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Lab bench instruments and laptop earthing

Discussion in 'Electronic Design' started by powerampfreak, Apr 12, 2013.

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  1. Greetings all!
    I need to consult you guys for a concern regarding lab bench instrument's and laptop grounding scheme.
    I'm using the laptop (which is a class-2 device, non earthed) together withthe Pickit3 programmer/debugger. The Pickit3 is connected to the target application which is powered by an external powersupply. Connected to the application board is one oscilliscope and two waveform generators.
    So, how should I earth all this to prevent anything to burn.
    Between the laptop "ground" (measured on an USB connector shield)and earth I have about 80VAC when the battery charges is connected to mains.
    This should be due to some Y-caps in the charger SMPS. The charger has no connection to earth so those Y-caps are probably connected between primary and secondary ground (laptop ground).
    I have had the oscilloscope floating, supplied from an isolation transformer since I've worked on SMPS's lately. But now I rather have it earth connected to minimize the leakage current from the oscilloscope SMPS and it's Y-caps to my application board.
    So, my question is: Is it safe for the laptop and Pickit3 to connect to the application board if I earth all the instruments. What worries me is, will that 80VAC potential cause any damage to the Pickit3 or even worse, the laptop?
    Thanks for your suggestions, and have a great weekend you all!
     
  2. Phil Allison

    Phil Allison Guest

    "powerampfreak"

    I have had the oscilloscope floating, supplied from an isolation transformer
    since I've worked on SMPS's lately.

    ** What a fuckwit thing to do.

    Common sense says to use the ISO on the SMPS - not the damn scope.

    But you have never shown any sign of sense.



    .... Phil
     
  3. mike

    mike Guest

    Stuff breaks...but if your stuff ain't broken, it's typically OK to
    connect the chassis/safety grounds together.

    The problem happens when the safety ground is on the connector that
    your plugging/unplugging.

    I have an external hard drive with eSATA connection. A cable can connect
    it directly to the motherboard. And it's FAST compared to USB.
    Problem is that I am unwilling to risk my motherboard on
    a glitch during plug/unpluging it. Or a static discharge that
    has too much inductance to the safety ground path.

    I once trashed a motherboard by plugging in a printer cable with
    nothing on the other end.
    As a percentage of the times I'd plugged something in, it was about zero.
    But that's no consolation when your motherboard is dead.

    You balance convenience with risk and take whatever chances you can
    live with.

    As for floating the scope, don't do it.
    Use isolated probes or make a differential measurement with
    probes that can stand the worst case excursion.

    The use of an isolation transformer when testing any equipment
    is a good idea for the same reason you use safety glasses...in case
    you screw up and accidentally touch something live. It's not a license
    to to connect a wire between ground and something with a lot of relative
    volts on it.
     
  4. Phil Allison

    Phil Allison Guest

    "mike"
    ** Or just float the circuitry with an ISO tranny.

    ** Not it isn't and there is no such comparison.

    Routinely using an ISO tranny on *all* equipment under test /repair is
    STUPID and DANGEROUS !!!

    This has been thrashed out here several times now and there is NO
    justification for the practice.

    ONLY if you need to scope the AC line side of a device ( PSU etc) should an
    ISO tranny ever be used at all.

    Technicians must never get the idea that it is safe to handle ( or ground )
    the live parts of a circuit - or one day it will kill them.

    Plus, using an ISO hides serious fault conditions in equipment, either pre
    existing or due to errors by the technician.

    So, once the need for the ISO had gone, put the item back on the regular AC
    supply immediately and place the ISO out of sight so there is no confusion.


    .... Phil
     
  5. Den lördagen den 13:e april 2013 kl. 04:29:07 UTC+2 skrev Phil Allison:

    Phil - give me a break! You talk about sense... please direct your rude comments to those who deserve it.
    I must say you don't show much of a sense yourself really
    Reason for ISO the scope is simply I have not a ISO tranny of 20kW which would be needed to power the SMPS.
    But that's not what the topic was all about in this thred, maybe you got that wrong.
    In order to connect my class-2 laptop together with the pickit3 debugger which connects to my application board which has several instruments hooked up to it. What is the best way to earth everything to minimize the risk of damage to the laptop or instruments, was the meaning of this thred.
     
  6. Neon John

    Neon John Guest

    Just killfile him and he'll never annoy you again.
    The short answer is, you don't.

    The long answer is this. I'm in pretty much the same situation. The
    induction heater logic boards that I design run at line voltage.
    Therefore there must be galvanic isolation between it and everything.

    The most important piece of kit is a USB isolator. More than once
    I've forgotten or gotten in a rush and plugged a heater into line
    power to do a quick firmware update. The USB isolator saved the day.

    Sorry I can't give you a brand name. My partner who does all the
    procurement got it from E-bay I think. It is a blob about the size of
    an Aruino covered in thick heat shrink. I hi-potted it to 1500 volts
    before using it.

    I run my scope on an isolation transformer but that's just to protect
    me from stupid. I can't make isolated measurements that way because
    of the huge capacitance of the ground side vs the signal side of the
    probe. For scoping, I use a differential probe similar to this

    http://www.ebay.com/itm/New-Profess...457?pt=LH_DefaultDomain_0&hash=item5892516b41

    If that link doesn't work just search for "differential probe". This
    instrument will allow me to see things like the gate drive signal of
    the high side FETs in an H-bridge.

    Finally, I DO have a 20kVA isolation transformer. Actually 2 10kVA
    Topaz transformers in parallel. Keep an eye out on sleazebay. They
    occasionally show up and are cheap.

    I have a 20kVA variac, a set of contactors and the isolation
    transformers between the line and my connection to the unit under
    test. The contactors are operated by a foot switch so that the UUT is
    energized only when I explicitly command it to be. THAT has saved my
    butt many times.

    John
    John DeArmond
    http://www.neon-john.com
    http://www.fluxeon.com
    Tellico Plains, Occupied TN
    See website for email address
     
  7. Phil Allison

    Phil Allison Guest

    "powerampfreak"

    Phil - give me a break! You talk about sense... please direct your rude
    comments to those who deserve it.


    ** For ****'s Sake Fool - have a look which post you are replying to !!

    This one was NOT directed to you !!!
    ----------------------------------------


    I must say you don't show much of a sense yourself really
    Reason for ISO the scope is simply I have not a ISO tranny of 20kW which
    would be needed to power the SMPS.


    ** That makes NO SENSE AT ALL !!

    But that's not what the topic was all about in this thread, maybe you got
    that wrong.


    ** This is now a sub thread - fuckwit !!!!

    Try learning to FUCKING READ !!!

    And fix your damn news reader and STOP it doubling all the line spaces.

    You are an IDIOT - piss off.


    ..... Phil
     
  8. Phil Allison

    Phil Allison Guest

    "Neon Nutcase John"
    ** YOU are a PRIZE **** aren't you ?



    ** No YOU are 100% stupid to have a floating scope.

    I sincerely hope it kills you.
     
  9. I thought this was a place of engineering knowledge spreading, not a place to throw sand on each other in the kindergarden. :)
    Of course my intention was not be gain enemies here...
    Phil, I don't think you could judge me being an idiot or not.
    Anyway, John it was interesting to hear about your high power UUT testing.
    An USB isolator sounds good, but still I need to power my debugger from USB power, so I cannot use it.
    Anyway, I will figure out something.
    Thanks guys
     
  10. Phil Allison

    Phil Allison Guest

    "powerampfreak" <

    ** ROTFLMAO !!!!!!!!!!

    Your idiotic fake name, your "hotmail" addy and

    VERY SINGLE THING you post

    SCREAMS:

    Warning: NUT CASE ON the LOOSE !!!

    **** off.
     
  11. Oh we do both here.
    Don't take it personally, there is indeed a lot of engineering knowledge
    to be had.
     
  12. It's quite humoristic how angry you are Phil.
    I hope it's not affecting your blood pressure too much, because I believe
    you are, after all, quite a nice guy :)

    Cheers
     
  13. Guest

    any real usb isolator would also isolate the power

    -Lasse
     
  14. Guest

    still need to power the pi so you are likely to get similar problems

    the laptop you can just unplug and run on battery

    -Lasse
     
  15. Jasen Betts

    Jasen Betts Guest

    Beaglebone gives you an easy isolated power over ethenet option, rPi not so much.
     
  16. Neon John

    Neon John Guest

    That's usenet for ya. Killfile half a dozen people and the group
    becomes generally pleasant.
    My isolator has a barrel jack on it for connecting 9 volts DC to
    supply the isolated side. I've never had to use it, as the isolator
    passes through enough power to run my JTAG-ICE. The blob gets a
    little warm so it's obvious that they're using transformer isolation
    to get the power across the barrier.

    If you need a specific brand, I can ask my buyer where he got it.

    John
    John DeArmond
    http://www.neon-john.com
    http://www.fluxeon.com
    Tellico Plains, Occupied TN
    See website for email address
     
  17. Phil Allison

    Phil Allison Guest


    ** There is nothing pleasant about NGs filled with freaks and trolls.

    People like you, that is.

    Fucking half witted ostrich.



    ..... Phil
     
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