How to Size a DIN Rail Power Supply Correctly
A DIN rail power supply feeds PLCs, sensors and actuators in the control cabinet with stable 24 V DC. This guide shows how to size the output current with reserve, use a wide-range input and select supplies confidently by MTBF and DIN EN conformity.
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How do I size the output current correctly?
First add up the current demand of every load on the 24 V rail: PLC, input and output cards, relays, sensors and actuators. Onto this sum add a reserve of 20 to 30 percent so the supply never runs permanently at its limit and future extensions stay possible.
Watch the inrush current. Capacitive loads and motors briefly draw a multiple of their rated current. Supplies with a power reserve (power boost) deliver up to 150 percent for a few seconds to cover such peaks and to trip circuit breakers reliably.
Why does the wide-range input matter?
A wide-range input accepts input voltages of roughly 85 to 264 V AC without any switching. The same supply is then usable worldwide and tolerates mains fluctuations and short brownouts without the output voltage collapsing.
- Single-phase wide-range inputs cover 100 V (Japan) to 240 V (EU) in one device.
- Three-phase supplies (320‑575 V AC) make sense for high power from about 480 W upward.
- An active power factor correction filter (PFC) reduces mains distortion and is often mandatory above 75 W under EN 61000‑3‑2.
- Check the permissible temperature range; power derating usually starts around 45 to 60 °C.
What do MTBF and DIN EN conformity tell you?
MTBF (Mean Time Between Failures) statistically describes the mean operating time between two failures across a large population of units. Values of 500,000 to over 1,000,000 hours per MIL-HDBK-217F are common. Important: MTBF is a failure rate, not a service-life guarantee for a single unit.
Service life is often limited by the electrolytic capacitors and strongly driven by operating temperature. A lower internal temperature, enough clearance inside the cabinet and moderate loading extend the real lifetime considerably.
Our specialists size your supply including reserve and redundancy.
Request adviceWhen are redundancy and buffering worthwhile?
For critical processes two supplies are connected in parallel through a redundancy module with decoupling diodes (n+1). If one unit fails, the second takes over the full load without any voltage dip.
- A redundancy module with MOSFET decoupling instead of plain diodes cuts power loss.
- Buffer modules bridge short mains dropouts from milliseconds to seconds.
- A DC UPS with battery keeps the controller alive and enables a safe shutdown over minutes.
- A signal contact (DC-ok) reports undervoltage early to the PLC.
Frequently asked questions
How much reserve should a DIN rail power supply have?
Plan 20 to 30 percent above the summed rated current of all loads. That leaves room for inrush currents, ageing and later extensions without permanently overloading the unit.
What does wide-range input mean?
The supply operates across the whole range of roughly 85 to 264 V AC without switching. This makes it usable worldwide and tolerant of mains fluctuations and short brownouts.
Does a high MTBF equal a long service life?
No. MTBF is a statistical failure rate across many units, not a guarantee for a single device. Real service life depends mainly on operating temperature and loading.
Which standard must a supply meet for CE marking?
Product safety follows DIN EN 62368‑1, together with the EMC standards EN 61000‑6‑2 and -6‑4. A SELV output ensures the touch-safe extra-low voltage.
Looking for the right DIN rail power supply?
We supply 24 V units with wide-range input, power boost and high MTBF - tested to DIN EN 62368-1 and matched to your control cabinet.
Standards tested
Supplies to DIN EN 62368-1 and EMC standards.
High MTBF
Selected units with over 500,000 h rating.
Usable worldwide
Wide-range input 85-264 V AC from stock.
Expert advice
We help size reserve and redundancy.