I have a question about a simple DC floating power supply: it is as simple as possible, secondary coil -> bridge rectifier -> filter cap, as shown on the figure below.

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If I connect the input of the oscilloscope between the negative terminal of this simple floating power supply and the earth ground (which is the negative terminal of the oscilloscope input too), I can measure 140 Vpp with the frequency of 50 Hz (figure attached).

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Why is this high amplitude voltage present between these two terminals? Can somebody explain to me the causes of this measurement result?

  • 2
    \$\begingroup\$ parasitic capacitance is a wonderful thing.... \$\endgroup\$ – Trevor_G Oct 20 '17 at 7:54
  • \$\begingroup\$ Mistake: the 230VAC supply isn't floating. Also, every transformer is a capacitor, where the primary coil is one plate, and the secondary coil the second plate. \$\endgroup\$ – wbeaty Oct 20 '17 at 8:24
  • \$\begingroup\$ Of course, the 230 VAC isn't floating, in this sense the drawing isn't entirely clear. Beside this, to make the description more accurate, I should mention that the power transformer has an electrostatic shield between the primary and the secondary, connected to the earth ground. \$\endgroup\$ – kalaq Oct 20 '17 at 8:47
  • \$\begingroup\$ Shield wont matter much for a floating input. Ever hook up just a length of wire to a scope.... AC noise is all around us. \$\endgroup\$ – Trevor_G Oct 20 '17 at 10:05
  • \$\begingroup\$ If you think it is really live.. turn it off, hook in a resistor to ground, turn it on again and check there is no current through the resistor. \$\endgroup\$ – Trevor_G Oct 20 '17 at 10:08

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