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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).

enter image description here

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

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    \$\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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