How to Size an Industrial DC Power Supply

Define the load before choosing the supply

Start with what the load needs, in its manufacturer's own terms. Most industrial automation equipment, such as controllers, I/O modules, sensors, valve coils, relays and small DC motors, is a constant-voltage load: it expects a fixed DC bus voltage within a stated window and draws the current it needs. LEDs driven directly without their own regulator behave differently and need a constant-current driver; that is a separate selection method and is outside this guide.

For each device, record the nominal DC voltage, the window it accepts at its own terminals, whether its demand is steady or has a startup or pulsed component, and the document that states those values. If a datasheet is missing, mark the row as unknown instead of borrowing a figure from a similar device. An unknown row is a question for the equipment manufacturer, not a number to estimate.

• Bus voltage and the accepted voltage window at the device terminals
• Specified steady current or power, and whether the figure is a maximum or a typical value
• Startup, inrush or pulse current together with its duration
• Which devices can operate at the same time
• Source document and revision for every value

Build a steady-state load budget

The steady-state budget is the current the bus must deliver continuously while every device that can run at the same time is running. Multiply quantity by the specified current for each row, then add only the rows that are simultaneous. Keep the budget in amperes at the bus voltage: converting to watts helps when comparing with a rated power figure, but the supply has to meet the current at its actual output voltage.

Every value in the worksheet below is hypothetical and exists only to show the method. Replace each row with your equipment's documented values and keep the source column filled; the total is only as reliable as its weakest row.

Separate startup and transient requirements

Spare steady-state watts do not prove that a supply can start a load. Motors, valve coils and capacitive inputs can draw more than their running current for a short time, and a supply answers that demand through its overload behaviour rather than its rated figure. Take the startup current and its duration from the load manufacturer, then compare them with what each candidate's PDF actually documents.

The LRS-350 specification lists overload protection at 110 ~ 140% rated output power: the 3.3~36V models, including the 24V model, enter hiccup mode and recover automatically after the fault is removed, while the 48V models shut down and latch off until re-powered. Its Note 7 adds a 150% peak load capability for up to 1 second for the 12~48V models, after which the unit enters hiccup mode.

The NDR-240 specification lists overload protection at 105 ~ 130% rated output power by constant current limiting, with automatic recovery after the fault condition is removed. Its description presents the constant current mode as fitting inductive or capacitive applications, but no peak-load duration is listed, so a start above rated current has to be checked on the real load.

Setup and hold-up times matter when a controller must boot in sequence or ride through a brief mains dip. LRS-350 lists setup, rise time of 1500ms, 50ms/230VAC and typical hold-up of 16ms/230VAC and 12ms/115VAC at full load; NDR-240 lists 1500ms, 100ms/230VAC and 3000ms, 100ms/115VAC, with typical hold-up of 28ms/230VAC and 22ms/115VAC at full load.

Apply the actual operating constraints

Check the input first. LRS-350 accepts 90 ~ 132VAC or 180 ~ 264VAC, selected by a switch that must match the site supply, and its page 3 input curve plots the two ranges separately. NDR-240 accepts 90 ~ 264VAC without a switch, and its Note 4 warns that derating may be needed under low input voltage.

Then check temperature and mounting. Both working-temperature rows reach +70℃ but refer to a derating curve (LRS-350: -25 ~ +70℃; NDR-240: -20 ~ +70℃). The LRS-350 curve is drawn for HORIZONTAL mounting of a fan-cooled enclosed unit, and the NDR-240 curve for VERTICAL mounting of a convection-cooled DIN-rail unit. Read the usable load at your cabinet's internal ambient and orientation from the exact curve; the derating guide explains how to read one without over-reading it.

Altitude and spacing also change the result. Above 2000m, LRS-350 requires ambient derating of 5℃/1000m, while NDR-240 gives 3.5℃/1000m for fanless models and 5℃/1000m for fan models. NDR-240 recommends 40mm clearance on top, 20mm on the bottom and 5mm on the left and right when loaded permanently with full power, or 15mm next to a heat source.

Finally, check the market. LRS-350 Note 10 says the series does not meet the EU harmonic current requirement it names and should not be used in end devices in the European Union that connect to public mains of 220Vac or greater when the device's input power is above 75W or the supply is part of a lighting system, with listed exceptions such as professional equipment above 1000W total rated input. The NDR-240 description states conformity with the EU harmonic current norm.

Use a transparent worked example

Hypothetical example: a 24V machine bus made of the five worksheet rows above. The assumptions are that every device accepts 24V ±10% at its own terminals, all rows can run together, the pump starts while everything else is already running, and the cabinet ambient and mounting stay inside the full-load region of the chosen candidate's derating curve. None of these assumptions describes a real machine.

Steady state: 1.5A + 1.2A + 1.4A + 0.8A + 2.5A = 7.4A, and 7.4A × 24V = 177.6W. Startup: the hypothetical pump datasheet states 7.5A for up to 0.3 seconds, so the worst simultaneous current is 7.4A - 2.5A + 7.5A = 12.4A, or 297.6W at 24V.

Voltage: the 24V columns of both candidate specifications list ±1.0% voltage tolerance, which sits inside the hypothetical device window. That window applies at the device terminals, however, so cable voltage drop at 7.4A, and at 12.4A during the pump start, must be calculated from the real cable length and cross-section before the voltage check is complete.

Reserve for future loads is a project decision. Record the reserve your project requires as its own input; this guide does not apply a universal margin percentage, because the right reserve depends on the equipment, duty and expansion plan.

Compare candidate models and record exclusions

The matrix uses only the two exact models' PDF data and the hypothetical load. Each cell is a check to record, not a compatibility verdict: a hold means the candidate is not accepted without further evidence, and a pass means only that the documented figure clears the hypothetical requirement.

Writing down the reason for each hold keeps the decision reviewable. If the pump manufacturer later confirms a lower start current or a soft-start, the NDR-240-24 row can be re-checked without rebuilding the whole budget.

Prepare a quote-ready specification

A quote request gets a useful answer when it states the requirement instead of a guessed model. Send the load budget with its sources, the startup rows and the environment, and say which candidate models you have already checked and why any were held or excluded.

Every worked value in this guide is hypothetical. The equipment manufacturers' load specifications and a technical check of the final choice are still needed before ordering.

• Output: DC voltage, accepted window at the load, steady current, and startup current with its duration
• Duty: continuous or intermittent operation, and which loads run together
• Input and environment: site mains voltage and frequency, cabinet internal ambient, altitude, mounting orientation and airflow
• Approvals and market: required safety approvals and destination country, including whether the end device connects to public mains in the European Union
• Commercial: quantity, delivery destination and any preferred or excluded models