Touchscreen Backlight Flicker on Bently Nevada 120M8155-01 (HMI-DSP) - Excellent PLC

1. Equipment Information

Model: 120M8155-01
Device Classification: HMI-DSP (Human-Machine Interface – Display Unit)
Application Sites: turbine control panels, compressor vibration monitoring kiosks, auxiliary machine condition stations

2. Reported Field Symptom

Backlight brightness fluctuates in short bursts (0.5–2 seconds), especially during machine ramp-up phases. No full blackout observed. Image layer remains intact — only illumination changes.

3. Operating Context at Time of Failure

Ambient temperature: 47°C internal cabinet temperature
Cabinet cooling: fan only, no A/C
Power supply source: shared 24VDC with VFD control relays
Operating cycle: continuous runtime > 18 months
Flicker intensity: increases during startup of large induction motors

4. Technical Interpretation

Based on collected data from multiple industrial sites, the flicker originates from backlight subsystem instability, not the LCD panel itself.

Key contributing factors:

(a) Ripple on 24VDC Line
Measurement indicated ~0.9V peak-to-peak ripple induced by motor starters and VFD switching.

(b) Aging of LED Driver Components
Thermal stress on LED driver IC and associated capacitors reduces current regulation precision.

(c) Pulse Load Interaction
Motor startup events cause temporary voltage dips; LED driver responds with brightness modulation.

5. Field Diagnostics Conducted

The service team executed the following verification steps:

DC bus ripple measurement using differential probe
Thermal imaging of rear PCB zones
Isolation test with clean laboratory power supply
LED driver current waveform capture
Connector reseat validation

Result Highlights:

Ripple reduced to near-zero on lab PSU → flicker vanished
Backlight driver IC area showed hotspot at 86°C
Electrolytic capacitor ESR values above nominal specification

6. Corrective Measures Implemented

To stabilize the system, technicians performed:

Replacement of degraded driver-side capacitors
Installation of electrolytic and film decoupling combination on 24VDC input
Addition of EMI suppression ferrites on supply leads
Improved airflow via directed cabinet fan

Cabinet temperature dropped by 9°C and flicker ceased during subsequent motor startup events.

7. Preventive Notes for Future Installations

To avoid recurrence:

Avoid sharing HMI power with inductive loads or VFD logic
Maintain cabinet temperature under 40°C whenever possible
Inspect LED driver capacitor ESR every 3–4 years in high-heat environments
Include ripple monitoring when diagnosing intermittent display anomalies

8. Service Outcome

After corrective actions, the panel has been operating without flicker for 120+ days under full plant load conditions.

Troubleshooting