As I am exploring instruments and played with them, in order to understand them better.

I have digital oscilloscope and Lab DC Power supply.

With DC Lab Power supply set to 5V and tip to positive terminal and clip to ground terminal, to my surprise, the mean reading is 430mV while RMS reading is 75.5V.

Using the clip on ground pane of any DC circuit board is akin to ground terminal of the Lab Dc Power Supply. The question is how are they different?

What can I do do with ground terminal of power supply, besides rail to rail DC output? Can I use this ground terminal to connect to any ground pane of the circuit board under test with +/- or + power supplied?

Thanks in advance

  • \$\begingroup\$ Did you try loading the supply? \$\endgroup\$ Commented Jan 5, 2017 at 5:54
  • \$\begingroup\$ No, at the time, it was not loading under test. I was trying to read some voltages like I would do with multimeter. \$\endgroup\$ Commented Jan 5, 2017 at 5:59

1 Answer 1


Many lab power supplies have "floating" outputs - neither output terminal is connected to chassis or AC power ground. I would expect such supplies to have three terminals on the front panel: +, -, and Ground, with the Ground terminal connected to the AC power ground, and the actual power supply output connected between the + and - terminals.

If you connected your scope ground clip to the supply ground terminal, and the scope probe tip to the supply positive terminal, I would expect meaningless readings on the scope, as there is no defined relationship (no connection) between the supply + terminal and Ground.

  • 1
    \$\begingroup\$ And so the solution is usually to connect a jumper between the power supply ground and (-) output. Some supplies even have a jumper clip ready to go for this. Sometimes it's better to just ignore the supply ground connection to avoid ground loops and other sources of noise. \$\endgroup\$ Commented Jan 5, 2017 at 7:31
  • \$\begingroup\$ Thanks Paul Elliott, your comment saved my day. In my country (old electric installations) are never enforced to use proper ground and my power supply was floating. I jumped -vcc to GND and voila. Ripple reduced from 2Vpp to about 100mV pp. \$\endgroup\$ Commented Mar 9, 2022 at 23:16

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