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Can an oscilloscope be safley used to check a TV set?

Discussion in 'Electronic Basics' started by Blue News, Feb 18, 2004.

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  1. Blue News

    Blue News Guest

    Hi,

    cant remember where I read this but I thought there was a danger in using an
    oscilloscope to check a TV sets signal due to the chassis being directly
    connected to mains or something like that - is that right, if so how can one
    use a scope for the purpose?

    thanks
    Sean
     
  2. Not just o'scopes, any test device. Use an isolation transformer if
    there is ANY doubt, and even if there isn't.

    GG
     
  3. 10 minutes ago, I said to use an isolation transformer if there is any
    doubt as to the safety. I didn't say that the isolation transformer is
    to be used on the TV, not the scope. Isolating the scope would allow
    you to touch the scope ground to the TV BUT anything else (soldering
    iron, computer, stereo... ) WOULD STILL BE HAZARDOUS. If you're not
    aware of these issues, perhaps the TV is not the best thing to be
    repairing.
    GG
     
  4. Art

    Art Guest

    Agreed! One of the basic safety issues that most tech automatically do when
    diagnosing any tele.
     
  5. Sean,

    Back in the days of the old electron tubes, the TV was often powered
    directly by the mains. So all of its inside was connected to the mains and
    repair without insulating transformer was always dangerous. Connecting a
    non-insulated scope (or any other test device connected to protective
    ground) caused an outbreak of fireworks.

    These days TVs are powered using an SMPS. This SPMS contains a transformer
    that insulates its mains part from the other parts of the TV circuit. So you
    can do your measurements relatively safely in this parts of the TV. I write
    *relatively* because a TV has dangerous high voltages inside even when
    insulated from the mains.

    But... The SMPS can be cause of the problem quite often. Touching the
    non-insulated part of the SMPS - even shortly by accident - may still start
    the fireworks. So most of the times an insulating transformer is still
    necessary. Remind, even then there are dangerous voltages.

    petrus
     
  6. Blue News

    Blue News Guest

    Well after reading through all the responses, I guess I will just say NO! I
    have been in normal low voltage electronics for years but have always
    avoided TVs or monitors due to their high voltages and I have just been
    reminded of the reasons again. So, thank you all for your inputs and stay
    safe and I'll leave the TVs to the experts.
     
  7. DarkMatter

    DarkMatter Guest


    Yeah... bone up on Usenet too, as top posting is just as retarded
    as hooking up electronic instruments to HV sources.

    Sorry so succinct... NOT!
     
  8. Terry

    Terry Guest

    Not sure if my earlier post in sed got through (ISP news server has
    problems), so excuse possible duplication:

    Can you clarify how exactly the use of an isolation transformer helps
    with the sort of measurements I sometimes want to make on 240 V mains
    circuits please? My 'scope return probe remains grounded to mains
    earth, so if I apply both probes across a component (even if fed via
    an isolation transformer), it seesm to me that still risks shorting a
    hot main-level node to earth. For example, I might be looking at the
    voltage across an SCR or triac in a dimmer.
     

  9. Terry,

    Almost glad I'm not the only one whos provider has problems with news
    servers :)

    A insulating transformer is supposed to deliver mains voltage without a
    galvanic connection to the mains. So on the secondary side you can choose wh
    ich part of the circuit under test - if any - you connect to ground. I guess
    your 'scopes frame has been connected to its common and to protective
    ground, so you can connect the common of the circuit under test to the
    'scopes frame. No need to say the voltages are still dangerous.

    petrus
     
  10. Terry

    Terry Guest

    Thanks petrus. To make sure I really have this straight, could you
    take a look at the illustration I've drawn at
    http://www.terrypin.dial.pipex.com/Images/Isolation.gif
    and tell me if that setup looks OK please?
     

  11. Terry,

    That's the way I've set my 'scope and insulation transformer.

    petrus
     
  12. Bill Vajk

    Bill Vajk Guest


    At a test bench I (in the US) always have a solid mechanical ground
    drawn directly to the bench. We use a star system for power
    distribution in the US while in UK the common system is to
    have a "ring".

    The schematic you've provided clearly has a "ground loop" in
    it and that could be a significant problem. For some
    background take a look at this web page:

    http://www.epanorama.net/documents/groundloop/index.html

    and/or go a google search for "ground loop" (be sure to
    use the double quotes.)
     



  13. Terry,

    Third time I post this reply. Hope it comes through this time.

    That's the way I've set my 'scope and insulation transformer.

    petrus
     
  14. I fail to see a ground loop in this setup. Maybe if one of your mains wires
    has been connected to protective ground like seems to be done on the west
    side of the pond. When that's the case you have two wires from the 'scope to
    the mains earth. Even then I doubt it to give a problem but I have no
    experience. In Europe the neutral is never connected to protective ground
    but in the distribution station. Protective ground on the other hand is usua
    lly "earthed" near the place the mains cable enters the house.

    petrus
     
  15. Yes thanks, received at least twice! You sound as if you're having as
    much trouble as me with Usenet. Demon's service was down for 2 full
    days. Finally got back to something like normal this morning.
     
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