I connected a DC power supply (set to 706mV) to a 1kohm resistor as this:

enter image description here

The semantic is like:

enter image description here

However, strange things happen when I use the oscilloscope to view the voltage waveform at point A and B.

Below is what is shown on the oscilloscope - cyan wave for point A and yellow for B:

enter image description here

As you see the two points are all sine waves. The voltage difference between the two waves are always kept at 706mV.

I know the dc power supply is floating (not grounded to real Earth) and the oscilloscope here is (separately) measuring the voltage of the two points relating to the real Earth (because the oscilloscope is natively grounded) And I know that a floating circuit’s voltage related to the real Earth cannot be determined.

Yeh, this way truly the 706mV DC voltage is outputted between the two terminals of the dc supply - But, why is the dc power supply outputting two sine waves (relative to the Earth, one higher and one lower) to maintain a dc voltage? Why not straight output plain DC voltages? Is this for safety? As you can see in the oscilloscope picture above - the two sine waves are having much higher magnitudes related to the 706 mV desired voltage, which looks relatively unsafe though...

Also, I don't understand how a floating equipment can make such a voltage referring to the real Earth - previously I always thought there is a voltage difference from an object that does not touch the Earth to the Earth only when there is static electricity or an unequal amount of + - particles in the object.. Now I'm confused about this too...

  • 3
    \$\begingroup\$ The frequency of that sine wave is 50Hz, which im guessing is your local line frequency. \$\endgroup\$
    – Michael
    Dec 4, 2020 at 6:20
  • \$\begingroup\$ Yeh it is @michael \$\endgroup\$
    – gudako
    Dec 4, 2020 at 6:23
  • \$\begingroup\$ You almost always have weak coupling from the mains, which is why you'll need to ground the circuit under test to the scope. If you grounded the power supply output to its adjacent ground instead that would probably get most of it at low frequency, but you could still have a slight offset if there's any current in the mains ground (normally there should not be) and you will have susceptibility to high frequency noise when the scope and device share only a roundabout reference ground through the mains. \$\endgroup\$ Dec 4, 2020 at 7:59
  • \$\begingroup\$ @gudako: That picture of your work area makes me nervous. It looks to me like you are working with electrical stuff on a metal sheet desktop. That's asking for short circuits, and can be very hazardous if you do anything with line voltage. \$\endgroup\$
    – JRE
    Dec 4, 2020 at 9:00
  • \$\begingroup\$ @jre I have thick glass on the desktop \$\endgroup\$
    – gudako
    Dec 4, 2020 at 9:34

2 Answers 2


The output is isolated and floating, so it has very high impedance to earth/ground.

The oscilloscope inputs also have very high impedance to earth/ground, and the setup is a differential measurement where scope earth/ground is not connected to the circuit, so the strongest impedances that connect the supply outputs to earth/ground are the scope probes.

Any external disturbances, even small ones, are able to move the isolated output voltage with reference to earth/ground.

As the power supply has most likely a standard mains transformer inside it, even a weak capacitive coupling like for example the few picofarads of stray capacitance between transformer primary mains side and isolated secondary low voltage side can make the isolated output to have common mode voltage to be few volts of mains frequency sine wave in respect to scope earth/ground like what you are seeing here.

  • \$\begingroup\$ Did you mean the transformer inside the dc power supply? And, I just found the 5V output having the same COM as the second 30V output. By speaking 5V Did you mean that? \$\endgroup\$
    – gudako
    Dec 4, 2020 at 10:27
  • \$\begingroup\$ I also noticed that sine wave has the same shape and frequency as my residential power supply - Also, what transformer did you mention \$\endgroup\$
    – gudako
    Dec 4, 2020 at 10:34
  • \$\begingroup\$ Sorry I clarified. I meant the output you use, for some reasong I thought it was the 5V one. And yes, transformer inside the power supply. Sine wave at mains frequency would be expected waveform, yes. \$\endgroup\$
    – Justme
    Dec 4, 2020 at 12:51

Note ‘floating’ is relative - there is capacitance involved which couples the mains voltage. Earth your power supply 0V and the problems go away. You should have your cro probe earth connected to the power supply 0V


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