Frustrated and trying to understand ground.

Discussion in 'General Electronics Chat' started by AcousticBruce, Nov 27, 2011.

  1. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    0
    I have...
    -PC power supply with Gnd, 3.3, -5, +5, -12 and +12
    -Digital Oscilloscope Owon SDS6062 60Mhz 500Ms/s
    -Lodestar Function generator 0.2 - 2Mhz FG-2102AD


    I want to understand ground safety and grounding setups. What grounding is. What is isolation. What is safe. What is not safe. Why my scope do I not need my scope ground because its already connected somehow (with PC power supply).

    I am trying to watch a capacitor on the oscilloscope charge and discharge. I can do this successfully with batteries. But I CANNOT using the power supply. I believe this is because the ground is somehow connected.

    With 5v batteries and a 1 second time constant. I can clearly watch the oscilloscope screen and see the capacitor charge with a logarithmic curve at about 5 seconds

    With the power supply 5 volts when I connect the positive the scope immediately jumps to full 5 volts and if I disconnect the positive from the power supply it immediately shows the voltage at the caps potential difference.

    The batteries allow me to see the logarithmic charging curve. The power supply doesn't.

    Can you help me understand what is going on and how to set up my bench.


    about me. I am a hard core home study and I take notes. I am studying physics trig and grogs basic electronic book and taking great notes. It is not a matter of lazyness that I do not understand. It is a matter of not knowing how to learn about grounding my home electronics project bench.

    thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    When you connect 5V batteries to your capacitor, the internal resistance of the batteries slows down the charging. The internal resistance of the 5V power supply is very low and hence the capacitor comes up to full charge much more rapidly.

    The ground on your scope has nothing to do with what you are observing.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Part of the problem you probably are fighting is the word ground is slung around with abandon, and doesn't always mean quite the same thing.

    Ground as applies to a circuit is the common point where all other voltages and signals are referenced. In the case of two power supplies if the positive lead of the power supply is connected to "ground" it is a negative power supply, if the negative lead is connected to "ground" it is a positive power supply. There are many uses in electronics for ± voltages, especially for op amps.

    Ground as applies to AC power is a safety feature, and is not usually the same thing. It is possible, but not required, that earth ground and circuit ground are connected.
     
  4. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    I have a 220uF and a 4.7K resistor. Even if the PC power supply has no resistance it still should be at about 1 second RC time. Right?


    If I buy a power supply such as this. http://www.mastechpowersupply.com/hy1803d-mastch-power-supply/prod_1.html would the black be connected to earth ground?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I am not where I can freely browse the internet, so I can not follow the link you gave.

    In general a power supply should not have either side connected to earth ground. This is so you (the end user) can decide if it is a plus or minus power supply. If they ground one lead of the other then they are making the decision, and it may not work for your application.
     
  6. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    what you should take notice of is that you never,ever connect your scope up to any equipment which is not mains isolated and uses a switch mode power supply,or you will have a rather loud bang and damage to your scope and or damage to the power supply of the equipment you connected it to-you need to connect that sort of equipment to the mains via an isolation transformer then you can take measurements.i take it you have just got your self a scope?
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Hacked ATX power supplies are NOT isolated.

    Your scope probe is the same as your negative wire.

    Plug all of your equipment into one unplugged power strip), use a multimeter on continuity or diode mode and you will see which wires on your equipment are connected together, the ATX case and black wires and scope ground probably are.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    [​IMG]

    That is the same power supply I use, only mine is under the Vellamen brand. It is isolated from AC ground, so it can be plus or minus.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    3,245
    ATX supplies are isolated from the AC. Do you mean the output grounds (commons) are not isolated from each other?
     
  10. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    Here is my ATX hack http://photobucket.com/HackedATX



    I hacked an ATX power supply which IS a switch mode power supply. I know nothing about isolation (the reason for this post). I have read that you should never hook a scope to an isolation transformer. Even though I dont know what that is I have seen this, so are you saying add a isolation transformer to my ATX switch mode power supply?

    I did this and found that the ground of scope and the black wire (gnd) were in continuity. I asked another EE about isolating the atx power supply and he said it wasn't a good idea. Is it really so bad? I really want to be able to do these type of test because I am learning.


    I just do not understand. "ATX supplies are isolated from the AC" you said this and ThatOneGuy said "Hacked ATX power supplies are NOT isolated."
    The continuity test I did above might prove that it's not isolated. What do you think?
     
  11. Yako

    New Member

    Nov 24, 2011
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    2
    If there is continuity then it cannot be using an isolation transformer.
     
  12. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
    58
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    So what can I do about this?
     
  13. Yako

    New Member

    Nov 24, 2011
    245
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    I don't claim to know very much besides the basics about switch-mode supplies that run off the mains, but I think that at best the isolated ones have a transformer with a turns ratio of 1:1
     
  14. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    no i was referring to smps (switched mode power supplies )in general(crt-lcd plasma etc that are not at all mains isolated....with these an isolation transformer is essential....an isolation transformer is a 1 to 1 transformer which when fed from your mains supply isolates the equipment you are working on from the mains so you can safely connect your scope to take measurements without any risk of damage on switch on.....
     
  15. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There is some confusion here. Most power supplies for equipment such as computers including ATX types achieve isolation from the mains input live and neutral. In many cases nowadays this is done without using a transformer at the mains frequency. Switched-mode types often use a transformer running at a high frequency to provide isolation: this is necessary as otherwise their use with accessible uninsulated connections would be dangerous (and in many cases iillegal).

    Note however that in case of a fault developing, it is possible for such isolation to be lost, and unless some part of the circuit is deliberately earthed this may not at first be obvious. It is however quite common (pun noted) to connect one side of a power supply output to mains ground, possibly as a safety precaution, and/or to enable the use of more effective mains input filtering. You must therefore be very careful about where you put your oscilloscope "ground", or there may indeed be expensive and hazardous fireworks.

    Some switched-mode supplies used for such purposes as lighting, including certain display backlights, may not provide isolation. This is considered acceptable if the devices are not accessible to the user in case of normal operation. They require particular care for servicinf however, especially as some involve voltages higher than the mains input.

    Finally, as to the idea that "you should never hook your oscilloscope up to an isolation transformer", this is a good example of a half-truth: There are circumstances where an isolating transformer may make things very unsafe, e.g. if it allows the frame of an oscilloscope to be taken to a high voltage which the operator may contact. In other cases, an isolating transformer at the input to the device under test can be used to convert a non-isolated system into an isolated one, giving enhanced safety.

    This is, frankly, not rocket science, but unless you are able to understand these things very clearly, it may be better for you not to meddle.
     
  16. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    But I WANT to understand. I cant just except that answer. The reason for this post was to learn and learn where to learn. Frankly i have lots of electronic books and none of them talk about this safety. If they do it is too advanced and not a direct approach to safety.
     
  17. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    There are serious problems in the way of trying to explain safety matters on an Internet forum. For instance, we cannot be sure that whoever we are talking to will understand what we are saying. We are not (in general) professional instructors, nor can we apply the sort of checking that would be possible for example for an instructor working face-to-face with apprentice electricians.

    This becomes all the more concerning in a case where the inquirer states that they are having difficulty understanding things, despite having tried to access material describing them. I hope this does not sound condescending, but if you find their descriptions seem "too advanced", it may be that you would benefit from first studying some more basic information on electric circuits, such as may be found on the ebooks on this website.

    We would be doing you the opposite of a favour if we tried to explain matters which you are not in a position to understand, possibly leaving you with a dangerously false sense of confidence. This is my take on it anyhow. There are others on this site who make good and careful explanations, and perhaps they will be able to help you.
     
  18. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
    58
    0
    Nothing I have is too advanced. I meant maybe it explains it in the later advanced lessons. Even this sire has lots of free information.. but where is the saftey and proper set up of a home electronic bench? Its not there. Do you know books... web sites that show THIS specific subject? I try searching google and the terms I used didnt lead me where i needed.

    This might be my 5th time posting about this issue in various forums and again I am left to a brick wall and no information on how to learn about this information. This time i am going to persevere until I get at least a lead that explains this stuff.. weather I keep posting here or find it on my own.

    Please understand that I am grateful for all of your responses but I need some more help, weather it be a direct explanation, a phone call, Skype, book, website, youtube video, forum response.
     
  19. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Perhaps the main confusion is about the term "isolation". The common definition for that in consumer applications is that the mains power and neutral (not safety ground) are isolated from the internal electronics, that is, there is no internal path (or voltage) from the mains power to earth ground in the electronics. That is true for many devices such as computers and TVs. The AC safety ground may or may not be connected to the device chassis. If the device has exposed metal it often is (indicated by the power cord having a 3rd prong in 110V U.S. power plugs).

    Switching power supplies, such as used in computers, are isolated from the mains by the switching transformer as mentioned in Post #15. Because of the high switching frequency such transformers can be much smaller than line transformers for the same power level.

    1:1 isolation transformers are used when you want to more safely work on a line powered device that is not otherwise internally isolated from the mains. It prevents electrocution between the line power voltage and earth ground (safety ground). It does not, of course, prevent electrocution between the power line voltage from the transformer output and circuit common.
     
  20. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Note however that obtaining the mains input power for an instrument such as an oscilloscope via a high isolation transformer in order to allow its common line to be taken to a high potential is not a very safe thing to do. It is the sort of thing that engineers used to have to do until the advent of better ways. (I can remember using a scope in a thick perspex case to observe low-voltage circuits which were floated up to 12kV!)

    Perhaps this is what the OP may have been warned against doing. The difference is to my mind pretty obvious, and may be to the OP if he gives it a little thought. It would be interesting to hear his comments on that.
     
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