When a PLC fails, everyone notices. When an HMI goes dark, operators immediately escalate. But when a power supply starts degrading, the symptoms are often subtle—until suddenly they're catastrophic.
Industrial power supplies convert incoming AC to the DC voltages your control systems need. They're in every panel, usually mounted in a corner and forgotten. And they're responsible for more mysterious control system failures than most engineers realize.
How Power Supplies Fail
Power supplies don't usually die suddenly. They degrade over time, and the degradation causes problems that look like something else is failing.
Capacitor Aging
Electrolytic capacitors are the lifespan-limiting component in most power supplies. They dry out over time, especially in warm environments. As capacitance decreases, the supply's ability to maintain stable voltage under load diminishes.
Symptoms: Random PLC faults, communication errors under load, equipment that works fine in the morning but acts up in the afternoon (when the panel is warmer).
Output Voltage Drift
Power supplies have adjustment pots for output voltage. Over time, these can drift, or the supply's regulation circuitry can degrade.
Symptoms: 24VDC devices acting erratically. Sensors that were reliable start giving intermittent signals. I/O cards report values slightly out of range.
Overload Without Overcurrent
A degraded power supply may not trip its overload protection even when it can no longer deliver rated current. The voltage sags instead of the supply faulting.
Symptoms: Problems that correlate with certain machine states—when a particular valve bank energizes or when multiple devices demand power simultaneously.
Signs Your Power Supply Needs Attention
Measure your 24VDC (or whatever voltage your system uses) at multiple points in the panel, under normal operating conditions:
- At the power supply terminals: Should read close to nominal. If a "24VDC" supply reads 22V at the output terminals, it's not providing what it should.
- At the end of your longest wire run: Some voltage drop is normal, but if you're seeing more than 0.5-1V drop, investigate your wire sizing or check if the supply is struggling.
- During high-demand states: Check voltage when your heaviest loads are energized. A marginal power supply will show voltage sag that recovers when loads drop.
Common Industrial Power Supplies
| Brand | Common Models | Typical Application |
|---|---|---|
| Allen-Bradley | 1606-XL series | ControlLogix/CompactLogix panels |
| Siemens | SITOP series (6EP1) | S7 PLC systems |
| Phoenix Contact | QUINT, TRIO series | General industrial |
| PULS | Dimension series | Compact high-efficiency |
| Mean Well | SDR, NDR series | DIN rail general purpose |
When to Proactively Replace
Power supplies in continuous industrial service typically last 7-15 years, heavily dependent on operating temperature. Consider proactive replacement when:
• The supply is more than 10 years old
• The panel runs warm (consistently above 40°C/104°F)
• You're seeing unexplained intermittent issues elsewhere in the panel
• The supply shows physical signs of stress (discoloration, bulging capacitor tops visible through vents)
A replacement power supply costs a few hundred dollars. The downtime from an unexpected failure costs much more.
Stocking Spares
Power supplies are excellent spare parts candidates:
- They're universal—the same supply can often work in multiple panels
- They don't require programming or configuration
- Swap time is measured in minutes, not hours
- Shelf life is long if stored in reasonable conditions
Recommendation: At minimum, stock one spare of each power supply rating you commonly use. For critical lines, consider having a spare mounted in the panel on standby.