24V 4.5A Power Supplies for PLC Sensors and Relay Coils
Introduction: A 24V 4.5A supply is built around control electronics, so the practical question is which PLC, sensor, and relay loads fit inside its 100W budget.
Walk through any machine panel and the 24V rail is doing quiet work: it powers the logic that decides what the machine does and the devices that report back what happened. Design engineers new to low-voltage control loads often compare supplies by glancing at the current rating alone, then discover that a 4.5A figure says nothing on its own. What matters is the shape of the loads attached to it — steady, small, and predictable for electronics, spikier and much larger for anything that turns a shaft. Getting that distinction right is what turns a specification number into a usable design decision.
The Control-Load Side of a 24V 4.5A Supply
The loads that live on a 24V DC rail in a factory cabinet are the small decision-making and reporting parts of the machine: PLC CPUs, digital and analog I/O modules, proximity switches, photoelectric sensors, pressure and level transmitters, relay coils, indicator lamps, and buzzer circuits. IEC 60204-1 treats a 24V control circuit as the standard low-voltage control arrangement on factory equipment, which is why so many of these devices are built to run from the same rail. Individually they are modest. A sensor may pull a few tens of milliamps, a PLC CPU a few hundred, a small relay coil usually less than a tenth of an amp. Added across a realistic panel, the total often lands somewhere between one and three amps. That load pattern explains why a 24V 4.5A supply fits control electronics so naturally. At 24V, 4.5A works out to 108W, and the Schneider ABL2REM24045K is rated at 100W with a 108W figure on its output data — the same output envelope described in two ways. Most of that budget goes unused in a typical panel, which is deliberate: spare capacity is what absorbs devices added later, brief switching transients, and the small drift in current draw as sensors age and warm up. The regulated 24V output also holds the rail steady while loads switch on and off, and that stability is what keeps PLC input thresholds and analog readings consistent from shift to shift.
Matching PLCs, Sensors, Relay Coils and Indicating Devices to a Shared 24V Rail
Sharing one rail is standard practice, and it works because the loads involved are all low-power control devices rather than energy converters. Still, they do not behave identically. Two load families in particular deserve separate thought: the steady electronics and the switching coils.
1. PLC CPUs and I/O Modules Draw Steady Low-Current Loads
A PLC CPU plus its I/O modules is close to an ideal load for a regulated supply: it draws a largely constant current, varies little with the machine cycle, and does not produce large inrush. The practical sizing work is mostly bookkeeping — add the backplane consumption of each module, count the current drawn by input circuits from field devices, and account for output modules that source current to lamps or small valves. A panel with a compact CPU, a handful of I/O cards, and a mix of sensors usually sits well under two amps. That leaves room on a 4.5A rail for indicator lamps and a bank of interposing relays without pushing the supply anywhere near its ceiling.
2. Relay Coils and Solenoids Create Switching Peaks That Differ From Sensor Loads
Relay coils and small solenoids are still low-power control loads, but they behave differently from sensors. A coil is an inductor, so its current climbs rather than jumping instantly, and when the coil de-energizes the collapsing field produces a voltage spike that can disturb nearby electronics unless a flyback diode or suppression circuit handles it. Mechanically, contactors and larger relays also pull a higher pickup current for a brief moment before settling to a lower holding current. On a shared rail these peaks are short and small compared with motor starting, and a supply with spare headroom rides through them comfortably. Suppression at the coil remains good practice, since it protects the whole rail rather than only the supply.
Where the 100W Output Limit Stops Being Suitable for Motor Drives
Motor loads live in a different category, and the dividing line is not brand or build quality — it is the physics of converting electrical energy into torque. A DC motor can draw several times its rated current during acceleration, and the current stays high until the shaft reaches speed. A 24V motor consuming anywhere near 100W continuously would already sit at roughly 4.2A, leaving almost nothing of the 4.5A budget before starting current is even considered. Add a gearbox, a brake, or a load that jams, and the demand climbs further. That is why motors are normally paired with a dedicated drive or motor controller, often on a separate supply or a higher-voltage bus designed around starting current and regenerative energy. Relay coils and indicating devices are a different story entirely because their peaks are measured in milliseconds and in fractions of an amp, not in multiples of a motor's full-load current. The distinction worth carrying into a design review is therefore about load behaviour rather than device labels: steady electronics, low-current sensors, and suppressed coils belong on a shared 24V 4.5A rail, while anything that spins, drives a load mechanically, or produces sustained high current belongs on its own supply and protection scheme. Keeping those two worlds apart is what lets a compact 100W unit stay cool, quiet, and inside its ratings.
Conclusion
A 24V 4.5A industrial supply is a control-rail device. It suits PLC hardware, sensors, relay coils, and indicating circuits because those loads are small, mostly steady, and easy to add up, and because the 100W ceiling leaves headroom for growth and switching peaks. It is the wrong tool for motor drive, where starting current and continuous torque demand swamp a 4.5A budget. Reading the output rating against the actual mix of loads on the rail is the step that turns a datasheet figure into a confident design choice, and the ABL2REM24045K listing shows how that rating is presented for one specific model.
FAQ
Q: What kinds of loads can a 24V 4.5A power supply support?
A: It fits low-voltage control devices: PLC CPUs and I/O modules, proximity and photoelectric sensors, pressure and level transmitters, relay coils, indicator lamps, and buzzer circuits. These draw small currents individually and typically total one to three amps across a panel, leaving headroom inside the 4.5A and 100W envelope. Anything that drives a mechanical load belongs on a separate motor supply.
Q: Why are PLC sensors and relay coils different loads on a 24V rail?
A: Sensors and PLC electronics draw a steady, predictable current that changes little during the machine cycle. Relay coils are inductive, so their current rises gradually, they release a voltage spike when de-energized, and larger relays pull a higher pickup current briefly before settling. The rail handles both, but coils benefit from suppression diodes so their switching does not disturb sensitive electronics sharing the same 24V.
Q: Can a 100W 24V power supply drive a motor?
A: Not in practice. A 24V motor drawing close to 100W continuously would need roughly 4.2A, which leaves almost no margin in a 4.5A budget before starting current is counted, and motors can pull several times their rated current during acceleration. Motor loads are normally served by a dedicated drive or controller with its own supply sized around starting current and regenerative energy.
Sources / References
UL 508A | UL Standards & Engagement
Switch Mode Power Supply (SMPS) Topologies - Texas Instruments Application Report
Related Examples
Schneider Open-type Switching Power Supply ABL2REM24045K Features
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