How to Size a Power Supply for a DIN Rail PC

Most DIN rail PCs run from the cabinet’s 24 V DC supply. That supply is usually a DIN rail power supply: an AC to DC converter that clips onto the same DIN rail and turns mains into the safety extra-low voltage used by control circuits. Choosing its rating looks simple, but an undersized supply causes some of the hardest faults to trace: a computer that reboots when cameras switch on their infrared, fails to start on a cold morning or drops out when a relay pulls in.

Oversizing is safer, but not free. A much larger supply costs more, takes more rail space and runs less efficiently at light load. The aim is a rating that covers the worst case with sensible headroom, in the conditions the cabinet will really see.

Write down everything that will draw from the supply, using the maximum power figure from each datasheet rather than the typical one:

The PC’s PoE circuitry is not perfectly efficient, so add around 10% to the PoE budget for conversion losses. Then add 20 to 30% headroom to the total. Running a supply at about 70 to 80% of its rating keeps it cooler, extends the life of its capacitors and leaves room for one more camera or module later.

Here is a worked example for a cabinet PC that powers two PoE+ cameras:

A 150 W, 24 V supply delivers 6.25 A, so that is the rating to look for. If the next size up is 240 W, choose it only if you expect to add loads. For larger camera counts, our guide to PoE budgeting for multi-camera edge AI goes into more detail.

A power supply’s headline rating usually applies up to a stated ambient temperature. Above that, many DIN rail supplies derate, delivering less power for every degree, and the curve in the datasheet shows by how much. Read it at the temperature near the top of the cabinet, where the supply and the PC usually sit and where the air is hottest, not at the room temperature outside.

Mounting orientation and spacing matter too. Most supplies are rated for vertical mounting on a horizontal rail with a stated gap above and below, and a supply squeezed between other hot devices may need to be derated further.

Inrush current. A PC’s input capacitors draw a short surge when power is applied. Supplies that switch to a constant-current mode on overload ride through it, while some that use a hiccup mode can fail to start large capacitive loads. Some supplies also offer a short power boost above their rating for starting loads and tripping breakers.

Voltage drop. Long, thin cables lose voltage. At 5 A, a 3 m run each way in 0.75 mm² copper drops about 0.7 V. That is fine for a PC with a wide 9 to 36 V input, but it matters for devices that need 24 V within 10%, so keep runs short or use a thicker cable.

Branch protection. Fuse or breaker each branch so a short in one device does not take down the PC. Check that the supply can deliver enough current to trip the protective device, or use electronic circuit breaker modules that are designed for 24 V supplies.

For critical cabinets, two supplies with a redundancy module give N+1 protection: each supply must be sized for the full load on its own, so the system keeps running if one fails. Some DIN rail PCs accept two DC inputs directly for the same purpose.

A supply’s hold-up time covers only brief mains dips, typically tens of milliseconds. To ride through longer cuts or shut the PC down cleanly, add a DC UPS between the supply and the computer. Our industrial UPS range includes supercapacitor modules for the rail, and our comparison of supercapacitor vs battery UPS explains which suits each site.

We supply our DIN rail PCs with the operating system, storage and wireless modules fitted and configured as standard, ready to install on the rail.