Connecting an Oscilloscope to a AC Circuit without earth.

Discussion in 'General Electronics Chat' started by dannhughes, Sep 13, 2016.

  1. dannhughes

    Thread Starter New Member

    Feb 11, 2016
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    So I am trying to test a small pulsing board connected to a thermal actuator to see if the pulsing board actually reduces the overall current used. I think the current surge every time there is a pulse might outweigh the benefit of the off time. (BTW I don't have a great deal of electrical knowledge, i'm a learner)

    The board is 24 VAC so I have a Voltage regulator supplying 24VAC from mains to the board. I input a small resistor after the thermal actuator to use as a shunt resistor and hopefully get my readings from (Not sure if this is the right thing to do).

    So the question is how to I connect up my oscilloscope to get a reading without blowing the fuse? Here is my diagram with the oscilloscope as I incorrectly wired it.

    Can any one help? How do I connect it? Why is it shorting as it is? is there a better way of doing it?

    Thanks in advance!!

    upload_2016-9-13_13-40-29.png
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC
    Is the pulsing circuit isolated from Mains ?
    Why can't you put the scope across the resistor ?

    I am not sure what you are trying to find out.
    Need a better explanation.

    By the way ...You should not connect a scope to the mains without an isolation transformer. It might create ground loop which is dangerous to the user and the equipment,
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    What is this 24VAC regulator? Is its output isolated from its input?
    There would be no problem if the 24VAC came from a transformer driven by the mains as it would provide isolation.
     
  4. DGElder

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    Apr 3, 2016
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    Scopes don't have + / - channels. Where are you connecting the probes to the O-scope?
    The probes should be connected to two different channels. The ground clips should Not be connected to anything. And the probes should be connected across the resistor if you want to know the current into the actuator. Then use the Math function Ch1-CH2 to get the voltage across the resistor. Make sure the pulse voltages do not exceed the maximum scope/probe ratings.

    I don't know what effect the test resistor has on your circuit (quite a bit I imagine), but you could use a much smaller resistor, e.g. ~1 ohm - depending on the current and scope sensitivity.
     
    Last edited: Sep 13, 2016
  5. DGElder

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    Apr 3, 2016
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    He should NOT connect the O-scope to an isolation transformer! It should be plugged into the standard 3 prong mains outlet. Isolating the O-scope, besides providing potential for equipment damage, could kill him. Especially, dangerous for a newbie.
     
    Last edited: Sep 13, 2016
  6. AlbertHall

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    In what way is connecting a 'scope to the output of an isolating transformer dangerous to equipment or persons?
     
  7. DGElder

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    Apr 3, 2016
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    If the operator connects the ground clip to a hot point in the DUT then the external connections of the scope become hot, e.g. the ground sleeve where the probes are connected.
     
  8. AlbertHall

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    Jun 4, 2014
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    Ah, I see. I was taking it that the isolation transformer was feeding the equipment being tested then it makes it safe
     
  9. R!f@@

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    Isolation transformer prevents earth loops. I did that when trouble shooting old CRT's.
    But if one do not understand earth loops and non isolated supply's, one aught not to use scope if one need to connect the probe to mains.
    The concept of Scope and Isolation transformer should be studied before attempting such.
    As the TS did connect and it seems he blew a fuse, it seems he is unaware of the issues.

    when using isolation transformer and probing live circuit the operator need to be away from scope chassis and use proper probes with separate ground leads. when you said ONE should not connect, it was somewhat funny. Cause I believe there is no other safer way if you need to probe live circuits. Which I did so many times. But I agree a newbie should not even play with mains much less connect a scope probe to mains
     
    Last edited: Sep 13, 2016
  10. R!f@@

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    They Don't ? Never knew tht.. o_O I always thought there should be a return path or a GROUND..
    Or do you mean Ch A, Ch B, etc
    If I do not connect ground of my scope, All I get is a weird wave forms. o_O
    What kinda scope are you talking about :confused:
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    Connecting the scope earth clip to mains neutral is a quick way to find out if you have ELCB units on the mains board.

    You probably need to power the equipment via a mains isolating transformer before putting a scope on it.
     
  12. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The differential probe is the ideal way of doing this. Although they are a bit pricey, I broke down and finally BUILT myself one. I can't overstate how useful it is.
     
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    This is the problem, the scope should always stay at ground potential, and you should use an isolation transformer to isolate the measured circuit. It seems to me that often when people talk about isolation transformers it is not quite evident if it is the scope that should be isolated or the device.

    It can be done the other way (with the scope lifted above ground) and it is done a lot when your isolation tranformer cannot handle the power needed by the measured circuit and you don´t have high voltage differential probes, but as you can see this should be done only when you are absolutely certain you know the implications of what you´re doing, and even then a second person around to question your setup and do CPR if necessary is very welcome.
    As we all know brain farts happen in an instant and with this kind of measurement they can have very deadly consequences.
     
  14. dannhughes

    Thread Starter New Member

    Feb 11, 2016
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    Hey Guys, Thanks so much all for your help, judging from everyone's responses it looks like I might leave it for now and come back when I have a better knowledge of what I am doing, a few too many "could dies" in there :eek:.

    Thanks again!
     
  15. AlbertHall

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    Jun 4, 2014
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    Mains is dangerous for the unwary and even for the knowledgeable and wary in a moment of brain fade.
     
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  16. R!f@@

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    I cannot always get my hands on an isolation TX that could power the unit I am testing. So I ended up a one capable of running the scope.
    Of course I need to be away from the scope + probe ground when handling. I use a separate ground fixed securely at both ends and use one hand while probing.
     
  17. R!f@@

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    @ TS
    Do you need to connect to the mains.
    You did not explain your situation well enough.
     
  18. KeepItSimpleStupid

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    Mar 4, 2014
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    You should be able to use two probes, 2 channels in A-B mode.
     
  19. AlbertHall

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    This works but has a limitation when the probes are connected to something at mains potential. Each channel sees the mains voltage as its input and if you are trying to see a small signal and so have the attenuator set to a low setting, the large input voltage can overload the 'scope input amplifier giving meaningless results on the screen.
     
  20. DGElder

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    Apr 3, 2016
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    Smart move. The joy is in the learning.
     
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