The humble 24 V DIN-rail power supply feeds everything that thinks in a modern panel — PLC, HMI, sensors, relays, network gear — and its failure takes all of them down at once. Sizing one properly is ten minutes of arithmetic; skipping that arithmetic is one of the most common root causes of “random” panel faults we see. Here is the method.
Step 1: Sum the real load — including inrush
List every 24 V consumer with its rated current: PLC rack, I/O (count sensor supply per point), HMI, valves/solenoids, relays, switches, network devices. Separate continuous from switching loads. Solenoids and contactor coils draw multiples of rated current at pull-in; DC motors and capacitive loads (drives’ control inputs, some HMIs) have real inrush. Apply a sizing margin: a supply loaded to 60–70% of rating runs cooler, survives inrush without folding back, and leaves room for the I/O that always gets added later. A PSU run continuously at 95% of rating is a scheduled failure.
Step 2: Choose the protection behaviour
How a supply behaves at overload matters as much as its rating. Hiccup-mode supplies shut down and retry — clean for hard shorts, but a motor inrush can leave the rail bouncing. Constant-current / power-boost supplies ride through inrush by limiting current — the better choice for panels with solenoids and motors. For branch protection, pair the supply with electronic circuit protectors or correctly rated breakers per circuit: a single 20 A supply behind ten unfused sensor branches turns one crushed cable into a whole-panel outage — and note that tripping a thermal-magnetic breaker on a current-limited DC supply requires the supply to actually deliver the trip current; selectivity needs checking, not assuming.
Step 3: Environment and lifetime
Electrolytic capacitors inside the PSU age with temperature — the same 10°C-halves-the-life arithmetic as everywhere else in power electronics. Mount supplies with their specified convection clearance, out of the hot top zone of the enclosure, derate per the datasheet above 40–50°C ambient, and treat manufacturer lifetime figures (often quoted as expected life at rated load and temperature) as the design variable they are. A quality supply in a cool panel outlives the machine; the same supply above a heat-generating drive does not.
How 24 V supplies actually fail
- Gradual capacitor aging: output ripple rises before output fails — unexplained PLC brown-out resets and HMI reboots are the classic early symptom. Measure ripple under load on any panel with mystery resets.
- Fan failure on fan-cooled units — prefer convection-cooled designs in dusty environments.
- Overload operation: supplies run at or above rating fail in a fraction of design life.
- Surge events: lightning-adjacent supply transients kill input stages; panels in exposed sites deserve surge protection upstream.
Redundancy where downtime is expensive
For critical machines, two supplies with a redundancy/decoupling module give N+1 on the 24 V rail for modest cost — the cheapest availability upgrade in the panel. Buffer modules that ride through brief mains dips solve the “whole line reboots on a 200 ms flicker” problem at similar effort.
PLC North stocks tested DIN-rail supplies from Siemens (SITOP), Phoenix Contact, and other major brands, plus redundancy modules — each load-tested before dispatch. Send us your panel’s load list and we will sanity-check the sizing for free. Browse our tested power supplies.