Noob scope error. How bad can it be?

Discussion in 'General Electronics Chat' started by Marcus2012, Feb 8, 2016.

  1. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Yeah so ass the title suggest I made a really bad noob error with my scope today and I was hoping you guys could help me out please :) It's an old D83 scope and I went ahead and measured across my induction heater coil running at 12V.........unsurprisingly there were sparks and the probe cable got hot.......quick. Of course I was using an ATX power supply and neglected to realise it is a main referenced supply (really embarrassing). Now I am a little worried, I luckily avoided shocking myself but now I'm concerned for the scope and my heater. At the moment the heater seems to work still (not sure if I could of damaged an individual cap or something?) and the scope works. BUT I am picking up interference from the probe now. At least I assume it's the probe as when I unplug it from the scope it returns to a flat line.

    So basically that was my screw up for today, hopefully I haven't wrecked anything more than a probe at this point but I wanted to ask if there is anything else I could have broken and should check?

    Thanks for reading/helping guys hope It's not too bad lol
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You are probably okay. The path would be the ground of the probe to the case to the green wire ground. Not much to break, but maybe burned some insulation off the green wire in the line cord.
    Be careful! :eek:
     
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  3. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Phew! Thanks :). It does seem to be working ok at the moment minus the interference when I move the probe near to the CRT. I guess the probe took most of the damage. Thanks....lesson learned and I guess it definitely could of been worse. :)
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    I agree, the probe took the worse of it. Lucky for you that you are still functional. Probes are cheap, you, a little more.
     
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  5. SLK001

    Well-Known Member

    Nov 29, 2011
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    I did that once with the old TVs that had a live chassis. Tried to ground my scope and "BAM"! Sparks flew everywhere and my scope ground clip got mostly vaporized. Since the path was to ground through the probe, the only damage was to the probe. I started using a 3-prong to 2-prong grounding adaptor (this is a US thing) to take my electrical ground out of the circuit.
     
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  6. DickCappels

    Moderator

    Aug 21, 2008
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    Maybe drifting toward the edge of the topic but:

    Floating a scope as SLK001 mentioned is not recommended for those who do not have a lot of experience working around dangerous voltages or tend to be reckless or absent minded, or who sometimes works alone.

    An isolation transformer reduces the danger significantly but does not eliminate the danger.
     
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  7. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Thanks everyone, I really appreciate your advice and concern regarding this :) I see I did somewhat dodge a bullet here. (for me and my scope). In my case I think it's a lack of experience AND tendency to be absent minded :), and I should probably write a start up checklist for equipment like. So, as you mentioned, I guess an isolation transformer the way to go forward from here and if so is it best to isolate the test circuit or the scope? I have heard some people mentioning grounding your scope probe on the neutral of the power supply to measure mains as it would be at the same potential as my scope ground but I am unsure as to how safe this is now with my limited understanding.
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Quite apart from any ground path issue, please be advised that you were attempting to directly measure a large signal RF source! --- Such is inadvisable for the rather high EMFs encountered via 'transformation' secondary to unreckoned resonances, transmission line phenomena, etc. --- IMO you're fortunate the scope's input channel (at least) escaped instant destruction! --- Note, also, that inasmuch most induction heating systems are, in effect, power oscillators, any connection to the 'work coil' will result in 'detuning' and, hence, erroneous readings -- as a practical matter such measurement is seldom if ever necessary!

    Best regards
    HP
     
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  9. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Duly noted thanks :) I was trying to see how far I could extend the work coil from the capacitor bank before the efficiency of the LC circuit tanked. I thought this might have been a useful way of determining that other than using it directly on a work piece.......boy was I wrong. :)
     
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  10. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Ironically, shorting the PSU probably saved your scope:cool:

    Best regards
    HP:D
     
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  11. Johann

    Senior Member

    Nov 27, 2006
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    For measurements on mains powered equipment, a power scope is the safest option. It has true isolated inputs for this purpose.
     
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  12. Johann

    Senior Member

    Nov 27, 2006
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    Isolate the test circuit. Don't ground to the neutral!
     
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  13. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Yeah I am considering myself a very lucky boy. I can't help but think if it were a modern DSO I would've killed it. Also it shames me to admit but I am currently designing a linear supply for more power (48V 25A) and I was going to test the output to see if I had any ripple.........so so so very glad I learnt this lesson now. :oops:
     
  14. bwilliams60

    Active Member

    Nov 18, 2012
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    There is a good Youtube video on this subject titled appropriately "How Not to Blow Up Your Oscilloscope". It is worth watching and he explains in easy terms what is going on. Good luck, play safe.
     
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  15. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Haha great video thanks :) I do have a question now though if anyone could help please. After watching that I wonder if I would be ok debugged my new power supply with a mains references scope. The transformer I am using is toroidal with no earth connection at all so would this then be considered isolated and safe to test? I'm not sure if my earth and neutral are tied together somewhere in the house.
     
  16. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If I understand your question correctly the transformer would provide the isolation from the mains. But to be sure ---- maybe a drawing?
     
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  17. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Hey :) Same PSU you helped me out designing the other day :) Plastic case with no earth connection on the transformer or anything at all.

    psu adjustable 2.png
     
  18. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Cool!:) Yes, the transformer provides isolation from the line. Draw a little ground symbol on the long line at the bottom and that can be your reference for logic ground.
    PS.
    If the transformer puts out 40 volts RMS X 2 windings that will give ~ 112 volts when you filter it. A lot of "stuff" won't like that. :(
     
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  19. DickCappels

    Moderator

    Aug 21, 2008
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    Back in the 1970's we would float the scopes on the line for occasional tests, but that was in a lab with other people around who knew and understood the danger and could perform cadio-pulminary respiration (CPR), and standing on a rubber floor mat. These days I think it would be a pretty dumb thing to do since good isolation transformers are available that would allow one to ground the scope and many points in an AC operated circuit under test.

    If you are going to be doing work on off-line power supplies or line powered circuits invest in a really good isolation transformer -one with very low leakage current and capacitance between the windings. It could easily save your life -what would that be worth to you and your family?
     
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  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I can remember when I was first out of school working on a triac circuit to control like a 2 HP 220v motor. I hated it every time I hooked up the ground clip.:eek:
     
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