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oscilloscope question

Discussion in 'General Electronics Discussion' started by markm6164, Feb 21, 2012.

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  1. markm6164

    markm6164

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    Jan 22, 2012
    Can i safely scope mains voltage? Can it be done with the correct probes and if so what rating probes does it require?
     
  2. GonzoEngineer

    GonzoEngineer

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    Dec 2, 2011
    If you have to ask that question, I highly suggest you not try it unless you know exactly what you are doing.

    What are the spec's of the scope?
    What country are you in, and what configuration is the mains voltage?
    Is it two wire, or three wire?
    Where are you going to ground the scope probe lead?

    Not knowing the answers to all these questions could result in severe damage to the scope, as well as to yourself!
     
  3. markm6164

    markm6164

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    Jan 22, 2012
    I am new to electronics. I'd like to scope the primary side of a psu circuit to see the effect of mains filters. I would never connect anything to mains without being 100% sure it's safe. I live in the uk with 240v 50/60hz mains. We have 3 wire live, neutral and earth wires. I have a Phillips analogue 100mhz scope.
     
  4. davenn

    davenn Moderator

    13,576
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    Sep 5, 2009
    hi again Mark

    I have owned a scope for many yrs and to be honest i have never though about sticking it across the mains to do any measuring

    I do notice on my Tektronix scope tho that direct in it can only handle 200V AC or DC max, But using the x10 switch on the probe would extend that.
    I just dont know if a normal probe could handle that sort of voltage, I do know there are hi voltage probes available for scopes and if I was you I would be looking into going that way

    Presumably you are referring to the mains filter in that PSU you and I were talking about in the other thread. ?
    IF and ONLY IF you find a safe way to scope the mains.... what you are looking for is hi freq ~ 20 - 100 KHz noise on the mains circuitry.

    I dont know what your knowledge of SMPS Switch Mode Power Supply(s) is like, but basically, the incoming AC mains is rectified and smoothed producing ~ 320VDC. That DC voltage is applied to a transistor/coil arrangement oscillating/switching at hi freq ~ 20 - 100kHz. That coil(s) is the primary side of the step down transformer to the secondary side from whence the low voltage DC rails are sourced.

    Why do they use high freq you ask ? ... the higher the freq the more efficient the transformer becomes, it needs less windings and a smaller iron/ferrite core and this is huge advantage of a SMPS over a linear PSU with a huge heavy transformer that is running at 50/60 Hz.

    cheers
    Dave
     
  5. markm6164

    markm6164

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    Jan 22, 2012
    Thank you Dave i really appreciate your help. As i am learning i was wondering about the primary side. I have learnt alot about transformers and what happens on the secondary side and how the wave form is rectified but not so much the primary. It would be nice to see whats happening on the primary side with the scope but after googling there is little info on this and some people say they have scoped mains. I don't want to stick my oscilloscope probes into the mains outlet but i would like to see what is happening at various points on the primary side of the psu but of course i was checking safety before doing anything of this nature. What might seem a silly question to someone who already knows the answer, for someone that doesn't its the only way to learn by asking the question.
     
    Last edited: Feb 21, 2012
  6. duke37

    duke37

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    Jan 9, 2011
    It may be a Philips !00MHzscope.

    An analog scope will not show non repetative signals, you will need a storage scope for this.
    The scope will have its case earthed so that you can get a mains short if you are not careful. I suggest that you use an earth leakage trip and an isolating transformer and be very careful.
     
  7. markm6164

    markm6164

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    Jan 22, 2012
    Its actually a Philips 100MHz scope.

    Maybe this was a very silly question judging by the response and i think i will take it no further.

    Thanks for everyones replies. ;)
     
    Last edited: Feb 21, 2012
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,374
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    Jan 21, 2010
    No, it's a reasonable question, and the short answer is *NO*

    The long answer is: Maybe, if you know what you're doing.

    If you have to ask the question, see the short answer. If you can find someone who knows how to do this safely, ask them if your equipment is appropriate and how to go about doing it (and have them demonstrate and/or supervise you).
     
  9. markm6164

    markm6164

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    Jan 22, 2012
    I have been reading various things and learning abit and i have another question. Can i power a circuit board via an isolation transformer to allow me to scope the primary side of a psu???
     
  10. davenn

    davenn Moderator

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    1,866
    Sep 5, 2009
    That does make it a lot safer for you :)
    BUT you still need to make sure that your scope can handle the voltage safely, as commented in an earlier post
    else you make see your scope go up in a puff of smoke

    Dave
     
  11. Rleo6965

    Rleo6965

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    Jan 22, 2012
    I can use my old Leader 1021 Oscilloscope measuring 220V AC main. Because I use 2 prongs AC plug for my scope. I removed the 3rd pin or ground prong. But I don’t touch any metal part of Oscilloscope casing or probe when measuring AC mains.
    But I still suggest that you must use isolation transformer for your oscilloscope to be safe.
    As we all know. In order to measure ac mains. We need to connect ground clip of oscilloscope to one side of ac line and test probe to other side of ac line. Now. If your using 3 prong ac plug or with ground prong . It will be disastrous if you placed your ground clip to wrong side of ac lines.

    Note: I seldom measure ac line with scope. Unless it's necessary. I use Multimeter instead for safety.
     
    Last edited: Feb 28, 2012
  12. markm6164

    markm6164

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    Jan 22, 2012
    Thanks. I'm not going to do it but I'm just curious.
     
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