1 A Touch and backlight draw
Inductive proximity sensor 0.01–0.05 A
Photoelectric sensor (through-beam) 0.03–0.1 A (emitter+receiver)
Solenoid valve (single coil) 0.2–0.8 A Inrush higher
Relay (24V DC coil) 0.05–0.2 A
Indicator light (LED) 0.01–0.03 A

Important: For outputs that switch DC loads (solenoids, relays, indicator lights), the load current may be drawn from the same power supply. Calculate carefully. For large contactors or motor starters, use a separate power supply or an interposing relay.

Example:

  • PLC CPU (0.5 A) + 2 digital input modules (0.2 A) + 1 analog module (0.1 A) + HMI (0.7 A) + 10 inductive sensors (0.3 A total) + 4 solenoid valves (2 A total) = 3.8 A continuous.

2.2 Accounting for Inrush Current

Many DC loads draw a high initial current when first energized (inrush), lasting milliseconds to tens of milliseconds. Examples:

  • Capacitive loads: The input capacitors of PLCs, HMIs, and drives create inrush.

  • Solenoids and contactors: Magnetic inrush until armature seals.

  • Incandescent lamps: Low cold resistance.

A power supply may shut down or voltage dip if inrush exceeds its short-term capability. To mitigate:

  • Select a power supply with peak current capability: Many industrial supplies can deliver 150% of rated current for 5 seconds or more.

  • Stage the startup: Use delay timers or the PLC to enable loads in sequence (e.g., enable outputs 500 ms after power-up).

  • Add buffer modules: See Section 5.

Rule of thumb: Size the power supply for the maximum steady-state current plus 50% for inrush margin, or specify a supply with explicit peak current rating.

2.3 Derating for Temperature and Mounting

Power supplies are rated at a specific ambient temperature (usually 40°C or 50°C). Above that, the output current must be derated.



Temperature Derating Factor Example: 10 A supply
40°C 100% 10 A
50°C 80% 8 A
60°C 60% 6 A
70°C 40% 4 A

Additionally, mounting orientation affects cooling:

  • Standard horizontal (input terminals at bottom): Rated airflow.

  • Vertical (input terminals on side): May require 10–20% derating.

  • DIN rail mounting with limited space: Adjacent heat-generating devices (VFDs, large relays) increase ambient temperature. Derate accordingly.

Action: Determine the maximum temperature inside the control panel (measure during summer). Derate the power supply accordingly, or upsize to compensate.

2.4 Recommended Sizing Margins

For reliability and future expansion, add margin beyond calculated load:



Application Sizing Margin Rationale
Non-critical, simple machine 25% Allows for minor additions
General industrial 50% Accommodates future sensors, HMIs, or I/O
Critical process (no redundancy) 100% (double) Run two supplies at 50% load each; if one fails, the other can take full load
High ambient temperature 50–100% extra to compensate derating Ensures sufficient output at elevated temperatures

Part 3: Types of Industrial Power Supplies

3.1 Standard Single-Phase Supplies

Most common for control panels fed from standard 120V or 230V AC outlets.



Form Factor Power Range Typical Applications
Miniature (e.g., 40–100 W) 1–4 A Small panels, single machine control
Standard (e.g., 120–480 W) 5–20 A Most machine control panels
High-power (e.g., 960 W+) 20–40 A Large panels with many loads, distributed I/O systems

Example models: Delta DRC series, DRP series (up to 960 W), DRL series (slim).

3.2 Three-Phase Supplies

For 400V/480V three-phase mains, often found in larger machinery or facilities where single-phase is not available. Advantages:

  • Higher power density (up to 2000+ W).

  • Reduced input current for same power, smaller circuit breakers.

  • Better resilience to phase imbalance.

Note: Three-phase supplies are not inherently redundant; they still have a single output stage.

3.3 Redundancy Power Supplies

For critical applications where a single supply failure cannot be tolerated (e.g., emergency shutdown systems, data logging), use redundant power supplies.

Configuration:

  • Two identical power supplies.

  • A redundancy module (also called decoupling module) with two input diodes (or MOSFET ideal diodes) and one output.

  • If one supply fails, the other continues to power the load without interruption.



Redundancy Type Method Advantages Disadvantages
Passive (diode) Two diodes in series with each output Simple, low cost Voltage drop (0.7–1 V per diode), wasted heat
Active (MOSFET) Ideal diode controller with ultra-low drop No meaningful voltage drop, less heat Higher cost, more complex
1+1 with load sharing Both supplies share current equally; each sized for 60% of full load Supplies run cooler, longer life Requires special load-sharing units

Application: A 10 A load: use two 10 A supplies and a redundancy module. Each supply sees ~5 A steady-state. If one fails, the other handles 10 A within its capacity.

3.4 Buffer and Capacitive Storage Modules

Buffer modules (also called capacitive storage modules or energy storage modules) provide short-term power backup when input AC dips or fails. They contain banks of electrolytic capacitors that charge from the power supply and discharge when input voltage drops.

Specifications:

  • Hold-up time: Typically 20–2000 ms depending on load current and capacitor size.

  • Buffered output: The buffer module sits between the power supply and the load.

Use cases:

  • Bridge the gap between an AC power glitch and the response of a UPS (uninterruptible power supply) or generator.

  • Allow a PLC to execute a controlled shutdown after an AC power loss.

  • Absorb high short-term loads that would otherwise cause voltage dip (e.g., multiple solenoid valves energizing simultaneously).

3.5 Industrial UPS Systems

For longer backup (minutes to hours), use an industrial UPS, which contains batteries (usually sealed lead-acid or lithium). Unlike office UPS units, industrial UPS are designed for DIN rail mounting, wide temperature ranges, and communication with automation systems.

Types:

  • Offline/standby UPS: Switches to battery when AC fails. Small delay (2–10 ms) – may cause PLC reset if supply hold-up is insufficient.

  • Line-interactive UPS: Regulates voltage continuously, faster transfer.

  • Online (double-conversion) UPS: Inverter always powers the load; AC input charges batteries. Zero transfer time, best for sensitive equipment.

Integration: Industrial UPS often provide signals to the PLC indicating "AC fail" and "battery low," allowing the PLC to save data and shut down orderly.

Part 4: Protection and Diagnostic Features

4.1 Overload and Short-Circuit Protection

All industrial power supplies include some form of output overload protection. The behavior matters:



Protection Type Behavior on Overload Best For
Foldback current limiting Voltage collapses, current drops to low value (e.g., 20% of rated). Load must be removed or reset to recover. Sensitive loads that could be damaged by prolonged overcurrent
Constant current limiting (hiccup) Voltage drops, current limited to set value (e.g., 110%). Supply attempts to restart periodically. If short persists, continues hiccup. Most industrial loads; self-reset after fault removal
Trip and latch Supply shuts down and stays off until manually reset (power cycle). Critical systems where automatic restart is unsafe

Short-circuit tolerance: Reputable power supplies can withstand a dead short on the output indefinitely without damage (in hiccup or foldback mode).

4.2 Overvoltage Protection

If the internal voltage regulation fails, output voltage may rise above safe levels, damaging downstream devices. A secondary protection circuit (typically a crowbar or OVP) clamps the voltage to a safe level (e.g., 30–32 V for a 24 V supply) and may blow an internal fuse.

Select: Verify the supply includes OVP. Not all low-cost units do.

4.3 DC OK Signal and Remote Monitoring

Many industrial power supplies include a "DC OK" or "Power Good" output. This is an isolated transistor output (or relay contact) that changes state when the output voltage falls below a threshold (typically 90% of nominal) or the supply experiences a fault.

Uses:

  • Connect DC OK to a PLC input for remote monitoring.

  • Annunciate power supply failure on HMI.

  • Automatically switch to redundant supply if primary fails.

4.4 IO-Link Power Supplies (The Smart Trend)

Emerging in 2025–2026 are power supplies with IO-Link communication. These supplies provide not just DC OK, but a rich set of diagnostic data:

  • Output voltage and current (real-time).

  • Internal temperature.

  • Operating hours.

  • Number of overload events.

  • Impedance / aging status (capacitor health).

  • Predictive failure alerts.

With IO-Link, maintenance teams can monitor power supply health from the control system or cloud, enabling condition-based replacement rather than reactive failure or arbitrary preventive replacement.

Example: Delta DRP-IO series with IO-Link.

Part 5: Installation Best Practices

5.1 Panel Layout and Cooling

  • Mount vertically (input terminals at bottom) for natural convection cooling.

  • Leave clearance: 40–50 mm above and below for air flow.

  • Keep away from VFDs, large contactors, and braking resistors that generate heat.

  • Use a dedicated 24V DC bus bar for distributing power to multiple devices, rather than daisy-chaining with small wires.

5.2 Wiring and Fusing



Segment Protection Sizing
AC input circuit Circuit breaker or fuse Based on supply input current (e.g., 4 A for 480 W supply at 230V)
DC output circuit Fuse or circuit breaker per load branch Typically 1.5× load current, but not exceeding wire ampacity
DC common (0V) Do not fuse; must be continuous N/A

Use twisted pair for DC output: Especially for long runs to sensors, a twisted pair (two wires twisted together) reduces EMI.

Wire color code per IEC 60204-1:

  • Dark blue: 24V DC + (positive)

  • Blue: 24V DC common (0V)

  • Black: DC control signals (inputs/outputs)

5.3 Grounding (Earthing)

Proper grounding is critical for EMI immunity and safety:

  • Secondary (DC) ground: The 0V terminal of many power supplies is internally connected to the protective earth (PE) through a capacitor or resistor. Do not directly connect 0V to PE unless the supply is specifically designed for it (e.g., "functional ground" terminal).

  • Follow manufacturer guidelines: Some supplies require a connection from the 0V terminal to a separate ground terminal to suppress common-mode noise.

  • Ground the DIN rail: The metal rail should be connected to the panel ground bus.

5.4 Dealing with Parallel Operation

Some power supplies allow parallel operation for increased current capacity (e.g., two 10 A supplies to supply 20 A). Requirements:

  • Supplies must be identical model and firmware version.

  • A load-sharing wire (typically a dedicated terminal) must be connected between units.

  • Output connections must be of equal length and gauge.

  • Do not parallel standard supplies without load-sharing capability — one supply will carry most of the current and overheat.

Part 6: Sizing Examples and Common Mistakes

6.1 Sizing Example: Conveyor Control Panel

Loads:

  • PLC CPU + 2 input modules + 1 output module: 0.6 A

  • HMI 7": 0.7 A

  • 12 photoelectric sensors: 0.5 A total

  • 8 solenoid valves (6 W each at 24V = 0.25 A each): 2.0 A

  • Ethernet switch: 0.3 A

  • Relay outputs for motor contactors (coils): 0.4 A

Total continuous = 0.6 + 0.7 + 0.5 + 2.0 + 0.3 + 0.4 = 4.5 A

Add 50% margin = 6.75 A → select a 10 A power supply.

Derating for panel temperature (panel internal 55°C, supply rated 10 A at 40°C, derating factor ~70% at 55°C) → 10 A × 0.7 = 7 A available. Since 7 A > 4.5 A, supply is still sufficient. Margin is reduced but acceptable.

6.2 Common Mistakes



Mistake Consequence Correction
Undersizing for solenoid inrush When multiple solenoids energize simultaneously, voltage dips, PLC may reset or outputs drop out. Add buffer module or oversize supply for peak current. Sequence solenoid activation via PLC.
No redundancy on safety system Single power supply failure disables emergency shutdown functions. Use redundant supplies with decoupling module; each sized for full load.
Ignoring derating for temperature Supply fails prematurely after one summer. Measure panel temperature; derate accordingly or add panel fan.
Powering motors or actuators from same supply as PLC Motor start-up current causes voltage dip, resetting PLC. Use separate supply for motors/actuators (or use a buffer module).
Ground loops from multiple supplies Unstable analog readings. Establish a single-point 0V ground reference; use isolated converters if needed.

Part 7: Maintenance and End-of-Life Indications

7.1 Lifespan Expectations

Industrial power supplies use electrolytic capacitors that gradually dry out over time, reducing capacitance and increasing ripple. Typical lifespan:

  • 10–15 years in cool, clean environment (30°C, light load).

  • 3–7 years at 50°C continuous full load.

  • 1–3 years at 70°C or extreme vibration.

Rule of thumb: Replace power supplies after 8–10 years of service in normal conditions to prevent unexpected failure.

7.2 Indicators of Approaching Failure

  • DC OK LED dim or flickering (not just off).

  • Unstable output voltage under steady load (measured with multimeter or oscilloscope).

  • Excessive ripple (>5% of output voltage).

  • Audible high-pitched whine (indicates capacitor degradation).

  • Elevated case temperature (above 70°C even at moderate load).

7.3 Condition Monitoring with IO-Link

If using an IO-Link power supply, monitor these parameters periodically:

  • Output current vs. baseline (increase indicates added loads or degradation).

  • Internal temperature (rise above historical average).

  • Capacitor ESR (equivalent series resistance) if provided.

Set alerts when drift exceeds 20% of initial values.

Part 8: Selection Checklist

Use this checklist when specifying an industrial power supply:

Sizing and Load

  • Total continuous load current calculated (including all devices).

  • Inrush current accounted for (peak load of solenoids, capacitive loads).

  • 25–50% margin added for future expansion.

  • Derating for maximum panel temperature applied.

Functional Requirements

  • Output voltage (24V DC standard; verify any special voltages needed).

  • Single-phase or three-phase input? Match facility power.

  • Redundancy required? If yes, plan for two supplies + redundancy module.

  • Buffer or UPS required for short hold-up or extended backup?

  • Communication/monitoring: DC OK only, or IO-Link/fieldbus for diagnostics?

Protection and Environment

  • Overload protection type (hiccup preferred).

  • Overvoltage protection (OVP) included.

  • Short-circuit protection (must be tolerant indefinitely).

  • IP rating (IP20 for panel mounting; IP66 for standalone outdoor).

  • Operating temperature range matches panel environment.

  • Certifications: CE, UL/cUL, ATEX (for hazardous areas), marine, etc.

Installation

  • DIN rail mounting (standard 35 mm).

  • Adequate clearance for cooling.

  • Wire sizes and terminal types (screw, spring, push-in).

  • DC OK signal wired to PLC input.

  • Separate distribution fuses for each branch circuit.

Conclusion: The First Line of Defense

The industrial power supply is not just a commodity component; it is the first line of defense against electrical disturbances and the foundation of reliable automation. A properly sized, correctly installed, and adequately monitored power supply will outlast most other components in the panel and cause the least downtime. Conversely, a marginal or poorly selected supply will manifest as mysterious resets, analog drift, intermittent sensor failures, and premature component aging — all of which are difficult to trace back to the power supply.

Invest the time to calculate loads accurately, apply derating, consider redundancy where critical, and utilize diagnostic features like DC OK and IO-Link. The incremental cost of a higher-quality, correctly-sized power supply is minimal compared to the cost of an unplanned production stop.

At PLC ERA, we stock a full range of industrial power supplies: standard single-phase, three-phase, redundancy modules, buffer modules, and IO-Link smart supplies from Delta, DRP series, DRL slim series, and other leading brands. Whether you need a compact 40 W supply for a simple machine or a redundant 20 A system for a critical process, our team can help you select the optimal configuration. Visit us at plcera.com for technical data, quotes, and application support.

References and Further Reading

  1. Delta Electronics. (2026). DRP/DRL Series Power Supply User Manual.

  2. PULS. (2025). Power Supply Basics and Selection Guide.

  3. Siemens. (2026). SITOP Power Supply Catalog.

  4. Phoenix Contact. (2026). TRIO and QUINT Power Supply Handbook.

  5. IEC 61204. (2025). Low-voltage switch mode power supplies.

  6. Rockwell Automation. (2026). *1606-XL Power Supply Selection Guide*.

  7. Schurter. (2025). Power Supply Sizing and Derating for High Temperature.

Article Tags

#IndustrialPowerSupply #24VDC #DINRailPowerSupply #RedundancyModule #BufferModule #IOLinkPowerSupply #PanelDesign #ControlPanel #PowerSizing #Derating #ElectricalProtection #PLC #AutomationReliability #DRPSeries

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