Why 24 V DC Is the Industrial Automation Standard
Industrial control panels combine signal circuits, actuators, and protective devices that all need reliable power. 24 V DC established itself as the standard for several reasons: it is high enough to switch relay coils reliably and drive PLC digital I/O, yet low enough that contact with terminals poses no risk to personnel. Most sensors (inductive, capacitive, photoelectric) accept 10–30 V DC, so 24 V sits comfortably in the middle. PLC manufacturers — Siemens, Schneider Electric, Veichi, Mitsubishi — design their modules around 24 V as the primary supply voltage.
What Draws Current in a Typical Automation Panel
Before selecting a power supply, list every 24 V DC load and its maximum draw:
- PLC CPU and I/O modules: typically 0.3–1.5 A depending on the number of expansion modules.
- HMI panel (7–10 inch): 0.5–1.2 A.
- Sensors (inductive, capacitive, photoelectric): 10–60 mA each; 20 sensors = 0.2–1.2 A.
- Interposing relays: 24 V coil draws 40–80 mA each.
- 24 V DC solenoid valves: 100–200 mA each.
- Indicator lights, buzzers: 20–50 mA each.
Sum all currents and add 20–30 % headroom. The full calculation method with coincidence factors is covered in How to Calculate Power Supply Wattage.
24 V Model Comparison for Automation Panels
| Model | Current, A | Power, W | Mounting | PFC |
|---|---|---|---|---|
| HDR-60-24 | 2.5 | 60 | DIN rail | No |
| MDR-60-24 | 5.0 | 60 | DIN rail | No |
| NDR-120-24 | 5.0 | 120 | DIN rail | No |
| LRS-100-24 | 4.5 | 108 | Enclosed | No |
| LRS-150-24 | 6.5 | 156 | Enclosed | No |
| NDR-240-24 | 10.0 | 240 | DIN rail | Yes (active) |
DIN Rail Mounting: Why It Matters in Automation Panels
Automation panels are built on DIN rail — PLCs, I/O modules, relays, and MCBs all snap on without drilling. A DIN-rail power supply takes 1–3 slots next to the controller and connects directly to the bus. HDR-60-24 and NDR-120-24 are the classic choice for medium-size panels with 60–120 W loads. MDR-60-24 delivers twice the current (5 A) at the same wattage, which helps when module startup draws spike above steady-state.
When PFC Is Needed in a PLC Power Supply
Active power factor correction (PFC) reduces harmonic distortion on the supply network. For small panels with total loads under 150–200 W, it is generally unnecessary. But if the cabinet connects to a sensitive distribution board, or the project specification references EN 61000-3-2 (harmonic limits), use NDR-240-24 with active PFC — it meets that standard. NDR-240-24 is the natural choice for large panels: 10 A / 240 W / DIN rail / active PFC in a single unit.
Step-by-Step 24 V Power Supply Selection
- List every 24 V DC load in the panel.
- Find the maximum current for each item from the datasheet or nameplate.
- Sum totals, then multiply by 1.25 (25 % headroom).
- Up to 5 A: HDR-60-24 or NDR-120-24.
- 5–7 A: LRS-150-24, or parallel MDR-60-24 units if needed.
- Above 8 A or PFC required: NDR-240-24.
Further Reading
For guidance on choosing between 12 V, 24 V, and 48 V for different applications, see Which Voltage to Choose for a Power Supply. The full range is in the Industrial Power Supplies section. 24 V models specifically are listed at 24V Power Supplies. All Mean Well products at Mean Well.
Send us your panel load list and we will recommend the right supply within one working day.
