Bench power supply

Discussion in 'General Electronics Chat' started by cages, Jun 20, 2011.

  1. cages

    Thread Starter New Member

    Jul 3, 2010
    19
    0
    I have come accross a dual bench power supply that has both -ve oupts terminalls connected to Earth(the case) through a 6.8M ohm resistor.
    I have always assumed that electronic bench power supplies had floating outputs.
    This is the first time I have come accross this on a PSU. I noticed because it was affecting my measurments with a scope.
    Is there any reason to connect the -ve output to earth and is it common?

    Thanks,
    Cages
     
  2. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    I think it is for something like static electricity...building up a voltage difference between the case and the circuitry. I found one in my 'scope. Had to remove the case ground from the power cord to get the resistor out of the circuit.

    One point of view says removing the case ground is stupid and dangerous. Another point of view says blowing the ground clip off your probe when measuring the mains is stupid and dangerous.

    C'mon, the whole scope is running on its own isolation transformer if you can just get the case disconnected from the circuitry! Is this only about government regulations?

    I don't know.
     
  3. cages

    Thread Starter New Member

    Jul 3, 2010
    19
    0
    Thanks,
    I am use to the clip on the scope earthed but have never come across a PSU with -Ve output earthed by default.
    The PSU has a bannan socket that is earth. On other power supplies I have used if you need the -ve output to be at earth you would connect a cable between the earth output and -ve output.
    I have found the resistor on the PSU that is earthing the output was wandering why it would be manufactured that way.

    Cages
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Personally, I want my power supplies' outputs to be floating, but have a ground connection near so I can ground + or - output if I wish. On one HP supply I have I installed a toggle switch to do just this -- and wondered why manufacturers have never provided such a cheap convenience.
     
  5. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Personally, I build, hack, or modify anything I understand enough to be safe with my changes. Fourty years of experience keeps me and my mods safe. I can't recommend that beginners do this. I would probably get banned from the site if I did. The best I can say here is, be safe, electricity can kill you.
     
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