Powering 12V field devices from a 24V PLC cabinet

Short answer: keep the 24 VDC supply on the processor and add a second DIN-rail supply at 12 VDC for the field. Size it so the total continuous low-voltage load sits at about 80% of its output rating — the worked example's 1.96 A list lands on a 2.5 A unit — feed it from AC through its own fuse, land it on labeled fused terminals, and wire the relay commons and sensor powers to that bus. The load list in the worked example below is illustrative; pull the real currents from each device datasheet before you order anything.

What the existing supply can and cannot do

The 24 VDC supply in a PLC cabinet runs the processor and the I/O backplane, and that is all it should run. The cards distribute no field power of their own. A relay output card carries dry contacts, so it closes a loop that you power from somewhere else. A DC input card senses a voltage between its input point and its common, and the sensor builds that voltage from its own supply. The field voltage is whatever you wire to the commons, and the cards take it as long as their voltage windows cover it. That makes three tempting shortcuts fail in predictable ways.

The resistor row deserves the emphasis. Holding a 12 V drop at a 2 A load burns 12 V × 2 A = 24 W continuously, a heater strapped to the rail. A switching supply delivering the same 24 W at 85% efficiency wastes about 4 W instead. If your cabinet genuinely needs a DIN-rail resistor for some other duty, the wattage and mounting arithmetic in the DIN-rail resistor selection guide is the same one that just sank this shortcut.

What you need

• A DIN-rail 12 VDC supply, sized in the next section. Plan roughly 70 mm [2.8 in] of rail for a 30 W unit and about 6 mm per fused terminal, typical compact DIN widths.
• Fused terminal blocks that take 5 × 20 mm glass fuses, in their own group labeled as the field bus.
• 18 AWG [0.82 mm²] machine-tool wire in a color the existing wiring does not use.
• Wire duct sized so the added wiring keeps the fill at or under 50% of the duct's interior cross-section, the NFPA 79 percentage-fill rule behind the wiring-duct fill tables (the backplate and DIN rail layout guide carries the layout details).
• Labels for both DC buses and for the new AC branch fuse.
• Rated cable glands for the field entry, since a sealed box stays sealed only through its entry parts.

Adding the supply, its clearance gap and a terminal group eats rail length and depth in a small box. The minimum-depth method in the minimum enclosure depth article tells you within a few millimetres whether the enclosure still fits.

Size the supply from the load list

Sum the continuous currents of everything that can run together, then divide by 0.8. DIN-rail supply vendors publish the 70-80% headroom target themselves: a supply run there stays cooler, its capacitors last longer, and one more device fits later. Read currents from device datasheets rather than wall-adapter labels, because the adapter label describes the adapter.

Worked example, illustrative machine:

1.96 A ÷ 0.8 = 2.45 A, and the standard DIN-rail output ratings run 1.3 A, 2.5 A, 5 A and 10 A across most supply lines. The example machine takes the 2.5 A, 30 W unit and sits at 78% of rating. On the AC side it draws 24 W ÷ 0.85 = 28 W, about 0.24 A at 120 VAC, which is why the datasheet's input-fuse line lands at 1-2 A to ride through switch-on.

Then check surges, not just the running total. An incandescent lamp pulls 10-15 times its cold running current, a small DC motor pulls its stall current — often 5-10 times rated — and an LED driver lands one capacitive spike of a few milliseconds. Most DIN-rail datasheets quote a boost, commonly 120-150% of rated current for a few seconds. The example machine's worst simultaneous surge is the stack light driver's spike, comfortably inside the 3 A boost of a 2.5 A unit. Add the surge figures from your datasheets; if the switch-on sum outruns the boost, stagger the outputs in the program or give that load its own supply.

Any nameplate reading 12 VAC needs an AC supply instead; a DC unit is the wrong part for it.

Wire the new bus in

1. Lock out the cabinet's AC feed and verify absence of voltage at the existing supply's input terminals before any wire moves.
2. Mount the second supply on the rail beside the first with the convection gap its datasheet calls for, about 2 in [50 mm] of free space above and below on compact units. The clearance and airflow points in the switching-supply mounting article apply to both units, and the backplate rail mounting article covers the rail itself.
3. Feed its AC input from its own fused tap, sized to the datasheet's fuse line. One AC entry feeding two DC outputs is a normal layout; the one-power-input layout article shows the branching, and a fused IEC power-entry module puts the branch fuse right at the connector in a small box.
4. Run +12 V and 0 V to the labeled fused-terminal group in the second wire color, and keep the duct they use at or under its 50% fill. The field bus stays off every processor-side terminal; each rail keeps its own labeled group.
5. Relay outputs: feed the card's common from the fused group, run each output point to its load's + lead, and return each load's 0 V side to that group. Add what shares a common: the example's two 0.5 A valves plus the 0.4 A tower put 1.4 A through one common, and the card datasheet's per-contact and per-common ratings must both carry it.
6. Sensors on the field bus: power each from the fused group, wire the signal to an input point, and tie the input common to that rail's reference as the card's sink/source diagram shows. Confirm the card's guaranteed-ON window actually contains 12 V, since a card specified for 24 V only can leave the sensor between the ON and OFF windows.
7. Fuse each branch just above its continuous current and at or under the wire's ampacity: 0.5 A glass for the 0.3 A sensor group, 2 A time-lag for the 1.0 A valve pair so a switching spike never becomes a nuisance blow.
8. Bring field cables in through glands rather than a naked drilled hole. Our stainless wall-mount boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and the assembly keeps that rating only through entry parts rated to match. An M20 gland clamps roughly 6-13 mm of cable OD, the range most gland dimension tables print. The rating-after-cutting article walks the kept-rating question point by point.

Checks before the door closes

• Measure the field bus at the fused terminals with no loads. Most units ship trimmed slightly above nominal, so expect 12.0-12.5 V.
• Measure the processor rail at the CPU. It should sit within a tenth of a volt or so of the pre-work reading; a moved reading means the two supplies share something they should not.
• Force each relay output and trigger each sensor, and confirm both the card LED and the program tag change state.
• Measure at the device end of the longest run with its load running. The example's 25 ft [7.6 m] 18 AWG round trip to a 0.5 A valve drops 0.5 A × 0.32 Ω = 0.16 V, about 1.3% of 12 V, using the standard copper figure of 6.4 Ω per 1000 ft at 20 °C. Hold every run under 5%, or 0.6 V on this rail, unless a device datasheet demands tighter, and add about 8% for a 40 °C (104 °F) cabinet, because copper resistance rises 0.39% per °C.
• Switch on everything that can run at once and re-measure the field bus. It should stay within about 0.6 V of the no-load reading; a bigger sag or a hiccuping supply means the load list was short or a surge outran the boost.
• Run 30 minutes at full load. A case you can still hold a thumb on, roughly under 60 °C (140 °F) by the shop rule, is normal for a unit at 78%; a case too hot to touch points to crowding or a missing clearance gap.

Mistakes that undo the fix

• A 24 V wire landed on the field group. The most common kill in mixed-voltage panels. Separate labeled groups and the two-color rule prevent it.
• Sizing from the wall-adapter label. The label describes the adapter, not the device; datasheet currents are the ones that sum.
• A floating input common. The sensor lights and the input will not, or it flickers. The tie in step 6 fixes it for good.
• An overloaded relay common. Per-contact ratings get checked and per-common sums get missed; the example's 1.4 A common has to fit both limits.
• One input card reading both rails. A card whose common sees sensors from both supplies needs one reference, so tie the two 0 V rails; with one voltage per card, leave the buses separate.
• An entry hole closed with whatever was in the drawer. The weakest entry part sets the assembly rating, so an unrated plug drags a Type 4X panel down to that plug's rating.

The decision, by load size

Every row uses the same pattern: the processor keeps its own rail, the field gets its own fused bus, the commons carry the field voltage, and each branch fuse is sized to its wire. Measure the all-on bus voltage and the case temperature before the door closes, and the fix holds for years.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions