Grounded metal desk problem

Discussion in 'General Electronics Chat' started by magnet18, Apr 5, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, I recently converted an old computer PSU into a bench power supply, and it's metal, sitting on my metal desk.
    In doing this, I unknowingly created a very good ground for my desk. I realized this when I rested my arms on my desk and shocked myself every time my finger touched the bare metal where my A key should be (I really should fix that...)
    In any case, it's enough to be painful, and I would like to know that the best way to stop it is. I currently just plug in the USB cable to my printer, which is also grounded, so it shorts the problem out, but I don't really like the fact that theres transient voltages on my computer frame.

    Also, my voltmeter didn't pick anything up, either AC or DC.

    Thoughts?
     
  2. debe

    Well-Known Member

    Sep 21, 2010
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    If the shock is painful, then Id be woried. Obviously there must be a fair bit of leakage through your computer. Sugest you fix/ insulate the A button. Im not a great fan of metal desk tops for electronics, to risky for my likings.
     
  3. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    It's not just the A key, it's anything metal on the laptop (theres a decorative, brushed steel faceplate around the keyboard)

    And I am considering cutting a piece of plywood and putting it on top of it, just to keep from shorting out my circuits. (don't worry, if it's above what my 18Vdc supply puts out I do it in the garage)

    But getting a different desk doesn't change the fact that theres a substantial voltage on my computers frame. :(
     
  4. mbxs3

    Active Member

    Oct 14, 2009
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    Is it a 2 prong or 3 prong power cord for your PSU? If 3 prongs, is the E prong connected to the metal chassis to provide an earth ground?

    You could also ground the metal frame of your desk while you are at it.
     
  5. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Two prong, which is probably where the problem lies.
    I'm not worried about it reaching dangerous levels, since it goes through a converter to feed DC in (I mentioned that it's a laptop, right?), so is it possible for the mains neutral line to have a voltage potential?
     
  6. mbxs3

    Active Member

    Oct 14, 2009
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    Honestly, you probably know more than I do with regards to that. If I were in your position, I would start my properly grounding the metal desk frame. Then I would just ground the chassis of the PSU to the desk frame.
     
  7. debe

    Well-Known Member

    Sep 21, 2010
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    You got me thinking Magnet, We have 240V mains here & im using a laptop. Using a digital volmeter on AC range i measure 3.7V ac betwean Earth pin of mains & metal parts of the laptop. This i expect through leakage, in fact a small load drops the voltage to nearly zero & i dont get shocks. If you earth the bench i think it will be worse. The fact yours bites sounds like a fair bit of leakage through the powersupply. Went out & tryed several SWMP plug packs & they all read from 10 to30V AC betwean neg terminals to Earth, but put a 1M ohm resistor across it drops to about 2V AC. But they dont Bite.
     
    Last edited: Apr 6, 2011
    magnet18 likes this.
  8. eblc1388

    Senior Member

    Nov 28, 2008
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    Your post is not making much sense. If the grounding for the desk is good then you should not have received any shock from the leakage current.

    Your thread title is also misleading to say the least. What you really meant is "Minor shock from touching ungrounded metal desk"
     
  9. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    No, the desk is grounded, but the computer is not, so my arm creates a path between the two :(
    and yes, the title was poorly chosen, I apologize.

    I think I'm going to place a grounding pad on one of the feet for my laptop...
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    I see. Thanks for clarification.

    The leakage current is probably due to the safety capacitor between HV side and low voltage side.

    However, a grounded metal desk is a very bad idea for people experimenting with AC voltage. You should consider to use some form of desktop cover if possible.
     
    magnet18 likes this.
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    You might want to consider a piece of 'Plexiglas' instead of the plywood. It will last longer and gives the added benefit of a place to display things like charts and diagrams of often used components and things of that sort.:)
     
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  12. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, I'll just leave it alone then

    I think I will, how about cardboard (temporarily)?
    plexi glass it a good idea too...
     
  13. PatM

    Active Member

    Dec 31, 2010
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    Have you considered using a Isolation Transformer ?
     
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  14. t06afre

    AAC Fanatic!

    May 11, 2009
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    The laptop you are using is most probably powered by a power supply. And I am almost sure this power supply is A Class II or double insulated. So your laptop would be floating in respect to the mains. An electrical appliance which is double insulated does not have an earth wire fitted. There are also strict requirements relating to the maximum insulation resistance and leakage to any functional earth or signal connections of such appliances. Products of this type are required to be labelled "Class II", "double insulated" or bear the double insulation symbol
    [​IMG]
     
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  15. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    I don't understand exactly what your setup looks like, but I really don't like the idea of you getting a shock. It sounds like a leakage current and possibly a significant one. You need to track down where it's coming from and fix it or destroy the thing that's causing it.

    Your AC supply power should be on a GFI circuit. This does NOT fix the problem with the leakage current, but it can go a long way in protecting you from getting hurt by it in the meantime.

    Leakage testing isn't hard to do, but it's not really something that a beginner should attempt, mainly because it can be hazardous if you don't understand what you're doing, have the right equipment, and have some training -- and then know what to do with the data once you have them.

    Unless you have the skills necessary to troubleshoot this, I'd suggest getting a professional involved like an electrician. That leakage needs to be FIXED.
     
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  16. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Yes, my power supply is double insulated, and yes, I have everything on a GFCI.

    I'm fairly good at assembling/disassembling laptops but I'd rather not go poking around inside...
     
  17. roadey_carl

    Active Member

    Jun 5, 2009
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    Can you take a picture of your setup please? Also where have you supplyed the power to your bench from? This would help me understand more about the equalpotentiol diffrence of your earthing system. Switch mode power supplys leak current to earth as part of there function. we call it a Functional earth in the UK! it sounds like there is more to this fault as just earthing all of your metal together! pics would be a great help!
     
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  18. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I'll post a couple pics in a sec.
     
  19. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, heres the pics

    In the first pic you can see the modified PSU (the one that isn't double insulated, and that has had the warranty ripped to shreds about 1000 times over, IS plugged in to the GFI), next to my laptop (double insulated powersupply, 2 prong, not plugged into the GFI), next to my axillary monitor (also not on the GFI, I thought more was in the GFI, Ima fix this...).

    Plugging in the auxiliary monitor or the USB cable to the printer will stop the shocking because it provides a chassis ground connection.

    The second pic shows the printer (not on the GFI, as you read this I'm plugging the monitor and printer into the GFI), and the power strip that powers my laptop (no GFI, should I go get one for it or does it not matter since the laptop power supply is only 2 prong?)

    In addition, anything I plug in other than the stuff already mentioned is on a powerstrip that is plugged into the GFI (same strip as the modified PSU)
     
  20. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, any and everything that has a ground prong is now plugged into the GFI, with the exception of the mini-fridge, which doesn't touch anything.
    After doing this, the shock is no longer painful, and isn't noticeable unless I slide my arm back and forth, in which case I can feel... something... feels like it's 60Hz.
    I feel the same thing rubbing the metal on my jukebox when standing on the cement on the basement, and I know it's had a short for as long as I've been alive.
     
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