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Measurement Techniques without Earth Grounding an Offline smps

Discussion in 'Electronic Design' started by D from BC, Feb 21, 2007.

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  1. D from BC

    D from BC Guest

    I'm thinking about methods to test a non-isolated homebrew 130W
    offline smps.
    I need to detect RF bursts "birdies", ringing, spikes and overshoots.
    An earth grounded scope will blow a fuse.

    I'm trying out:
    Diode detector circuits with an analog meter.
    I suspect wrong readings from my digital DMM due to the HF.
    Also, it's only rated to 350Hz without true RMS. :(
    Just touching a pulse node with one probe scrambles the DMM display.
    (300Vpk square wave at 100khz.)
    I also get a nice hiss from my stereo speakers.

    How about...a pick up "sniffer coil" on a scope probe.
    If that works??
    Just a coil at the end of a scope probe.
    EMI is not such a bad thing... It's a test signal. :)

    I have extra isolated windings on the power inductors. That's safe to
    earth ground for scope use..

    I'll be interested in any nifty poorman test tricks...
    D from BC
  2. Gibbo

    Gibbo Guest

    You have a dual trace scope?

    Can it display an "A+B" trace?

    Can it invert one trace?

    If so then you can display the voltage difference between *any* two
    points anywhere in your SMPS
  3. It will not if you use an isolation transformer!

    Otherwise you are stuck with differential measurements, which suck because you
    use two channels on the scope for each trace.
  4. Eeyore

    Eeyore Guest

    A *real* differential input is better though.

  5. D from BC

    D from BC Guest

    I only have a single channel scope.

    The challenge is making measurements and test techniques without a
    isolation transformer nor an isolated scope to detect RF bursts
    "birdies", ringing, spikes and overshoots.

    I suspect it's a little bit like making radio tuner/demodulator ccts
    and measuring a DC out with a DMM.

    D from BC
  6. D from BC

    D from BC Guest

    Single trace. :(
    It's the stone age over here :)
    D from BC
  7. pointless ... like deciding to run the 100 m sprint with your left hand tied to
    your right foot behind your back?
  8. Joerg

    Joerg Guest

    That's how I do it. But not at the end of a scope probe. Take a coax but
    make sure it's 50ohms and not 75ohms, they can look deceptively similar.
    Then switch you scope to 50ohms input or terminate it via a T-connector.
    Now wind a coil of 1/2" diameter or less. About 3-4 turns, nice and
    tidy, and use wire that has an insulation that isn't easily worn or
    scraped off. Solder to shield and center at the end of the coax. Now you
    have an H-field sniffer. If ye olde Hickock scope has enough bandwidth,
    that is.

    The other trick is the E-field smeller: Take the same kind of coax,
    strip the shield about 1/4" but do not strip the center conductor.
    Insulate the tip well. I use Kapton tape. Anything that won't scrape
    off. Make sure the center doesn't poke through (which it easily does
    when bending the coax back and forth). Now you have an E-field probe.
    Same here, if the scope doesn't have enough BW you may not see a thing
    even when the birdie cometh along.

    Well, those are the tricks, pretty much. Since I do this stuff for a
    living I abandoned the poor man's method a long time ago and use an EMCO
    probe kit. But that would set you back by about $1000. Believe it or not
    but I managed to wear out one of its probes already.
  9. Rich Grise

    Rich Grise Guest

    If you're crazy enough, you could float the scope, but of course, then
    the case could be at line voltage - use one of these or equiv:

    I do not advise this, it is dangerous and can be lethal - it's up
    to you to decide if you want to take the chance. In any case, do NOT
    touch any metal part of the scope.

    Good Luck!
  10. Definately read AN70 from, nice bit about sniffers

  11. D from BC

    D from BC Guest

    On Wed, 21 Feb 2007 16:42:44 GMT, Joerg

    I'm hoping for low freq. Big Bird size birdies to see on that 40yr old
    25Mhz BW Hickock scope.
    I'm guessing my scope has a 6dB/oct attenuation.
    So....mmm.. down 24dB'[email protected] at 400Mhz....4th harmonic.
    Those birdies have a f limit somewhere...
    Maybe I can alter my PCB construction and make some "cats" :)
    I might try too..
    I didn't know it was an industrial product...
    So I did some googling.. and found some reading material on:
    About near field probes use.

    D from BC
  12. Joerg

    Joerg Guest

    With a 25MHz bandwidth I am afraid you may not see them. Unless there is
    some serious "slow" ringing going on. Think of humming birds, very
    small, very fast and cats usually can't catch them ;-)

    Yep, even came in a little black suitcase, with calibrations sheets and all.

    Can't see it. Seems to be one of those members-only sites. You'll be
    surprised what you see on the scope when you hold that probe near one of
    those laptops power supplies. Makes the toe nails curl.
  13. D from BC

    D from BC Guest

    I've been tempted..
    The scope I'm using has an all metal grounded face plate so turning
    knobs will be dangerous..
    I could do a back away, power up, power off, adj and repeat routine.
    Or maybe poke at the knobs with a stick :)
    With no earth ground connection to the scope there's no line power
    60Hz loop.. No fuses will blow.
    However, should I be concerned with my scopes high frequency isolation
    from line and neutral? Just wondering if it will throw off the
    measurements when testing the offline smps.
    D from BC
  14. D from BC

    D from BC Guest

    Found it...Page 55 on,C1,C1003,C1040,C1130,P1535,D4159

    That's cool... Sleek and compact...
    Looks like I could cruise over 30mil traces and not pickup too much
    crosstalk from other mag sources..
    Not only that...this app note has lots of other goodies too.

    In my OP, I called it a "sniffer coil". It's something I probably
    heard of some 15years ago. Then forgot all the details but the sniffy

    D from BC
  15. Rich Grise

    Rich Grise Guest

    Sounds like an excellent idea. :)
    If your probe has one of those little ground clippies at the end, that
    should provide a reference - the freq. response might be affected, and
    there could be some 60Hz pickup because the scope case will be capacitive
    to whatever's around it, kinda like an antenna, but if it's a decent scope,
    it should ignore that and just show what's between the probe tip and its
    ground clip.

    I've been known to do this, and I've seen others do it, but never while
    alone in the room. :)

    And put your other hand in your pocket, seriously. In the USAF, they
    _ordered_ us to keep our other hand in our pocket, at least if we were
    probing anything over, say, 28V.

    Good Luck!
  16. D from BC

    D from BC Guest

    What do you think about catching birdies with a tuner (bandpass) and
    diode detector cct.?
    In other words, this is the poor mans spectrum analyzer.
    Is there a module or inexpensive unit that can tune 100khz to say
    I'm doing a google on that now..
    D from BC
  17. Joerg

    Joerg Guest

    Won't work very well. Birdies are short bursts of oscillations that
    happen during the phase where the FET goes through its linear region.
    IOW the duty cycle is extremely low. Even expensive spectrum analyzers
    have a hard time diagnosing that. I had to do that a lot in pulsed
    ultrasound and had to build myself some fancy limiters to get any kind
    of information out out the analyzer. So yes, it can work but it's a
    whole lot of effort.

    Probably. People have used TV tuners for that. But I never bothered
    after finding out the dynamic range of the average TV set. Sometimes I
    wonder whether their designers actually knew what a dynamic range is.
  18. On Wed, 21 Feb 2007 19:59:02 GMT, in D from BC

    pick up "sniffer coil" on a scope probe.
    Think you will need some serious insulation on yours. Remember, always
    keep one hand your pocket playing with this stuff

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