Sizing a Control Panel Power Supply
Short answer
- Size a 24 VDC control panel supply by summing every device's continuous current, adding a 20 to 25 percent margin, and choosing a rated output above the total.
- The supply must also survive inrush peaks from capacitors and contactor coils, and its rating must be derated for ambient temperature and altitude.
- A switch-mode supply gives tighter regulation and higher efficiency than a control transformer with rectifier, and an electronic circuit protector suits a DC bus better than a standard MCB.
The first question you face when designing a control panel is: "What 24 VDC supply will reliably feed all the devices without dropping out or restarting?" The answer lies in a disciplined sizing process that looks beyond the average current draw and includes inrush behaviour, environmental derating, and protection strategy.
How do you calculate continuous load and add a safety margin?
Start by listing every 24 VDC device that will be powered directly from the supply. Include sensors, PLC I/O modules, indicator lights, small drives, and any auxiliary circuitry. For each item note the name‑plate current (often given as "max" or "continuous"). Sum these values to obtain the base continuous load.
Example:
- PLC power module: 1.2 A
- Eight 24 VDC contactor coils (0.3 A each): 2.4 A
- Analog input card (0.1 A): 0.1 A
- Signal LEDs (0.02 A each, 10 pcs): 0.2 A
Total continuous current = 3.9 A. A common engineering practice is to add a 20‑25 % margin to accommodate future expansion and minor tolerances, giving a design current of roughly 4.8 A. The supply’s rated output current should therefore be at least 5 A.
Do not rely on a supply that is sized exactly to the summed name‑plate values. Those values are often measured under ideal conditions and do not reflect real‑world variations such as cable losses or temperature‑induced derating.
Why does inrush current trip a correctly sized supply?
Even a perfectly sized supply can trip or fold back if the instantaneous demand exceeds its current‑limit capability. Two common sources of high‑frequency peaks are:
- Capacitive loads. When a large electrolytic capacitor is first connected, the initial charging current can be 10‑20 times the steady‑state current, lasting a few milliseconds. For a 1000 µF filter capacitor on a 24 VDC bus, the peak can approach 30 A.
- Contactor coils. A 24 VDC coil typically draws 5‑10 times its steady‑state current for 10‑30 ms as the magnetic field builds. A 0.3 A coil may momentarily demand 2‑3 A.
Supplies that rely on simple linear regulation often have a limited short‑circuit or inrush rating (e.g., 150 % of rated current for 0.5 s). Switch‑mode supplies usually specify a peak current capability; look for a value that comfortably exceeds the worst‑case sum of all inrush peaks. If the combined peak is 8 A, a supply rated for at least 12 A peak will survive the start‑up of all coils and capacitors without collapsing the bus.
One practical technique is to stagger the energising sequence of large loads using the PLC or a soft‑starter module, thereby reducing the simultaneous peak. However, the supply must still be capable of handling the worst‑case scenario because a fault condition (e.g., a shorted coil) could still present the full peak.
How do input power, temperature and altitude affect a supply?
Most panel‑mounted 24 VDC supplies accept either 120/240 V single‑phase or 208‑240‑480 V three‑phase input. Choosing the input depends on the plant’s distribution architecture.
- Single‑phase input. Simpler wiring, but the supply draws the full input current from one phase. In a 120 V system, a 5 A 24 V output may require 1.5 A input, which can be a noticeable load on a small branch circuit.
- Three‑phase input. The input current is divided among three phases, reducing conductor size and improving harmonic balance. For the same 5 A output, a three‑phase supply may draw only 0.5 A per phase at 208 V.
Power factor correction (PFC) is built into most modern switch‑mode supplies. A supply with a PFC rating of 0.95 PF draws less reactive power and reduces the burden on upstream distribution. If the plant requires strict PF compliance, verify the supply’s PFC class (e.g., IEC 61000‑3‑2).
Environmental derating is mandatory for reliable operation. Manufacturers typically provide a derating curve based on ambient temperature. A rule of thumb is:
- At 40 °C ambient, multiply the rated current by 0.8.
- At 50 °C, multiply by 0.6.
Altitude also reduces cooling effectiveness. For every 1 000 m (≈3 300 ft) above sea level, apply a 0.9 factor to the current rating. If the panel will sit in a plant at 1 500 m, the combined temperature‑altitude derating could be as low as 0.5 of the name‑plate rating. In such cases, select a supply with at least 2 × the required current before derating.
Enclosure selection influences temperature. A well‑ventilated wall‑mount enclosure with a 40 °C rating will keep the internal air lower than a sealed box, reducing the derating factor.
Control transformer with rectifier or switch-mode supply?
Two classic approaches exist for generating 24 VDC:
- Control transformer with a diode bridge. The transformer steps down the mains to 24 VAC, which is then rectified and filtered. The resulting DC has a ripple of about 10 % of the nominal voltage, and the overall efficiency is low (≈70 %). The advantage is ruggedness and inherent isolation. For heavy‑duty environments where electromagnetic interference (EMI) is a concern, a control transformer solution can be attractive.
- Switch‑mode power supply (SMPS). These units convert AC to DC at high frequency, achieving efficiencies of 85‑95 % and a tight output regulation (±2 %). They also integrate PFC, inrush limiting, and often include built‑in protection. The downside is sensitivity to high‑energy transients and the need for EMI filtering.
When the panel houses sensitive electronics (e.g., analog I/O, communication modules), the low ripple and tight regulation of an SMPS usually outweigh the simplicity of a transformer‑rectifier. However, if the plant already has a robust power and distribution transformer that can supply a dedicated 24 VAC tap, the transformer‑rectifier method may reduce component count and cost.
Consider also the fault‑current path. A transformer provides a clear short‑circuit point, making coordination with fuses straightforward. An SMPS often limits fault current internally, which can mask a downstream short and delay protective action.
How can a panel ride through a loss of 24 VDC supply?
Control panels that cannot tolerate a brief loss of 24 VDC should incorporate redundancy. The most common schemes are:
- N+1 redundancy. Install two identical supplies where each can handle the full load alone. The extra unit is the "+1" standby. Automatic transfer relays switch to the standby if the primary trips.
- Decoupling modules. Large electrolytic or film capacitors placed close to critical loads act as local energy reservoirs. A 10 000 µF, 35 V capacitor can sustain a 5 A load for about 2 seconds, enough to bridge a brief supply interruption.
- Battery‑backed UPS modules. For panels that experience brownouts or require a clean shutdown, a small UPS (e.g., 24 V, 10 Ah) provides minutes of runtime. The UPS also smooths voltage dips caused by motor starting elsewhere on the plant.
When implementing redundancy, ensure that the supplies share a common ground reference and that the wiring layout avoids circulating currents. Use a diode OR-ing arrangement or a dedicated power‑path controller to prevent both supplies from feeding each other.
Why does a standard MCB fall short on a 24 VDC bus?
Miniature circuit breakers (MCBs) are designed for AC circuits with a thermal‑magnetic trip curve. On a 24 VDC bus, the thermal element sees much less power for the same current, and the magnetic element may never reach its release point because the fault current is limited by the supply’s own current‑limit circuitry.
Consequences of using an MCB on a current‑limited SMPS include:
- Delayed or non‑tripping during a short‑circuit, allowing the supply to enter a fold‑back mode that can stress downstream components.
- Unreliable coordination with upstream protection, potentially exposing the supply to fault currents beyond its design.
Instead, select an electronic circuit protector that is rated for low‑voltage DC and provides fast electronic trip (typically < 10 ms). These devices sense over‑current based on instantaneous current, not thermal buildup, and they can be set to a precise trip threshold (e.g., 6 A for a 5 A supply).
Additional protection layers include:
- Input fuses sized to the supply’s maximum input current (considering inrush).
- Transient voltage suppressors (TVS) on the DC bus to clamp spikes from inductive loads.
- Isolation monitoring circuits that alert the PLC if the DC bus voltage drops below a safe level.
When designing the protection scheme, remember that the supply’s own over‑current limit is not a substitute for external protection; it is intended to protect the supply, not the rest of the system.
By following the steps above—summing continuous load, adding a safety margin, verifying peak inrush capability, accounting for temperature and altitude derating, choosing the appropriate topology, and applying proper DC protection—you can select a 24 VDC power supply that stays online under normal operation and survives the inevitable transients of an industrial environment.
If you need assistance locating a suitable supply, decoupling capacitor, or protection device, feel free to reach out for a cross‑reference or quote.
Related reading: what a control transformer is and how to size one.
Related: What Is a Transformer? Types and How to Size One; What Is a Surge Protective Device (SPD)?.
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Request a QuoteFrequently asked questions
How much margin should I add when sizing a 24 VDC control panel power supply?
Add a 20 to 25 percent margin to the summed continuous current of every 24 VDC device. In the example, a total of 3.9 A becomes a design current of roughly 4.8 A, so the supply's rated output should be at least 5 A. Sizing exactly to name-plate values is not advised because those values ignore cable losses and temperature derating.
Why does my 24 VDC power supply trip when contactors energise?
Contactor coils and capacitive loads draw short inrush peaks far above steady-state current. A 24 VDC coil typically draws 5 to 10 times its steady-state current for 10 to 30 ms, and a capacitor can draw 10 to 20 times. If the combined peak exceeds the supply's current-limit capability, it folds back. Choose a supply whose peak rating exceeds the worst-case sum, such as 12 A peak for an 8 A combined peak.
How much should I derate a power supply for temperature and altitude?
Multiply the rated current by 0.8 at 40 °C ambient and by 0.6 at 50 °C. Apply a 0.9 factor for every 1 000 m above sea level. At 1 500 m the combined derating could be as low as 0.5 of the name-plate rating, so select a supply with at least 2 times the required current before derating.
Why is a standard MCB a poor choice for protecting a 24 VDC bus?
A miniature circuit breaker uses a thermal-magnetic trip curve designed for AC. On a 24 VDC bus the thermal element sees less power for the same current, and the magnetic element may never release because the supply limits fault current. The result can be delayed or no tripping during a short circuit. An electronic circuit protector with a trip under 10 ms is the alternative.