I have a PC that controls some equipment, such as, cameras, solenoids, and DACs. There is also a 45KW nitrogen laser. The laser has its own service installed, but, when it turns on, there is a voltage spike that disrupts the USB devices on the PC. The PC is on a surge protector, but the spike still affects the PC in this way.

When this happens, shutters open, and the general operation is disrupted. One option is to avoid the issue, and shut down the PC, power on the laser, and then turn the PC back on. This works well, but is tedious.

I have talked with the building manager, and he will not take any measures to improve the quality of the electrical services available (such as hire an electrician to install a Whole-House Suppressor).

I am looking into putting ferrite beads on the existing USB cables, but those have not arrived yet. Are there other options for noise suppression of this nature?

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    \$\begingroup\$ You need to consider all of the power supply as you are, direct induction into the USB cables, and potentially also instantaneous differences in ground between USB devices that have their own AC supplies. Note that USB is an interface intended for consumer settings, not industrial ones. However galvanic USB isolators do exist, especially if you can tolerate the USB full speed (vs high speed or the more recent even faster standards). \$\endgroup\$ Feb 9, 2017 at 17:22
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    \$\begingroup\$ You could try a better PC ATX PSU - cheap ones may not be too stable. I would also try to look at the PSU's voltage lines with an oscilloscope when the laser turns on. \$\endgroup\$
    – Turbo J
    Feb 9, 2017 at 17:39
  • \$\begingroup\$ You can try a online UPS, it should help a lot. Or you can use a UPS to isolate the PC from the mains for a brief moment when the laser is turned on. \$\endgroup\$ Feb 9, 2017 at 18:08
  • \$\begingroup\$ What kind of PC do you have? Processor, chipset? \$\endgroup\$ Feb 9, 2017 at 19:09

3 Answers 3


Apparently your system is suffering from susceptibility to EMI and low immunity to transients. Obviously your overall setup (with all USB devices and associated USB cables) was never tested to IEC 61000-4-x standards, especially to 4-4 EFT (fast transient) section.

The position of your building manager is understandable. Before installing some extra equipment, you need to find the root cause of your issue.

If the interference is coming from AC power mains, a battery-powered UPS is an easy solution. But if your PC doesn't reboot, the power is not likely the cause.

If USB is disrupted, you might want to look into the scale of communication disruption, whether any individual USB devices are disrupted, or the entire PC root hub gets reset and/or massive disconnects are happening.

If the interference (fast transient) is coming in a radiated way (coupling into USB cables), the simple ferrite blobs on USB cable coming to PC might help. However, most regular PCs have the shielding routed in completely wrong manner (shield is usually directly connected to signal ground plane on most mainboards at USB connectors), the ferrite beads might be not enough. In this case a good total (optical) isolation will solve the problem, see USB3.0 optical cables from Corning Communication. This would be the best, although a bit pricey.

[CORRECTION: The Corning cable does not provide galvanic isolation between grounds on connectors, as some audiophiles have discovered, and tolerance of data signals to common-mode is not specified or known. Corning didn't return the request for clarification]

USB has a limited range of how much of common-mode shift it can tolerate (officially about 1-2V, or less for HS). If the disruption comes from excessive bouncing of common ground across multiple power sources supplying your USB devices, you may want to reduce the ground loop area by consolidating all power bricks into one area if possible, and use another local UPS to power them.

Overall, fighting Electromagnetic Interference is always a challenge, so good luck to you.


Your PC is probably suffering from a brown-out, a surge protector will not help with this issue.

If you do not need steep edges on your laser (a 45 kW laser is probably not modulated for communications?), then limit the inrush current to the laser. There are some thermistor based limiters available for the mains connection.

If you cannot change the mains connection (45kW are not plugged to the wall outlet, right?), you should consider an online ("continous conversion") UPS for the PC.

If it turns out that your PC really resets due to radiated electromagnetic interference, I would think the whole setup is not FCC compliant. That would be a bigger problem unless you are in a shielded lab environment.


Scenario 1: Brownout

Hypothesis: switching on the laser lowers mains voltage for a short time.

You don't say if your USB devices are bus-powered or self-powered. Even if the PC does not crash from a brownout, any USB device powered from mains could also experience a transient loss of power. Its internal microcontroller could go haywire or reboot. One misbehaving USB device can make an USB bus go into la-la-land.


  • borrow UPS from the IT guys and run everything (including all USB devices) from it. Does it fix the problem?...
  • inquire with laser manufacturer about soft-start

Scenario 2: EMC

This is difficult to debug. I won't go into details... Adding ferrites could be beneficial, but I would rather suggest USB isolators. For USB1.1 devices, these are available and cheap. Recently USB2.0 high speed have become available, they are more expensive of course.

For example, Intona makes a USB2 isolator. Icron makes an USB to fiber optics bridge, which can span long distances.

The famous Corning cable is useless as it does not isolate.


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