A display panel belongs on its own 24 VDC supply. Size it so the continuous rating covers the panel's steady draw plus 25 to 30% margin, and the peak covers steady draw plus inrush for the full pulse with the bus holding about 20 V. A controller's auxiliary 24 VDC output, sized for sensors at a few hundred milliamps, fails that test.
Buy the separate supply. A display panel's startup pulse is a real load event, and a controller's auxiliary 24 VDC output is sized for sensors, not for screens. Sizing the right supply takes two checks against the datasheet. The continuous rating must cover the panel's steady draw plus a sizing margin. The peak capability must cover steady draw plus inrush current for the full pulse, with the bus holding about 20 V or more at the panel's terminals. This article gives you both checks, the standards behind the numbers, a worked example, and the decision for a panel that passes and one that does not.
What inrush current is
At power-on, the panel's input capacitors charge from zero volts up to the nominal bus voltage. During that window the panel pulls several times its running current, then settles within milliseconds. Published inrush windows on industrial 24 VDC equipment are short: about 2 ms on a compact controller's own input, around 20 ms on larger drives and operator interfaces. The pulse happens once per start. It adds no measurable heat and takes no part in the watts budget; it is purely a question of whether the supply can source the amps while it lasts.
Two different inrush numbers appear in this job, and they are not interchangeable. The supply's datasheet lists an input inrush on the AC side; in typical industrial units a negative temperature coefficient (NTC) thermistor limits it. That figure concerns your breaker and branch wiring. The panel's datasheet lists an inrush on its DC input, and that second figure is the one you size the supply against.
Why the controller's output cannot carry the panel
A compact controller's documentation often lists a total input power budget near 30 W, and its auxiliary output is a slice of that, typically rated a few hundred milliamps. The output exists to feed sensors and encoders. A panel drawing 0.5 A steady takes 12 W of that bus before its startup pulse even asks for more.
When the panel pulls the bus down mid-boot, two things happen. The panel can reset inside its own startup sequence, and everything else on that output sees the same sag, from a photoeye to an encoder. A shared bus sags under other loads for the same reason: solenoid coils on the feed drag the voltage down every time they energize, and an HMI on that shared DC supply freezes with them. The fix in both cases is the same: give the panel its own feed.
The two checks that size the supply
Convert ratings the same way every time: amps equal watts divided by volts, so a 60 W supply at the nominal bus delivers 2.5 A, because 60 W / 24 V = 2.5 A.
Check 1, continuous: sum the steady amps of everything on the supply, then add 25 to 30% margin, the common sizing rule for DIN-rail supplies.
Check 2, peak: add the panel's inrush to the steady amps already counted and compare the sum with what the supply can deliver for the pulse's length. Many industrial supplies publish a peak or boost rating with its duration; if the datasheet publishes one and it covers the sum, that figure settles the check. If it publishes none, treat the continuous rating as the ceiling and buy continuous amps that cover the peak sum.
The voltage condition behind check 2 comes from IEC 61131-2, the standard that defines supply requirements for industrial control equipment: a device rated 24 VDC nominal operates from 20.4 to 28.8 V. Your pass condition is a bus that stays at 20.4 V or higher at the panel's terminals through the whole pulse. Sizing to the spike, not the average, is the same logic behind a note on peak current as a sizing requirement.
| Check | What you compute | Pass condition |
|---|---|---|
| Continuous | sum of steady amps x 1.25 to 1.30 | supply's continuous amps at least the result |
| Peak | steady amps + inrush amps | supply delivers the sum for the pulse length, bus 20.4 V or higher at the panel |
Worked example
Take a panel whose datasheet lists 0.5 A steady draw and 1.5 A inrush for 20 ms, sharing the supply with nothing else.
Continuous check: 0.5 A x 1.3 = 0.65 A, which at 24 V is 15.6 W.
Peak check: 0.5 A + 1.5 A = 2.0 A for 20 ms. Instantaneously that is 2.0 A x 24 V = 48 W, and the energy in the pulse is 48 W x 0.02 s = 0.96 J.
Pick a 60 W DIN-rail supply: 60 W / 24 V = 2.5 A. Continuous: 2.5 A against a 0.65 A requirement passes with nearly 4x margin. Peak: 2.5 A covers the 2.0 A sum, so the pulse sits inside the continuous rating, the output never leaves regulation, and the bus holds above 20.4 V at the terminals with no boost spec needed.
Now the case that fails. A larger panel draws 1.0 A steady with 4.0 A inrush, so the peak is 5.0 A, or 120 W instantaneous. The 60 W, 2.5 A supply fails the peak check outright. Stepping to 100 W gives 100 W / 24 V = 4.17 A, still under 5.0 A, still a fail. The decision: 120 W / 24 V = 5.0 A, so a 120 W supply whose continuous rating covers the peak clears both checks.
Install and verify
De-energize the cabinet, lock it out, and move the panel's feed to the new supply on its own wire pair; check polarity and terminal tightness against the supply's installation sheet before you restore power. At first start, measure voltage at the panel's terminals while someone watches the screen. If the bus holds 20.4 V or better through the startup pulse and the panel boots cleanly three starts in a row, the sizing is done. Write the supply's model, wattage and amp rating into the panel's maintenance record.
A panel that browns out repeatedly can also come up into a failed boot. If yours loses its project after a power cycle, that is a separate failure with its own fix, and the supply comes first.
The decision
Recommendation: Feed every display panel from its own supply and leave the controller's auxiliary output to its sensors and encoders. Size that supply with the two checks: continuous amps at the steady draw plus the sizing margin, and peak coverage of steady draw plus inrush for the full pulse with the bus holding 20.4 V or better at the panel's terminals. The 0.5 A example panel passes on a 60 W, 2.5 A DIN-rail supply. A panel whose peak sum exceeds the supply's amps moves up in wattage until the amp rating covers the peak, and you skip any unit that publishes no peak figure unless its continuous amps already cover the peak.
Questions
Can I parallel two 24 VDC supplies to cover an inrush peak?
Only if both units are documented for parallel operation, and supplies in this size class rarely are. Without load-share wiring, one supply regulates high while the other current-limits, and the pair can fight each other or back-feed. The documented approach is one supply sized for the whole load, peak included. If one unit cannot give enough amps, move up a wattage size instead of paralleling.
The panel datasheet lists no inrush figure. What do I do?
Measure it once. Put a DC current clamp with peak-hold, or a shunt with an oscilloscope, on the panel's feed at first power-up, and record the peak amps and their duration. Repeat the measurement cold and warm, because input capacitor behavior shifts with temperature, and use the highest reading in the peak check.
Does the 20 ms inrush count toward the cabinet's heat budget?
No. Heat comes from watts over time, and a 20 ms pulse once per start adds nothing a thermal budget can see. Size cabinet cooling and supply derating on the steady draw in the worked example plus the supply's own efficiency loss, and leave the startup pulse out of the heat calculation.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.