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An unusual Oscilloscope phenomenon

Discussion in 'Hobby Electronics' started by David L. Jones, Jun 25, 2009.

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  1. Rich Grise

    Rich Grise Guest

    Well, then I guess using Henkel Pattex glue isn't all that great an idea. ;-)

    Use just plain ol' ordinary common off-the-shelf "contact cement", and
    let it DRY before you start placing the shims. Come to think of it,
    when I was a video game tech, the shipping guy used to do it to the
    sides of video games when they'd convert a cabinet from one game to

  2. Joerg

    Joerg Guest

    Same in this here lab :)
  3. Rich Grise

    Rich Grise Guest

    I think half of us are talking about low-noise audio frequencies, and
    half of us are talking about RF. The way I heard it, you leave one end
    unshielded for audio preamps and stuff, but ground both ends for RF;
    I think it has to do with wavelength and, well, propagation
    characteristics. Of course, the thing that was drummed into me while
    they were teaching me this was "GROUND LOOPS!". :)

  4. Joerg

    Joerg Guest

    Trouble is, since we moved from Alexander Graham Bell's wall phone (with
    crank) to cell phones RF frequently turns into audio after getting into
    such single-end shielded cables. All it takes is an opamp with bipolar
    input structure.

    Of course, with a good old tube amp you'll continue to be fine :)
  5. Joerg

    Joerg Guest

    Unless the shield slowly commences a faint red glow, followed by some
    "amperage scent" wafting through the room ...

    Seriously, I've seen that happen.

    Yup. OTOH that provides a good business opportunity 8-D
  6. Rich Grise

    Rich Grise Guest

    Because that's what it said to do in the instructions that came with
    the kit.

  7. Nico Coesel

    Nico Coesel Guest

    I wish... I'm working on interfacing a current sensing coil from a
    welding transformer. During welding there is a couple of hundred volts
    everywhere. This project starts to give me a headache :)
  8. Joerg

    Joerg Guest

    Try differential transfer, shielded twisted pair or Twinax soldered to
    the burden resistor, from there to your measurement box, place a 2nd
    signal transformer there. Make sure the twisted pair is source- and/or
    end-terminated properly, usually 100-120ohms. And never, never use this
    connection as the burden resistor even if there is a fire extinguisher
    at hand ...
  9. Nico Coesel

    Nico Coesel Guest

    I already went for a differential input. Solves most of the problems,
    but not all but I'm pretty sure the inputs where overdriven as well. I
    fixed that, but didn't field-test it. A 2nd signal transformer won't
    fly because the pulses may be quite long (up to 100ms).

    The problem is that nobody seems to know what the pulses look like
    exactly. The transformer is pulsed by a 1000Hz PWM signal. This makes
    me assume the current sensor sees pulses as well. But the shape
    probably depends on the induction in the welding circuit while
    carrying many kA...
    That might be worth something to explore. The burden resistor (1k is
    specified) is now on the measurement board itself. I'd like to keep it
    there though to keep the installation procedure easy. The wiring in
    the test setup is pretty short (2 meters) but bad (not twisted,
    unshielded). I'll order some shielded twisted pair tomorow. I already
    figured this must be part of the problem.
  10. Joerg

    Joerg Guest

    Recipe against headache in cases like this: Proper matching of literally
    everything, shielding, a glass or two of Beerenburger.

    100msec? Is it a DC welder? Anyhow, 100msec is feasible if you use a
    small doorbell transformer, the kind with two compartments where you can
    scrap one of them out with wire cutters and without having to
    disassemble the core. Calculate it so it's well under the saturation
    limits at 10Hz. Commercial current transformers can also go that low if
    you use one that's totally oversized for your current.

    Unless the input diff-amp has a huge common mode range even for fast
    stuff such as spikes I am not sure you can do it without another
    transformer at the receiving end. Or rather, I've seen cases where it
    didn't work.

    If it's fed 1kHz then 1kHz should come out.

    Not good. Increases noise, big time, because the cable is mismatched
    because even regular cord won't have a Z of 1000ohms. And if the cable
    ever comes off ... tungggg ... phssssss ... *PHUT*

    Ook niet zo goed. In an environment like this you can't work without
    shielded twisted pair, definitely not across two meters. That's like
    trying to listen to a Mozart concerto on a Harley-Davidson.
  11. Rich Grise

    Rich Grise Guest

    Can you get decent yakisoba in SFO? I also like plain ol' ordinary
    fried rice, but I can make that at home; I wish I had the recipe
    that the Java Curry Shop in Shinjuku used - best curry I've ever
    had! :)

    Yes, Japanese curry. :)

  12. Nico Coesel

    Nico Coesel Guest

    Shielding sounds doable. The Beerenburger is out of the question
    though :)
    Yup. It's a DC welder. I already tried a small mains transformer but
    that doesn't work out very well in a test. Besides that would exceed
    budget and space constraints.
    I think I have a pretty decent diff-amp. It can handle about 800V p-p
    common mode.
    The cord coming off doesn't seem to be a problem. All similar systems
    I've seen so far have the burden resistor on the measurement board,
    not on the welding transformer. But I'll try it anyways. The old ways
    are not necessarely the best ways :)
  13. Tim Williams

    Tim Williams Guest

    Every CT I've built has had the resistor as close to the transformer (i.e.,
    on it) as possible. I've had quite awful results putting the resistor on
    any kind of length. Come to think of it, that may be more due to the
    resistor itself having length, which would be a problem. Still, better safe
    than sorry...

    'Course, I wind my own, encapsulated in masking tape, so embedding a burden
    resistor is no big deal. ;o)

  14. Joerg

    Joerg Guest

    I liked this stuff ;-)

    A doorbell transformer blows the budget on a welder? Man, and here I
    thought I was uncle Scrooge.

    In a welding environment 800V may not be good enough. Plus you'd have to
    check the CMRR for fast pulses, like when the arc starts and stops.

    Often they sure aren't. A burden resistor at the other end of a cable is
    usually a bad idea.
  15. terryc

    terryc Guest

    Which "welding" environments did you have in mind?
  16. Joerg

    Joerg Guest

    The usual. >100 amps, electrodes touching, arcing, getting stuck once in
    a while, thick cables floating around the room, ground potential of
    equipment bouncing, and so on. I once told someone to get his ham radio
    gear out before we start welding a broken gate. "Nah, this stuff is
    tough" ... "Better get it outta here" ... "No, really, it's ok, just get
    cracking on the gate" ... "Ok then" ... bzzt, bzzt, bzzzzzzzzzzt ...
    beep ... *PHUT*
  17. terryc

    terryc Guest

    Umm, have you ever put a volt meter across the cables?
    I was under the impression that that stuff matched the telephone and was
    in the vicinity of 40volts.

    I politely suggest that if it was around 800volts, then black and crispy
    would be a welder who made the wrong circuit.

    Hint, it is the current that causes the field.
  18. Nico Coesel

    Nico Coesel Guest

    The problem is that there is an existing board. The new board must be
    more or less budget neutral. I blew most of the budget on 0.1%
    resistors and instrumentation amplifiers.
    Its for a resistive spot welder. There is no arc. Just tens of
    kiloamperes! An arc would be very bad for the particular application.
    I hope I can get some testing on a real rig this week.
  19. Joerg

    Joerg Guest

    Hint: There's lots of inductances involved, wanted ones as well as
    undesired ones. Inductance -> current -> current suddenly stops ->
    inductance says "I don't think so!" -> tsssk ... *PHUT*
    And mains connected power supplies. That's what croaked in the radio.
    The spikes on a mains circuit of sufficient length (and thus impedance)
    look rather terrifying.
  20. Joerg

    Joerg Guest

    Then there's cost reduction potential :)

    No way to clamp anything so you don't need all these precision parts?

    Until the spot contact lets go at a not so good moment, or one of the
    panels to be welded had a bad spot there, or a cable breaks, and so on.
    Never assume things to remain as perfect as they are on paper or in the
    engineering lab.

    Good! I'd test for faulty conditions as well, like the ones above.
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