Correct way to see main ac wavefrom using CRO

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Apr 18, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    1. I have to see ac main waveform across motor as shown in attached circuit.
    2. Problem I see by connecting CRO in this way is that mains phase or neutral(one of them), is shorted with CRO earth, as in schematic.
    3. I have to see what is the waveform across motor after triac.
    4. I have looked on internet, people use isolated probe. But they are very costly.
    5. What is correct way to see the waveform across motor
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Glad you are on your toes -grounding Neutral other than where the electrical code specifies can be dangerous.

    I look at waveforms on the power line by looking at the mains differentially.
    Use two channels of the scope.
    Channel A to Line, channel B to Neutral, both channels set to the same volts/division.
    Set the scope to display A-B.
     
  3. Reloadron

    Active Member

    Jan 15, 2015
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    There are a variety of Active Differential Scope Probes manufactured which afford isolation.
    Depending on the scope you can also use the method Dick points out using two channels of the scope, I use that method here at home.
    There are also ways to isolate the scope using an isolation transformer but I won't get into that method. Dick's is the simplest and most common as long as the scope allows for it.

    Ron
     
  4. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    100% concurrence with DickCappels!

    As a point of interest the difficulty you describe is a principal factor in the popularity of 'battery operated' instruments in (fixed) labratory settings -- Trouble is, while battery operation or ground isolation 'solve the problem' a hazard is presented by the electrically 'floating' condition of the instrument's chassis...

    Best regards
    HP
     
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  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I've noticed the newer battery powered scopes have a built in isolation from both sides of an input to their chassis, but it is a rather low value (less than line voltage in the device I looked at).

    One way to do this and still keep both scope probes is to use an isolation connector on your scope, the ones that break the earth ground connection. Now the scope will not short out the mains but is as dangerous as a loaded gun.

    If you do this proceed cautiously and keep one hand in your pocket while the power is on.
     
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  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    or go old school and use some high value resistors(meg ohms) and build a voltage divider of 100:1 minimum value. Scope the signal at the low end of the divider.
    A 10Meg in series with a 100K would be my choice if using a scope to view mains waveform.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What do you do with the scope ground? If you make a single-ended attenuator, then the scope chassis can still wind up connected to line?
    I dont see how this solves the isolation problem. Draw me a picture...

    Is'nt there likely already a 10X probe on the scope to make it capable of viewing ~200V?
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    For less cost that an active, isolated scope probe you can buy an isolation transformer for the scope AC power. Look at the power rating on the label, and get the smallest thing that will work.

    The problem with a differential connection using two standard probes is that both probe grounds are (usually) connected to the scope chassis ground, the third pin on the AC plug. So no matter how you connect them to the device, they still ground something. The classic solution, totally unsafe and never seriously recommended and used for over 50 years, is to use break off the ground pin on the scope AC plug or use a 3-pin to 2-pin adapter.

    ak
     
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  9. #12

    Expert

    Nov 30, 2010
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    Read the label on the scope and find a transformer or 2 with which to make an isolated power supply for it. You might find that as little as 6 watts are needed.
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The scope ground is connected to ground. One should double check the electrical connections on all equipment before attempting any tests. One does not perform any tests without being sure about live and neutral and ground to begin with. The habits these youngsters have of just hooking up test instruments and THEN discovering the power outlet is reversed, or probing with a multimeter and not checking to be sure if it is in voltage or current setting first is the leading cause of all these new safety features which confound and frustrate those of us trained to be more diligent and methodical in our approach to such things. This all stems from a "I'll sue" cadre of ignorant amateurs and greedy lawyers and is not being helped by piss poor teaching methods and ..ah hell... you know the rest of this rant already. I'll spare you the predictable finale and hope everyone else spares me the stale "but what about...", and "they are not..." type responses.
    We all see this type of post on a regular basis. I believe it to be indicative of a larger problem with basic electronics education methods used in this modern, highly advanced society our self appointed 'leaders' are so proud of.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    What I would do would probably get me banned from the forum - an alternative would be to run the scope from a 1:1 isolating mains transformer and earth it's chassis to the lowest potential part of the system you're measuring.

    If you're doing it in a workplace - you have to keep the H&S Nazis at bay by errecting barriers around the project and excluding anyone who's not fully informed about the procedure.
     
  12. piw

    New Member

    Mar 14, 2014
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    Could you harvest a mains rated isolating step down transformer from an old appliance and use it to safely view the waveform?
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Even better, do it the correct way and put the isolation tranny between the line and the device under test. Now you can leave the scope chassis grounded, connect the scope ground to any part of the circuit under test with impunity, and not wind up with the scope chassis at line potential.
     
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  14. ian field

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    Oct 27, 2012
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    Phase control can "dent" the waveform that makes it through a transformer, so what you see on the scope may not be exactly what's happening.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    No one has asked the nature of the motor.
    If it is universal, you could even use a 220v/120v isolation transformer and still run the motor for test purposes, that is if rpm is not important for the test.
    Max.
     
  16. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    @DickCappels After connecting one probe to phase & other to netural.
    What to do for ground/earth probe of CRO? Should it be disconnected all together.
    If yes, how would each probe pick up signal since there is no reference(ground) connection for probe?
     
  17. DickCappels

    Moderator

    Aug 21, 2008
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    Yeah, I was wondering whether anybody was going to ask about that.

    The scope ground connections should go to earth.

    In my case, the scope probes are grounded through the earth pin on the power plug on my scope and this arrangement works well for this purpose.

    By the way, make sure the scope probe connected to Line is rated to handle the high peak voltage (1.41 x RMS, plus some possible glitches). Differential connections work best when both probes are the same make and mode, and are properly frequency compensated.
     
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  18. ian field

    Distinguished Member

    Oct 27, 2012
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    Probably worth adding, that if scope probe earth clips are connected to the mains neutral, it completes a loop through the earth and neutral lines that will trip any ELCBs for that circuit on the consumer panel.

    The solution is either; expensive isolated probes, or run the scope from a 1:1 isolating transformer and ground its chassis to the lowest potential on the circuit under test.

    In a factory environment, the latter would require barriers and warning signs around the work area.
     
  19. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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