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PSU negative output to ground (earth)?

Discussion in 'General Electronics Chat' started by earckens, Oct 4, 2017.

  1. earckens

    earckens Member

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    I have a PSU with isolated outputs. When I connect my scope ground to the PSU negative sometimes (more often than not) the overcurrent sense kicks in and draws the positive to 0V (although no overcurrent or short circuit is present, it is just the overcurrent circuit that is very sensitive).
    When I connect a 10R 50W resistor between earth and the PSU negative I measure 0,000Vdc and 0.00175Vac (or less) over this resistor. When I measure between earth and PSU negative there is 0Vdc and about 46Vac, fluctuating.
    It is very annoying when doing measurements with my scope on sensitive electronics circuits that my PSU kicks out or generates oscillations on its output.
    Should/may I either place a large ceramic or metalfilm capacitor between earth and the PSU negative? Or provide a removable link between both?

    Edit: when I remove this resistor, the PSU keeps working. When I connect this resistor while the PSU is on, more often than not (again) does the PSU kick out. When I switch it off and then back on again with the resistor connected, all is well.
     
  2. ci139

    ci139 Active Member

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    yes, but what the meter shows when both probe terminals are floating in the air
    there is also that https://en.wikipedia.org/wiki/Earthing_system
     
  3. earckens

    earckens Member

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    Nice to read but not what I asked. Given my measurements I think I can safely say that the PSU negative is floating nicely. But I would like to resolve the consequences.
     
  4. dave

    Dave New Member

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  5. ci139

    ci139 Active Member

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    is it like static building up a mega-ohm discharge - just perhaps
     
  6. earckens

    earckens Member

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    No static build-up, there is no static anywhere around the PSU: the earth is well connected to the chassis and the negative output is well connected to the PSU electronics. The issue is about a floating PSU negative: consequences of its disconnect from earth. If you read my first post you notice from my measurements that there is virtually no potential difference and certainly no high impedance issue as would be the case with "static electricity".
     
  7. JimB

    JimB Super Moderator Most Helpful Member

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    I had a similar problem with one of my home made power supplies.
    The one shown in post #16 of this thread:
    http://www.electro-tech-online.com/...atile-bench-power-supply.127461/#post-1059392

    The output was floating from earth and could cause problems in circuits under test, with a large AC voltage being developed between the 0v side of the supply and earth.

    To fix the problem, I simply connected a capacitor and resistor in parallel, and connected them between the negative outlet terminal and the case of the PSU which is earthed.
    The capacitor presents a low impedance to the stray AC, which is caused by the interwinding capacitance of the PSU.
    The resistor prevents the build-up of charge on the capacitor, due to ??? who knows what effects in some future use.

    The values of the resistor and capacitor?
    Cannot remember, and it looks as though I did not go back and annotate the PSU schematic with the values either.
    At a guess, the capacitor was a 1uf polyester type, and the resistor was 100k Ohm.

    Have a bit of an experiment and see what works for you on your PSU.

    JimB
     
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  8. earckens

    earckens Member

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    Good. But I was thinking more in the line of a separate (yellow) output plug for earth, as you can find on industrial PSU's where yellow and black output plugs can be shorted (or left open) so that the PSU negative is at the same potential as earth.
    And install your proposed polyprop cap and 100k resistor to reduce stray potential differences.
     
  9. JimB

    JimB Super Moderator Most Helpful Member

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    A good idea.
    Many commercially produced bench PSUs have an arrangement like that, often with a "shorting bar" which can be slid so that the PSU can be configured for positive or negative earth as the situation requires.

    JimB
     
  10. ci139

    ci139 Active Member

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    the switching isolated flyback can only define electrically related signals by coupling through TF - the single RC may be insufficient with PFM (as well as it and may be incase of PWM)

    also the large or "wrong" C value × high potential difference to "ground" , any significant potential excursion of the "ground" (← it's why the wiki article) may cause secondary side instabilities
     
    Last edited: Oct 5, 2017
  11. schmitt trigger

    schmitt trigger Active Member

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    Many moons ago, when audio meant turntables and LPs, I had a similar problem with a preamplifier I had built. It sounded very nice, but had lots of hum. It would drive me crazy!

    One day, I saw on a commercial unit that it had a capacitor/resistor combination from the PSU's common to chassis, which itself was grounded thru a 3 prong connector.
    In desperation, I copied the values (can't recall the exact values, but were in the vicinity of 100k, 1000pF), installed them in my preamp, and the hum subsided!

    At the time, I could not explained why the fix had worked.....it took many years afterward for my to surmise that it must have been the transformer's interwinding capacitance.
     
  12. earckens

    earckens Member

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    Just to let you know: I used your solution, connected a 100k or so resistor and polyprop 1uF cap in parallel between earth and the supply negative and never again had this issue.

    I have the yellow earth output ready for installation too.

    Grts,
    Erik
     
  13. JimB

    JimB Super Moderator Most Helpful Member

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    Good to know.
    Thank you.

    JimB
     

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