HMI screen dim on PLC power: check the supply drop

Why the screen dims on PLC power

Sensor-side power on a PLC is sized for inputs, and it is current-limited. A Class A IO-Link port is specified at 200 to 500 mA per port, which is the same order as a PLC's sensor output. A panel manual that permits running from that output carries the reduced-brightness note for exactly this reason, and the note prints no number.

When total demand passes the limit, a current-limited source answers by pulling its voltage down. The panel keeps running, less power reaches the backlight, and the screen reads dim. There is a line under all of it: IEC 61131-2, the standard for industrial control equipment, defines the normal operating band for a 24 V nominal supply as 85% to 120% of nominal, which is 20.4 to 28.8 V. A reading below 20.4 V at the panel means the equipment is outside its normal band, and reduced backlight is the visible half of that.

A panel that lights up on PLC power tells you the output held at startup. It says nothing about the margin left once contactor coils, lamps and sensors switch in.

Symptoms and what each points to

• Dim on the PLC feed, normal brightness on its own supply. The shared output is at or past its current limit. This is the case the manual's note describes.
• Dimming that deepens when a contactor pulls in. Other loads are eating the output's capacity. A coil in the 3 W class at 24 V alone draws about 130 mA.
• Bright near the supply, dim at the far end of a long cable. Conductor drop. Compute it before blaming the output.
• Dim with resets or flicker while the panel polls its devices. The sag is hitting the whole panel. A backlight that flickers in step with polling points at the supply, and that pairing has its own check in HMI backlight flickers when polling.
• Dim on every source at the same setting. The feed is fine; the brightness setting or backlight age is the subject.

Check the supply before you change anything

1. Read both datasheets. The panel's input voltage range and current demand, and the sensor output's current rating. Check that the panel manual permits the PLC-feed arrangement, and note what it says about brightness.
2. Measure at the panel's power terminals, not at the supply. Screen showing a typical page, contactors and sensors in their normal running state. Compare the reading with the printed range.
3. Compute the cable loss. Round-trip resistance, both conductors, times the panel current. Standard copper tables give the ohms per 1000 ft for the wire size; the example below runs the arithmetic.
4. Hold the brightness setting unchanged through steps 2 and 3. A changed setting masks what the supply is doing, and you lose the comparison.

Example: a 500 mA output, a 10 W panel, 60 ft of 18 AWG

Example numbers for the walkthrough: the panel's datasheet lists 10 W maximum at 24 VDC with an input range of 20.4 to 28.8 V, the PLC's sensor output is rated 500 mA, and a 60 ft run of 18 AWG copper, about 6.4 Ω per 1000 ft at 20 °C, carries the feed. The output already feeds a contactor coil of 3.2 W at 24 V, about 133 mA, plus 150 mA of sensors.

Panel current: 10 W ÷ 24 V = 0.42 A. Total demand: 0.42 A + 0.13 A + 0.15 A = 0.70 A. Output rating: 0.50 A. The demand is 0.20 A past the limit.

A 500 mA current-limited output facing 0.70 A of demand pulls its voltage down before the panel's terminals. The backlight, the panel's largest single load, dims first, and the terminal reading can fall under the 20.4 V floor.

The wire adds its own loss:

One way: 6.4 Ω × 0.060 kft = 0.38 Ω. Round trip: 2 × 0.38 Ω = 0.77 Ω. At the panel's 0.42 A: 0.42 A × 0.77 Ω = 0.32 V.

On a healthy 24.0 V feed the panel sees 23.7 V at its terminals, 24.0 V minus the 0.32 V drop, inside the range. In this example the wire was never the problem; the 500 mA output was.

The dedicated supply sizes like this: running load 0.42 A, and keeping the steady load at or below 80% of the rating asks for 0.42 A ÷ 0.8 = 0.53 A, so a 1 A (24 W) supply covers the panel with headroom for inrush.

Move the panel to its own supply

1. Put the machine in a safe state and isolate the panel's feed before moving any wire.
2. Wire the panel's power terminals to a dedicated supply exactly as its manual shows, and check polarity before energizing.
3. Keep the new supply's output separate from the PLC's sensor output. Two supplies wired in parallel fight over the bus voltage, each regulator trying to set it on its own, and the result reads like the sag you just fixed. Feed sensors from the sensor output, the panel from its own supply.
4. Energize, display a typical screen, run the machine's loads, and measure at the panel's terminals. Expect a reading inside the printed range with the cable loss included; the example above reads 23.7 V. Where inrush belongs in the sizing, sizing a separate 24 VDC supply for HMI inrush current walks the arithmetic.
5. Leave the brightness setting where it was for the comparison. Once the terminal voltage reads right, adjust it for the room's light and check every operator screen from the normal position and lighting. If the voltage holds inside the range and the screen is still too dim, a dim display brightness adjustment fix on a large HMI walks through the setting side.
6. Write the measured terminal voltage, the supply arrangement and the brightness setting into the panel's service record, so the next shift reads a correction and not a temporary patch.

The decision, case by case

• Terminal voltage inside the printed range at worst-case load, and the screen reads clearly from the operator position: keep the existing feed and move on.
• Inside the range, but dimmer than the same panel on its dedicated supply at the same setting: the reduced-brightness note applies. Move the panel to its own supply.
• Below 20.4 V at any moment, or dimming that deepens when other loads switch in: the output is past its limit. A dedicated supply is the fix, and a brightness setting cannot raise a current limit.

If the shared output sags far enough, the trouble stops being cosmetic; sensors drift and the PLC can drop out of run mode, and PLC drops out of run mode starts that check at the AC supply.

Recommendation: measure the DC voltage at the panel's power terminals with the screen lit and every usual load running before you touch any brightness setting. Below 20.4 V on a 24 V panel, or dimming that deepens when coils and sensors switch in, the sensor output is past its limit: give the panel a dedicated supply, sized for at least about 80% headroom on running current plus inrush, and leave the sensor output to the sensors. If the reading holds inside the printed range and the screen reads clearly from the operator position, the existing feed is fine as it is.

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