Grounded benchtop power supply and USB port

Discussion in 'General Electronics Chat' started by yardleydobon, May 3, 2009.

  1. yardleydobon

    Thread Starter Member

    Apr 19, 2009
    Please read carefully as my questions are a bit specific.:D

    I have a Parallax USB oscilloscope that draws power from the USB port only. The probe grounds (black IC grabbers) have a connection to the outside of the USB plug (all the way up to the host side). For the rest of this post I refer to the outside of USB plug as the shield connection, please let me know if this is wrong.

    My laptop power supply only has _two_ nonpolarized prongs on the AC side.

    I have a AC to DC converter I'd like to use as a power supply for electronics projects. It has a _three_ prong plug on the AC side and two wires on the DC side. Using an ohmmeter I see that there is a low resistance connection between DC return and safety ground. Therefore the DC output is not floating.

    So if I use this AC to DC converter to power my project and use the USB oscilloscope, the result would be a connection from safety ground through DC return into probe ground through the oscilloscope through the USB shield into my laptop (then to what? laptop chassis ground? laptop common?). Is this a bad thing? I'm worried about destroying my laptop somehow!:confused::eek:

    Which is better for electronics projects, a floating or grounded supply?

    I understand why appliances have safety grounds, but in this particular case would it be a horrible idea to disconnect the grounding prong from the above mentioned AC to DC adapter so that I may get a floating DC output. Would this ruin the adapter's EMC?

    Thanks for reading. Any enlightenment anyone could give me would be very appreciated.
  2. David Bridgen

    Senior Member

    Feb 10, 2005
    They're one and the same thing, and yes, the USB lead's shield will be connected to the computer's chassis.
    Most, if not all, bench power supplies do not have either pole of their outputs or the mid-point of a bi-polar supply, connected to Earth, but some have an Earth terminal on the front panel so that any one of its output terminals may be connected to Earth if desired.

    During a lifetime in electronics I don't believe that I ever encountered a supply in which any of its ouptut terminals were, as a matter of course, connected to either the unit's case or its Earth connection.

    Your laptop will come to no harm by leaving the Earth connection of the converter intact.
    On the other hand, you may come to harm if you disconnect it.
  3. AdrianN

    Active Member

    Apr 27, 2009
    As David points out, it is a bit unusual to have the DC return of a lab supply connected to chassis (earth). Still, you said you measured it. Therefore, in your system, you have one single earth ground. Yes, the USB interface has its shield connected to the laptop chassis, but your laptop is isolated from earth. The AC connector has two prongs which are isolated from earth. Your oscilloscope will float. Everything should work well.

    In a different case, if you connect the oscilloscope to a desktop, its ground will be connected to earth through the USB shield and desktop chassis. In this case you will have two connections to earth. Still no problem with it, with the exception of measuring high gain low noise circuitry. When two earth grounds are involved you might have ground loops, which will bring high levels of noise in your circuitry. When I have a case like this, I connect all the AC cords into an extention cord, and then plug the extention into the AC outlet. That way I create an earth star.
  4. AdrianN

    Active Member

    Apr 27, 2009
    One more thing:

    You should always have at least one earth connection to your system, for safety reasons. So, leave your DC return connected to earth. Let the USB oscilloscope be connected to chassis via probe and PCB under test and power supply DC return.