Troubleshooting Screen Blackout in a Large Industrial Drive System

Recently, a client experienced a nuisance issue with their product in the field. The problem was described as an occasional human–machine interface (HMI) screen blackout. As the issue occurred intermittently, it was suspected to be EMI-related.

A simplified system diagram is shown in Figure 1. The unit is a three-phase bi-directional power electronics system. From the initial review, we believed the noise was generated internally rather than from external sources.

The intermittent nature of the problem made troubleshooting particularly challenging. The investigation approach is outlined below:

Figure 1 A Simplified system diagram

 As with any troubleshooting process, the first step is to reproduce the failure mode before identifying the noise path and implementing solutions. To trigger the screen blackout, a noise source was introduced near the small computer module to replicate the issue observed in the field.

This was achieved using a Langer E1 set. The Langer SGZ 21 burst generator was used, as it can generate high-level burst pulses with two selectable rise times. The amplitude of the pulse can also be adjusted, allowing control over the level of noise injected into the product under test.

In terms of coupling noise into the system, several methods are available, including magnetic and electric field coupling using the accessories provided in the E1 set. In this case, a near-field probe was used.

Here, we demonstrate the noise seen by different types of circuits. In Figure 2(a), a small wire loop is connected to the 50 Ω input impedance of the oscilloscope. This represents a low-impedance loop circuit commonly found on a PCB, which can readily pick up magnetic fields. When a small level of noise is injected using the BS 02 magnetic field loop probe, a peak-to-peak voltage of approximately 6 V is observed on the oscilloscope—sufficient to disturb a 3.3 V or 5 V circuit.

In Figure 2(b), a 10:1 passive probe is connected to the 1 MΩ input impedance of the oscilloscope, representing a high-impedance circuit. When the ES 02 electric field coupling probe is used, the return path of the noise generator is connected to the passive probe’s grounding sleeve. This arrangement allows observation of the noise seen by a high-impedance circuit. With the same low-amplitude setting, peak-to-peak spikes exceeding 10 V were observed.

Figure 2 Coupling transient noise into (a) low impedance circuit (b) high impedance circuit

It was observed that when the magnetic field probe (BS 02) was positioned at the side of the small computer module, the failure mode could be triggered.

To couple noise into the cables connected to the small computer module, a simple method was used by effectively shorting the output terminals of the noise generator. This forces the generated pulses to flow through a wire. The wire was then wrapped around the cables under test, creating strong magnetic coupling between the generator loop and the cables.

This approach allowed individual cables to be tested one by one to identify those most susceptible to noise.

Figure 3 (a) Using the BS 02 magnetic loop probe, connected to the SGZ 21 burst generator, to inject noise into the small computer module.(b) Wrapping a wire around the Ethernet cable to form a current transformer, allowing transient pulses to be coupled into the cable under test.

It was found that the power leads are robust, whereas the Ethernet cables represent the weakest points. This aligns with our experience in similar systems.

So far, the approach has focused on susceptibility. However, the key question remains: why is the small computer module particularly sensitive to noise within the system? To answer this, it is necessary to understand the noise coupling path in order to develop an effective solution.

We then used our emissions troubleshooting kit. In this case, a Langer LF1 set was selected. This set is specifically designed for low-frequency noise pickup and is highly sensitive. Power electronics systems such as this typically exhibit noise from a few kHz up to tens of MHz, with contactor or relay switching introducing transient events, making this probe set well suited to the application.

In addition, as this is a high-voltage system, probing must be carried out at a safe distance. The high sensitivity of the probe therefore becomes particularly important, allowing effective measurements to be made at a safe distance.

Figure 4 Performance of the Langer near-field probe LF-R 400 compared with other probes of the same loop size, showing its superior performance in the lower frequency range. Noise was injected from a function generator into a small wire loop, representing a typical “hot loop” on a PCB.

 As shown below, the LF-R 400 probe was taped to a safety-insulated rescue stick, allowing safe probing of the areas of interest. The results are convincing: the area where the small computer module is located is very noisy. Measurements were taken in continuous mode, and transient noise was also observed during start-up and shut-down.

Figure 5 Emission measurements using a near-field probe inside the cabinet, allowing identification of noisy coupling paths and relatively quiet areas.

Proposed Actions

Based on the findings from both the susceptibility testing and emissions mapping within the system, we propose the following actions:

• Relocate the small computer module to a quieter zone within the cabinet. The proposed location has been verified through measurement to exhibit relatively low noise levels.

• Add a ferrite core (material 31) to the Ethernet cable to improve the noise immunity of the module.

Conclusion

By using appropriate test equipment and applying practical field experience, we were able to identify the noise coupling mechanisms and implement effective mitigation measures to improve system robustness.

This approach demonstrates that, even for intermittent EMI issues, a structured troubleshooting method combined with targeted measurements can lead to clear root-cause identification and practical design improvements.