How to measure AC waveform using scope?

Discussion in 'General Electronics Chat' started by simpsonss, Jan 20, 2010.

  1. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
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    hi,

    ABCD0010.jpg

    For more easy understanding i attached the diagram above. Can anyone teach me how to measure the A,B,C,D point by using oscilloscope? To where should i clip the black(earth) of the scope so that i can measure all the points?

    thank you.
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Anywhere. Volatge can be read between any two points. You may need to refine your question. ( while anywhere some scopes are connected to ground on their input, if so you must connect its ground to a grounded point on your circuit if any )
     
  3. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    hi,

    ok.if let say i want to get the ripple waveform from the POINT B? how should i connect the black color wire of my probe? or do u mean i dont have to connect the black color wire but just measure the point using the probe?

    thank you.
     
  4. 3ldon

    Active Member

    Jan 9, 2010
    82
    3
    this circuit just isn't going to work for you without an isolation transformer, or make very sure your o'scope isn't grounded or you have a differential oscope probe (dubious/expensive)
    Or feed your o'scope from an ungrounded isolation transformer, or use a digital unit that connects to a laptop and unplug it while probing on a line connected device.

    i've learned some of this the hard way repairing power supplies.
     
    Last edited by a moderator: Jan 21, 2010
  5. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
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    So if i don't have any isolation transformer, is there any other ways to measure it.( i very sure that my scope is grounded).

    Can u explain about this way in more detail? What is the digital unit that u mean?

    thank you.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    If you have a dual channel scope, check to see if there is an "ADD" function where the two inputs are added together. If so, use one as the "common" and measure with the other. Scope remains grounded and safe. Please be aware that playing with "mains powered" circuits without fully understanding what is going on can be very dangerous....even deadly:eek:
     
  7. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
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    Hi Bill,
    i have a dual channel scope and it has the "ADD" function too.what u mean by use one as the common? ok let say i have CH1 and CH2. so i push the switch of the scope to add function. So black color wire of the probe for CH1 and CH2 go to ?

    thank you.
     
  8. creakndale

    Active Member

    Mar 13, 2009
    68
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  9. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    All signals must be referenced to something. In your drawing, you could simply use the built in scope ground and only one channel to look at the various points. Your Point D, for example, would show very little signal other than what may be seen across the dual inductor in the line filter. Point C would show the incoming line voltage. Point B would show negative going full wave rectified voltages and Point A would show Positive going full wave rectified voltages. (The signals would only be seen if you took the filter cap that is between A and B out of the circuit. Otherwise, without a load, it will charge to the peak voltage of your incoming line and just show DC levels.)

    If you want to look at the output of the bridge, connect Chan 1 to A, Chan 2 to B, Invert B and ADD. Again, with the cap in the circuit, you will only see DC. If you disconnect it, you will see Positive going full wave rectified sine waves.

    Messing with line operated circuits without understanding them is a dangerous thing to do. If you want to experiment with bridge rectifiers, get a filament transformer and hook your bridge to that. Much safer and will show the same waveforms without having to worry about where you hook your scope ground.

    Also, are you sure you want to tie the output of the bridge to the primary of a transformer? What are the desired end results?
     
  10. BillB3857

    Senior Member

    Feb 28, 2009
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  11. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    simpsonss, please heed the warnings here about measuring circuits with a scope that does not have a differential input (i.e., the majority of scopes out in the real world). That (typically) black lead with the alligator clip is grounded and if you don't know what you're doing, you can clip it in to a power circuit and cause a short to ground. You can also expose yourself to a shock by doing this. You have to understand the basic principles before undertaking such tasks.

    The most convenient way to make this measurement is to use a differential amplifier. The older 7000/5000 Tektronix scopes could take differential amplifier plug-ins and devices such as the Tektronix AM502 stand-alone differential amplifier plug-in were available. These can still be found on ebay, as well as the older vacuum tube scopes with differential amplifiers. You can buy a differential amplifier; here's one example -- but hobbyists may find that they cost more than their scope. They almost are a must if you're going to be making lots of scope measurements on circuits with line voltages or higher voltages on them.

    The usual technique for a user of a two channel scope without a differential amplifier is to use two probes, one into each channel, then subtract the signals. For this to be accurate, the probes need to be compensated correctly and the amplifiers need to be set to the same input parameters (coupling, gain, etc.).

    If you have a digital single channel scope, you can put your probe on one point, save the waveform, then put it on another point and save the waveform. Then you can use programming tools such as python/numpy to subtract the two waveforms, giving you potential difference. Of course, this assumes you save the data in some suitable form (e.g., ASCII or CSV), not as a bitmap.

    Finally, if all the signals are sine waves, you can simply estimate the RMS values of the waveforms from the scope, then subtract them. If you're looking at AC line waveforms, this is easier to do with a digital mulitmeter.

    I strongly concur with BillB3857's suggestion of investigating behavior with a low voltage transformer.
     
  12. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    ok,evrything become clearer n clearer. I understand y'all advice. I understand clip the grounded wire wrongly will have a bad 'result'...ha...cz i tried it before. Ok,i'll go for a dual channel scope 1st, as bill said. And someone dad, your explaination on digital scope really helps.thank to y'all.
     
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