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TDS 210 - worrying leakage current !

Discussion in 'Electronic Design' started by Eeyore, Apr 15, 2007.

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  1. Eeyore

    Eeyore Guest

    I've been struggling with this utter POS scope again (it seems determined to
    trigger on any HF energy and reject LF for example) and ended up dragging my old
    CRT scope along in its place.

    When I got a trace on the conventional CRT scope that made sense / I could use
    (long story there) I decided to see if the TDS 210 could redeem itself. I hooked
    it up to the audio analyser we were using and it 'upset' things.

    I detached the BNC lead between the bTDS210 and the audio meter and it calmed
    down again. On attempting to re-attach it both I and my colleague saw a tiny
    blue spark. Simply touching the BNC bodies together caused this.

    I measured 0.15mA leakage current from the TDS210 ground to our EUT ground yet
    there is apparently continuity (0.1 ohms) from the TDS210 BNCs to the mains
    earth pin on the power lead. I've noticed before that it causes trouble when
    hooked up to other gear btw.

    Any thoughts (aside from having it checked). I noticed there was a recall on
    these for an earth problem but I can't see how it relates to what we're seeing
    here.

    Graham
     
  2. Phil Allison

    Phil Allison Guest

    "Eeyore"

    ** It may very well be related.

    http://www.tek.com/service/safety/tds200/index.html

    There was a high the risk of damage to the scope if (when, really ) the
    signal ( BNC) earth was connected to a source of significant energy - ie
    the AC supply, a large PSU etc. Doing this would blow open fairly light
    gauge copper tracks on the analogue PCB, destroy the scope and possibly
    electrocute the user.

    Only the latter prompted a safety recall.

    The fix was to add a length of 1.5 sq mm copper earth wire from the input
    BNCs to the AC supply ground near the IEC inlet.

    Depending exactly where this new ground wire was fitted, it may well pick up
    HF magnetic noise from the SMPS.

    Wound that explain your leakage current and misbehaving synch ??




    ......... Phil
     
  3. Eeyore

    Eeyore Guest

    That's certainly a possibility. I'll check later.

    Thanks Phil.

    Graham
     

  4. Sounds like a classic ground loop. You can use the differential
    voltage using the two channels (without grounds connected) right?


    Best regards,
    Spehro Pefhany
     
  5. john jardine

    john jardine Guest

    Funny you should mention it.
    A week ago I was noting to a customer their equipment had an earthing
    problem as I was picking up the occasional 'tingle' on my forearm. From
    mucho experience I know this would have been about 55Vac.
    Seemed odd, as I know their kit and the massive amount of metal and earth
    bonding that holds it all together.
    Common factor is their TDS scope I was using.
     
  6. Mike Monett

    Mike Monett Guest

    Isn't there a filter on the ac line that has capacitors to gound from each
    leg?

    All my computers, and TEK and HP equipment have it. Any time I lose the
    ground pin on the ac line I get that old 55V tingle.

    Maybe the TDS 210 has especially large filter caps.

    Regards,

    Mike Monett
     
  7. Ben Jackson

    Ben Jackson Guest

    Maybe this is too obvious, but many Tek scopes have that exact behavior
    as a triggering option.
     
  8. Eeyore

    Eeyore Guest

    Creating 0.15 mA ?

    You know I'm so annoyed at it I hadn't even tried ! I'll check that out.

    Graham
     
  9. Eeyore

    Eeyore Guest

    Oh ! Interesting.

    Graham
     
  10. Eeyore

    Eeyore Guest

    True. There normally is in any gear with an SMPS.

    But if the chassis is bonded to ground...... !! ??

    Graham
     
  11. Phil Allison

    Phil Allison Guest

    "Eeyore"

    ** 1.5 mV into 1 ohm = absolutely typical.

    Now, say how you made that current measurement.

    With a DMM or a scope.




    ........ Phil
     
  12. It could read that on an ammeter. One indication would be that the
    current reading does not change proportionally when you switch range
    on the meter.

    Best regards,
    Spehro Pefhany
     
  13. Eeyore

    Eeyore Guest

    I own up It was a quick check with a Fluke 75 simply to establish that there was
    something there. I stopped using the TDS at that point.

    Graham
     
  14. john jardine

    john jardine Guest

    Yes, a lost earth was exactly what I was thinking at the time. Customers
    equipment is 110Vac operation, fed from a 240V to 55-0-55, 10kVA
    transformer, the "0v" solid copper linked to mains earth. Power cabling
    feeding the equipment has a 6mm^2 earth cable and a separate, earthed,
    overall braided screen. (plus a 'health and safety' earth tested sticker ;).
    Changed the 'scope power cable, no effect. Prob' was a side issue so didn't
    pursue.
     
  15. Mike Monett

    Mike Monett Guest

    Only 55VAC to earth ground? That's no fun. When I make a mistake, I want to
    see a huge flash, BANG!, and lots of smoke.

    Regards,

    Mike Monett
     
  16. john jardine

    john jardine Guest

    My dad was a power station electrician. He came home one day looking a bit
    'grey'. Casually mentioned he'd been thrown across a sub station room after
    touching some unisolated 10kV switchgear. As a kid I didn't think much of
    it, parents after all are indestructable!. But now, (older and wiser maybe)
    I cringe at the thought ;).
     
  17. Mike Monett

    Mike Monett Guest

    You are lucky you still had a Dad.

    Come to think of it, you described a 55V-0-55V system earlier, and a
    tingling feeling that you knew well.

    Do you have any kids?

    Regards,

    Mike Monett
     
  18. Jim Thompson

    Jim Thompson Guest

    Good grief! Electric shock only gets the juices flowing. I got it
    arm-to-arm one time when I was a kid... before GFCI's. A quick
    thinking neighbor kicked my arm loose ;-)

    ...Jim Thompson
     
  19. Phil Allison

    Phil Allison Guest

    "Eeysore Fucking IDIOT TROLL "

    ** Use a bloody scope - you ass.



    ......... Phil
     
  20. krw

    krw Guest

    A week-and-a-half ago I got it across my chest (front to back); 300J
    worth. I don't remember a thing that happened for about 10min. ;-)
     
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