I live in Europe and we have 230V coming out of the power outlet. I have several consumers on one outlet: 5V DC supplies for Router, HDMI-Switch, Ambilight, one Computer, one Macbook and a fan). The Ambilight is just some LEDs controlled by an Arduino. When I turn on the fan the Ambilight flickers for a short moment.

I found this kind of strange and used my oscilloscope to measure the output of my 5V DC power supply when turning on the fan:

5V @ 200ns

The result is quite shocking. I added a 330uF capacitor and the result got better but is still not satisfying (notice that the graphs have different scales one is 5V @ 200ns and the second is 2V @ 500ns):

2V @ 500ns

I also measured another 5V DC power supply and the result was almost the same (amplitude was a little bit lower).

EDIT: After a few MS the output voltage returns to a steady 5V DC.

Is this a problem with my power lines (too many consumer) or just some very bad 5V power supplies?

What could I do to smooth the output of my 5V power supplies (more capacitors? what size?)

  • 1
    \$\begingroup\$ Try shorting the o-scope leads and repeating. \$\endgroup\$
    – Andy aka
    Aug 11 '14 at 10:14
  • \$\begingroup\$ BTW 330uF seems way too big. Your noise signal is in the MHz range here. A 330uF has almost no effect at theses frequencies which are probably above the self resonant freq of the capacitor. Try with small ceramic caps instead. \$\endgroup\$
    – Blup1980
    Aug 11 '14 at 10:22
  • \$\begingroup\$ You say the scope picture is the output of a 5VDC supply. Yet the peak to peak value exceeds 12 volts. Something doesn't make sense. \$\endgroup\$
    – Barry
    Aug 11 '14 at 12:54
  • \$\begingroup\$ After a few ms the voltage is back to a normal 5V DC. I can't explain the peak to peak voltages, this is why I posted it here ;) But after adding the capacitor it got a little bit better. \$\endgroup\$
    – cgross
    Aug 11 '14 at 17:18

I initially wanted this as a comment but it seems I don't have enough reputation for this.

A few questions that might help cornering the issue :

  • How much power does your fan require? Have you tried measuring the current it draws when powered on? Is it significant compared to the max Amperage you can have on your 230V? (I shouldn't think so but maybe your fan is some kind of huge thing).

  • Have you tried replacing the fan with something of similar power needs (or higher) to see if the effect is the same? Things like a boiler, a toaster or a vacuum cleaner could be good tests since they can draw quite a lot of current.

  • To see if the problem is your 5V power converter, when the fan is not running, is the output really a "smooth" 5V? Could you measure the AC input of the 5V converter when the fan is on? Just to see if the peak-to-peak value of your 230V AC decreases when the fan is on.

  • You say that when your turn on the fan, the Ambilight flickers for a moment. Does it mean that after some time, even if the fan is still on, the 5V goes back to a normal behaviour?

  • \$\begingroup\$ Hey, thanks for the reply. I don't know how much power the fan draws, but other consumers like my electric blinds also cause this behavior. The output is smooth until I turn the fan on, then it will oscillate for some ms and after that it's back to a stable 5V. \$\endgroup\$
    – cgross
    Aug 11 '14 at 17:16
  • \$\begingroup\$ I sort of guess that the current rush when turning on a device might cause your 230V AC to be altered for some time, likely the peak-to-peak voltage decreases during a few ms causing the oscillations on your 5V. In other words I don't really have an answer to give you.. (It could maybe help to see what happens to your 230V when turning on your fan or electric blinds). \$\endgroup\$
    – DylanM
    Aug 12 '14 at 9:43

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