How to Choose a 24 V DC Power Supply (SMPS) for PLCs, Sensors and Control Panels

The short answer

A DIN-rail SMPS turns 230 V AC (or 400 V three-phase) mains into a regulated, isolated 24 V DC output for PLCs, sensors, relays, valves and HMIs. Five checks:

• Size by current. Add each load’s datasheet running current, multiply by 1.2–1.3, check that start-up peaks stay within the supply’s rating, and pick the next size up from our power supplies.
• Allow for heat. Output is derated above a stated ambient, and a closed panel is hotter than the room.
• Match the mains. A wide input (typically 85–264 V AC) copes better with voltage swings and generator or UPS changeover.
• Protect every branch. One shorted cable should not reset the PLC: use DC-rated breakers or electronic fuses per branch.
• Add redundancy where downtime is costly. Two supplies and a redundancy module keep the panel running.

How to size a 24 V DC power supply

Add up the real current

List every device on the rail: PLC CPU, I/O modules and HMI (see controllers), sensors, relay and contactor coils, and solenoid valves. Take each current at 24 V DC from its datasheet (the maximum, if a range is given); if only watts are listed, divide by 24. Don’t rely on label ratings, which may show something else, such as the load it can switch.

Amps, watts and margin

Power is voltage times current (P = 24 V × I), so 5 A is 120 W and 10 A is 240 W. Size on amps: rated output current must exceed your total. Then add 20–30% for datasheet tolerances and future additions. A supply that runs well below its limit runs cooler and usually lasts longer.

Peak versus continuous current

Continuous current is what the loads draw once running; the peak is the surge at switch-on. Some valve and contactor coils draw more while pulling in than while held (datasheets usually list both), and devices with large input capacitors draw a brief charging current. Some supplies allow a short overload to cover this and some do not, so check the datasheet. After load-shedding everything restarts together, so add up what starts at once or stagger the start in the PLC program.

Worked example: a small PLC panel

• Peak: the valves (assumed) draw 0.30 A each while pulling in, against 0.12 A held. The extra is 4 × (0.30 − 0.12) = 0.72 A, so the peak is 2.18 + 0.72 = 2.90 A.
• Margin and heat: 2.18 × 1.25 (25% margin) = 2.73 A. If the datasheet allows 80% of rated current at your worst-case panel temperature (assumed), the rating must be at least 2.73 ÷ 0.8 = 3.4 A.
• Result: a 2.5 A supply fails the margin and heat checks and has no room for the 2.90 A peak unless its datasheet allows a short overload. A 5 A (120 W) supply gives 5 × 0.8 = 4.0 A when hot, covering 2.73 A and the 2.90 A peak.

Temperature, mains input and efficiency

Temperature. A supply gives full output only up to a stated ambient (typically 40–60 °C, model-specific) and is derated above it. The ambient that counts is the air beside the supply inside the closed panel, which in a Pakistani summer can approach that limit. Size for the worst case.

Mains input. Many DIN-rail supplies accept a wide range, typically 85–264 V AC or 100–240 V AC, which keeps the output regulated while the mains stays inside the stated range (check for derating at low input). An interruption longer than the hold-up time (typically tens of milliseconds) can reset the PLC; a buffer module or DC UPS bridges longer gaps. Never connect a single-phase 230 V supply between two phases (400 V) unless the datasheet allows it.

Inrush and surges. At switch-on a supply draws a short inrush pulse, often many times its rated input current. Use the upstream MCB rating and curve the datasheet recommends (commonly B or C; C tolerates more inrush) rather than a bigger breaker, which must still protect the input cable. Add a surge protective device where lightning or heavy switching is a risk.

Efficiency and quality. Losses become heat in the panel: at 240 W output, 90% efficiency wastes about 27 W and 95% about 13 W. Compare regulation and ripple (output steadiness and residual AC), mainly for analogue inputs, and keep the voltage at the PLC within its datasheet range (IEC 61131-2 covers PLC supply tolerances).

Output protection, branches and redundancy

On overload the supply protects itself in one of a few ways. Constant current holds the output near a fixed limit while the voltage falls, then recovers by itself. Foldback lowers both voltage and current, which can stop a heavy or capacitive load starting. Hiccup switches off and retries repeatedly, and some models latch off until the mains is cycled. In every case the 24 V rail sags or drops out, so one short on a sensor cable can reset the PLC.

That is why one supply should not feed everything unprotected. Split the rail into branches (PLC and I/O, sensors, coils and valves) and protect each with an electronic fuse that cuts off the faulty branch while the rest stays up, or a circuit breaker rated for DC at 24 V. A supply that limits its fault current may not trip a large breaker quickly, so keep branch ratings low and runs short.

Where downtime is costly, use two supplies, each able to carry the full load (1+1), or N+1 (enough supplies for the load plus one spare), joined by a redundancy module so a failed unit cannot pull down the others. Feed each from its own breaker, and never join outputs directly unless the datasheet supports parallel operation.

Wiring and installation

• Wire size. Size conductors for the breaker that protects them, then check voltage drop, which matters more at 24 V: 5 A over 10 m (20 m out and back) of 1.5 mm² copper loses over 1 V (about 5%), against roughly 0.7–0.8 V for 2.5 mm².
• Layout and terminations. Keep + and 0 V conductors together, use distribution terminal blocks rather than daisy chains, keep runs short, fit insulated ferrules and tighten terminals to the datasheet torque.
• Separation. Keep 24 V DC cables out of ducts carrying 230 V AC or motor cables; use separate ducts or a barrier.
• Earthing. Connect the supply’s PE terminal if fitted. Earthing the 0 V is a design decision: follow the PLC and machine documents, and if it is earthed, use one point only.

Selection checklist

1. List every load and its datasheet running current at 24 V DC, and note the start-up peaks of coils, valves and capacitive loads.
2. Multiply the running total by 1.2–1.3, apply derating for your worst-case panel temperature, check that the peaks stay within the supply’s rating, and choose the next size up.
3. Match the input to your mains: single or three-phase, wide range, low-voltage derating.
4. Check the upstream MCB rating and curve, and the output mode (constant current, foldback or hiccup).
5. Plan branch protection and, for critical lines, redundancy or buffering.
6. Check rail fit, clearances, ripple and the approvals on the datasheet.
7. Plan the wiring: wire size, voltage drop, ferrules, separate ducts and earthing.

Browse our power supplies and controllers (PLC brands on the site include Siemens, Delta and Mitsubishi), or send us your load list on WhatsApp and our team will help you choose. For coil and PLC-output ratings, see relay vs contactor vs SSR; for wiring sensors to a 24 V PLC input, see PNP vs NPN proximity sensors.