How to size a switching power supply for a control panel: headroom for inrush, derating above 40 C, single vs multi-output, and when to choose a DIN-rail or enclosed unit.
A power supply is the cheapest part of the panel and the one most likely to be replaced. It is sized wrong in one of two ways: too close to the load, so it runs hot and trips at startup; or grossly oversized, so it wastes money and, at low load, may not meet its own efficiency or regulation figures.
Use the datasheet current for every device, not the "typical". PLC and IO modules, relays and contactor coils, sensors, indicator lamps, a display backlight, a modem or gateway. Add 10 % for wiring and terminal losses.
Then add the inrush requirement. Contactor coils, DC motors and capacitive-input loads draw a multiple of their steady current for tens of milliseconds. A supply with a comfortable average margin but no surge headroom will shut down or drop the 24 V rail every time a contactor pulls in — the classic intermittent fault.
Almost every industrial supply is specified at 25–40 °C and must be derated above that. A unit rated 150 W at 40 °C typically delivers about 80 % of that at 50 °C and around 60 % at 60 °C. Inside an unventilated enclosure the internal temperature is 10–20 K above room, so a 150 W supply in a closed box in summer may only be good for 100–110 W.
Two practical consequences: mount the supply low in the enclosure where air is coolest, and either ventilate the enclosure or specify the next frame up. Never mount a supply directly above a transformer or a VFD.
| Configuration | Use it when | Watch out for |
|---|---|---|
| Single output, e.g. 24 V | One rail feeding the whole panel | Voltage drop on long runs; use 4-wire sensing if available |
| Multi-output (e.g. 5 V / 12 V / 24 V) | Legacy equipment needing several rails | Shared return and cross-regulation; each rail's current is limited independently |
| Redundant (two units + diode/ORing module) | Continuous process, cannot tolerate a stop | Cost, and the module itself must be monitored |
| Battery-backed / UPS module | Controlled shutdown after mains loss | Battery chemistry and replacement interval |
Check that the input covers the worst case, including brown-outs: a "230 V" market with ±10 % tolerance needs a supply that holds regulation down to about 180 V. Wide-range input types (85–264 V AC) remove the risk of a wrong mains link being selected on site. Confirm the built-in protection set: short circuit, over-current, over-voltage and thermal shutdown. For outdoor or exposed installations add an external surge protective device; a power supply's own input protection is not a lightning arrester.
DIN-rail types suit panel building: no drilling, fast replacement, and the rail itself acts as a small heat sink. Enclosed types with screw terminals suit machine-mounted or standalone use. Open-frame types are for OEM products where the assembly provides the enclosure. Whichever you choose, leave free space above and below — convection cooling needs a chimney, and a supply boxed in on all sides is a de-rated supply.
ELEHPD supplies single- and multi-output switching power supplies, DIN-rail modules and LED drivers from 15 W to several hundred watts, with the matching enclosures and panel accessories.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.