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.
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.
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