Switch mode power supplies vs linear, help me understand these o-scope readings.

Discussion in 'General Electronics Chat' started by rjames711, Nov 8, 2015.

  1. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    Hello everyone, I was hoping that someone could help me better understand some of the different properties between switch mode power supplies and linear power supplies and also some some o-scope readings I took of these different power supplies.

    Below are my questions. I listed anything I think I understand as an assumption but please correct anything that’s incorrect. Sorry for weird formatting. I try to make the information digestible and I never sure if I made it better or worse than just typing out a long block of text. Thanks in advance.

    Question set 1:

    Please see the attached pdf for the O-scope readings I'm referencing

    Background: Circuit powered by 12 volt walwart, either switch mode or linear, which is stepped down to 6vdc with a linear regulator. O-scope probes are connected to the positive and negative outputs of the linear regulator (except last graph). Circuit is hooked up to sensor module as normal during testing. Current is very low ~6 mA.

    Assumption: The o-scope graphs show the voltage show the positive and negative side of the circuit oscillate with respect to ground which is left over from ac conversion

    Assumption: The SMPS has a much larger amplitude of oscillation as can be seen in the pdf. This is because it is regulated directly off of rectified mains voltage.

    Assumption: The voltage seen by the circuit is a consistent 6 vdc even with the smps since both channels (+ and -) oscillate together staying 6 volts apart.

    Assumption: Since this is happening at a low 60 hz frequency this is not related to switching noise and is a distinct phenomenon

    Questions:

    · Is there a name for the large amplitude oscillation with respect to ground of a dc circuit?

    · Is this phenomenon ever the source of interference issues (most of what I’ve read only speak about the issues caused by high frequency switching noise with smps’s)?

    · Are there any modern smps which mitigate this large amplitude oscillation?


    Question set 2:

    Background: Our electronics vendor (who are EEs but are in another country and we have communications issues with them) suggested grounding the negative side of the circuit. This solved the sensor issue and smooths the o-scope graph out tremendously (second to last slide). However I have some concerns and would like to understand better.

    When I measure between the negative side (or positive) of the circuit and earth ground when using the smps I get around 40 volts ac. However when I ground the negative side no current seems to flow (no sparks no reading on multimeter for ac current)

    Questions:

    · Should I be concerned about the ac potential between the circuit and ground ?

    · Would it be correct to say that the once the negative side is connected to ground it pulls down the reference ground in the circuit so that the smps and the linear regulator then regulate with reference to that ground and the ac potential disappears without much current flow?


    More Backround

    The Company I work for has recently started problems with a product we sell where the sensor would malfunction. Through testing we found the following factors:

    · The issue only arises when using a newer SMPS we had to switch to (the older linear power supply always works)

    · It is more likely to occur when the product is installed (product is plumbed so it generally would have a real connection to earth ground when installed.

    · The product is powered by a 12 volt dc walwart style which is then stepped down to 6vdc with a linear regulator inside an adapter plug (for compatibility with older units which were 12vdc)

    We decided to get an inexpensive usb O-scope to take a look at the output of the different supplies. One way I measured was with one probe connected to the positive and one probe connected to the negative and both ground wires unconnected (please advise if this is a bad way to measure).

    What I got was very interesting. All the power supplies I tested showed the two lines tracing with about 6 volts in between the two lines (makes sense). Both also oscillate at about 60 hz. The difference is that the output from the smps oscillates about +- 40 v and is very jagged whereas the linear psu is very smooth (looks like regular sine wave) and only oscilliate +- about 3 volts.

    So I’m aware that there is such a thing as switching noise but this is not what I’m seeing as switching noise would be seen at much higher frequency, correct? I did a bit of reading and it sounds like smps’s regulate directly off of rectified mains rather than stepping down. So I what I’m seeing I guess makes sense.

    Our electronics vendor suggesting grounding the negative side of the circuit. This solves the sensor issue and on the o-scope smooths the output greatly but my concern is that if I measure between the negative side of the circuit and ground I get about 42 volts AC ( this is after the linear regulator where there is only 6 vdc between positive and negative). Again our electrical vendor’s EEs suggested this but I’m still a bit concerned there is potential for shock or current flowing to ground. We’ve had trouble with communication with this vendor which is part of the issue. Its seems like the potential goes away when connected (no sparks, no current on multimeter). My thinking is that since that when the negative side is connected, it is pulled down to ground and then the smps and the linear regulator regulate on top of that pulled down earth reference and thus the ac potential goes away without current flowing. Does this make sense?


    Any help understanding any of this would be greatly appreciated. Sorry for such a long post! Thanks.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It is difficult to impossible to understand what you are doing without some sort of schematic. Words don't cut it. For example, what is the "ground" you speak of? That term has many meanings, one would have to guess what you mean by it.

    The oscilloscope images seem to be the same thing, and I have no idea what if first, last, in-between, what are they reference to, what voltage setting, time base, or what they mean period.
     
  3. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    Fair enough let me try to get a basic schematic together. I should mention I'm not an electrical tech but am trying to understand the concepts better.

    Ground in this case is would actually be piping which is connected to buried piping. My understanding is that this would be a good ground but please correct me if I'm wrong.
     
  4. dl324

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    Mar 30, 2015
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    It would help if you added the graphics in-line with your questions, cut out unnecessary data, and asked your questions more concisely. We only need to see one cycle of the waveform and don't need to see the scope controls; just annotate the pictures or state the relevant scope settings.
    I think you're referring to ripple voltage. Ripple is specified as peak to peak voltage, but not necessarily with respect to ground (e.g. 50mV peak to peak, but varying about the 6V supply voltage).
    All DC supplies derived from AC signals will have some ripple voltage. Frequency will depend on the circuit (half wave rectified, full wave rectified, switching frequency). Some frequencies are easier to filter than others. Ripple from any source can cause problems if it's magnitude is large enough.
    Yes. Even "old" switchers attempted to mitigate ripple.
     
    Last edited: Nov 8, 2015
  5. rjames711

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    Nov 8, 2015
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    Ok thanks for the feedback. I'm working on putting together something more sensible.
     
  6. rjames711

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    Nov 8, 2015
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    Ok, here's a basic diagram of the circuit and where I places the o-scope probes.
    [​IMG]
    This is what I got when I used an older linear transformer. Time division is 5 ms, volt divisions is 20 volts

    [​IMG]


    This what I got I plugged in with a switch mode power supply (I tried two brands and they were similar). Time division is 5 ms, volt divisions is 20 volts
    [​IMG]

    So basically I'm trying to get a better understanding of what going on here. dl324 mentioned above that what I might be seeing here is ripple. Does that still hold when looking at the above graphs? Is +- 40 volts normal ripple with a 6 volt dc supply?

    There's more information and questions but I'll limit it for now to avoid being garbled. Thanks and again any constructive criticism welcome, just trying to learn.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    Where are you placing the probe grounds? You have a great antenna for 60 Hz. from the power line. 3 and 1/3 divisions * 5 msec = 16.665 msec.!! That's a pretty obvious clue! You need to refine your measurement technique.
     
  8. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    The ground clips are disconnected. Sorry I was editing the drawing to add a note about that but now I'm not seeing the option to edit the post. Does this invalidate the reading? I've been reading and it sounded like you could do a differential or floating ground reading in some cases?

    I agree that its 60 hz. I may be using the O-scope incorrectly but here's my thinking: I always get this large amplitude jagged +/-40 v wave whenever a SMPS is plugged in and a small amplitude sinusoidal wave when using a linear power supply. So even if I'm using the o-scope wrong wouldn't such a consistent, repeatable and large difference between the two power supplies represent some sort of identifiable phenomenon?

    i did try placing the ground in different places. if I placed the ground clip on negative and the probe on positive I just got a flat line at 6 volts (for both the linear and SMPS ). I also tried connecting the ground clip to earth ground (buried piping). This worked fine when used with the linear supply but if I tried this with the SMPS it beeped and said out of range.

    This is all using a cheap USB scope btw.
     
  9. dl324

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    You need to have the scope ground clip connected to circuit ground. On my scope, I can add channels; to subtract, I invert one of the channels.
    You have Edit and Delete options in your post by the Report button.
    Just about all supplies will have some ripple, but it should be in the tens of mV or less. Whether that will cause problems depends on the circuit.
     
  10. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    Ok lets put aside the O-scope readings for a moment then. Using a regular Multimeter, if the SMPS is plugged in I get 6 volt dc if I measure between the terminals but I get 42 volts ac if I measure between either terminal and earth ground as shown below. However if I measure the same with a linear power supply plugged in I get only about 5 volts AC.

    [​IMG]
    Note: the above reading I put on the multimeter is just for the SMPS. With linear power supply I get 5 volts ac
    [​IMG]

    So this is the crux of my question I guess. One type of power supply shows a very large AC potential relative to ground. The other type doesn't. I was wondering if A: Theres a name for this type of variation B: Can this difference be a cause of performance issues or interference.
     
  11. dl324

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    You should study up on some basics on earth ground and isolated power supplies.

    Measurements between voltages, or ground, in your circuit relative to earth ground are meaningless unless circuit ground is connected to earth ground. Without a connection to earth ground, voltages in the circuit are relative to circuit ground; which may or may not be at the same potential as earth ground.
     
  12. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    Ok, I will do that. There are certain concepts with grounding and electrical circuits that I have trouble with even after reading over many times.

    That said I found someone who seems to have noticed a very similar phenomenon as what I've noticed.

    http://www.jestineyong.com/double-insulated-smps/
     
  13. rjames711

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    Nov 8, 2015
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  14. dl324

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    I did a quick scan of the info on that link and would caution you to not believe everything you read on-line. Anyone can post their opinion or belief, but that doesn't make it correct.

    The thing that jumped out at me was this guy was saying that a typical SMPS is isolated via a transformer; but what is typical?
     
  15. #12

    Expert

    Nov 30, 2010
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    Your references in posts 12 and 13 describe the capacitance between the primary and secondary sides of a switching power supply. This is not a, "phenomenon". This is not, "Spooky Energy". Electricity has known laws of operation and you seem to be investigating things like current through a small capacitance and how to use the ground leads of your oscilloscope as an antenna for local power line frequencies.

    The first thing you need to know is that every voltage is a voltage compared to some other place. That's why volt meters have two wires. That's why oscilloscopes have ground leads. When you go about trying to measure the voltages caused by tiny currents applied to a high impedance measuring device, you are measuring the current through the internal capacitor described in the article in your post #12 or the energy picked up by the unconnected ground leads of your 'scope.

    The, "phenomenon" seems to be about you trying to imagine that the voltages you measure are voltages compared to Earth ground when none of your test leads are connected to Earth ground. To paraphrase Frank Zappa, "That is the crux of the biscuit." You are merely imagining a reference point that does not exist in the way you connected your measuring devices.

    Instead of wondering how mysterious voltages show up, compared to a reference point that doesn't exist, you should be trying to figure out where the current is coming from, where it is going to, and how much current it must be to cause that reading as it passes through the input impedance of your measuring devices.

    Well educated and well seasoned electronics people know how this works and how to find the relevant paths. The, "phenomenon" in post 12 consists of about 150 microamps. I have personally chased (and found the cause of) leakage currents less than 1/1000th of that magnitude. Eventually, you will, too, but first, you have to get a clear mind about what your reference point is for each measurement.
     
  16. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    I don't think I've ever resorted to definitions to defend myself but here goes.

    Phenomenon: a fact or situation that is observed to exist or happen, especially one whose cause or explanation is in question.

    That's all I meant by the word. I meant I had observed something and was looking for the cause or name of what I was observing. I really appreciate you and everyone else's input but your characterization of me looking for "spooky energy" seems kinda unfair.

    I did measure with a multi meter between earth ground and + and - wires and got 42 vac as shown below (also above in post 10)
    [​IMG]

    I'm honestly asking, not trying to be smart ass at all. Does this mean anything or does it not matter because there is otherwise not a connection to ground. I'm actually very confused on this point.
     
  17. #12

    Expert

    Nov 30, 2010
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    Add the capacitor described in your link in post 12. Now do you see where the current is coming from?
     
  18. rjames711

    Thread Starter New Member

    Nov 8, 2015
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    Yes but wouldn't that also be an avenue for ac voltage to pass through? Which would the voltage relative to ground that I measured?
     
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