24 V DC control power systems

Why 24 V DC

Control circuits, PLCs, sensors and many actuators run on 24 V DC — a protective extra-low voltage that is safe to touch under normal conditions and standardized in IEC 60204-1 for machinery control circuits.

PELV and the 0 V conductor

Control circuits are typically PELV: the 24 V supply is safely separated from the mains, and the 0 V (M) conductor is connected to the protective earth at one point (near the power supply). This makes earth faults detectable (a fuse trips or an insulation monitor reacts) instead of creating unintended operation. SELV keeps the circuit unearthed. Follow the machine's electrical design and the PLC manufacturer's grounding instructions; do not create multiple 0 V–PE links.

Sizing the supply

Sum the continuous loads (PLC, modules, sensors, valve coils, relays) and add reserve (~25 %) and inrush of capacitive loads. A supply running permanently at its limit ages fast and its voltage sags at peaks.

Protecting circuits

Split the 24 V into protected circuits: PLC/CPU, inputs/sensors, outputs/actuators, field devices of different areas. Use miniature circuit breakers or electronic circuit protection modules that trip selectively even when the supply's short-circuit current is limited. A short in one valve cable should not reset the PLC.

Voltage drop

Long cables to field devices lose voltage: ΔU = 2 · L · I · ρ / A (ρ copper ≈ 0.0175 Ω·mm²/m). 50 m to a 1 A valve over 0.75 mm²: 2·50·1·0.0175/0.75 ≈ 2.3 V — the valve sees ≈ 21.7 V. Check the device's minimum voltage, or use bigger cross-sections / local supplies.

Separate supplies, redundancy, buffering

• Separate the PLC supply from inductive loads where possible (valve and contactor coils cause spikes).
• Redundant supplies with decoupling (diode/MOSFET module) for critical systems.
• Buffer modules or DC-UPS keep the PLC alive through short dips and allow a controlled shutdown.