help me understand grounding with scope and power supply

Discussion in 'General Electronics Chat' started by AcousticBruce, Nov 5, 2011.

  1. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    lets just say i make a resister LED circuit. I attach the positive to the annode of the LED. Then i attach the ground alagator clip to the resistor. I see the LED light up. so basically i have the positive power supply and the ground of the oscilloscope probe and some how this creates a potential difference.

    The scope and the power supply is connected on the same outlet.

    Should this be happening?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome Mate.

    How about u draw the circuit and show us, this way we will have a better view at the problem.
     
  3. MrChips

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    Oct 2, 2009
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    If you showed us a photo or model number of your power supply that might help.
    A single voltage power supply will normally come with two banana terminals. Sometimes there will be three terminals. A power supply can be either floating on non-floating.
    If the supply is non-floating, then usually the -ve terminal is connected to ground to give you a positive voltage supply. It appears that your power supply is of this variety.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What may be happening is the power supply you are using is not isolated from the AC line. Most O'scopes Ive dealt with are not isolated, meaning the ground opn the scope is connected to AC power neutral.

    Thus the power supply (-) output and the scope (-) are connected together back at the AC line, and this is making the connection when you connect the LED that way.
     
  5. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    I will take some pics and make a schematic when I get home tonight or morning.

    for now I have a Owon PDS5022s and I made a power supply with a atx computer power supply and it has ground 3.3 -5 5 -12 and 12.
     
  6. MrChips

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    If you are using a computer power supply, then your supply is not floating. So it shares a common ground with your scope.
     
  7. jimkeith

    Active Member

    Oct 26, 2011
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    Some lab power supplies have a removable grounding strap between the case and the neg output terminal--some (few) actually ground the neg terminal to the case.

    In some cases, to prevent ground loops etc, I am forced to float the scope via an open ground pin adapter--DO NOT FLOAT SCOPE WHEN WORKING WITH LINE VOLTAGE CIRCUITS--SEVERE SHOCK HAZARD!

    An ohmmeter is all you need to debug your situation.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    A wall wart or switching power supply other than a computer PSU generally are floating, they are not connected to AC ground. Your setup is a little unusual, but not necessarily bad.
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It would be a good thing for you to be clear in your mind about which connections are returned to mains ground.

    This may help you to avoid having a short-circuit accident at some time in the future.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    It appears the power supply and 'scope have there grounds connected together but it's likely through the third wire AC safety ground and not the neutral. No equipment for consumer use have their output commons connected to neutral which would be dangerous.
     
  11. Wendy

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    Mar 24, 2008
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    True enough, but the difference as seen from the outside is moot. If ground is used as a common connection then there can be unexpected shorts. This is why most decent power supplies are floating, there is no need to connect a 12VDC power supply to ground. If it has been then you need to be aware of it for future use.
     
  12. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    When you say "not floating" like no isolation transformer?

    What is line voltage? Do you mean like 120v rms household?

    What do you mean by unusual?

    I agree!



    I also would like to understand safety and ground much more, do any of you know good sources to learn this stuff?



    Ok as you can see Scope ground hooked to resister and the power supply +3.3v hooked to cathode of LED.

    Is this dangerous? I would rather this not happen if possible. What would you do?

    [​IMG]
     
  13. justtrying

    Active Member

    Mar 9, 2011
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    this is very interesting. I think if I did this is a lab at school they would not like it ;) Looks like the path is created by the fact that the scope and the power supply share a common ground. Oscilloscopes sometimes have a floating ground feature as well, but i don't know enough to comment on it except that floating ground is tricky and makes it easy to short things out (bad idea). I think a good idea would be to create a proper ground rail on your breadboard - would make it easier for referencing and avoiding shorts as well.

    p.s. if your oscilloscope probe is warming up, something is not connected right... been there, done that...
     
  14. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    Why would they not like it, because of danger?

    I need to learn about ground. I am going through a electronic text book, Grob Basic Electronics and surprisingly it didn't really explain ground. So the truth is I am a little ignorant to ground and safety.
     
  15. MrChips

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    Oct 2, 2009
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    The situation you describe is not unusual. There is no concern for safety. Oscilloscopes are usually grounded via chassis ground. That is also true with function generators. With a power supply, the important thing is that you are aware of whether it is floating or not floating. (You should not use the ground for transmitting power, as in your example.)

    Imagine a 9V battery on your bench. If you connect the -ve terminal to ground then you have a +9V supply referenced to ground. Conversely, if you connect the +ve terminal to ground, you have a -9V supply referenced to ground. If you do not make any connection to ground, you have a floating supply. There is no safety issue in any of the three cases.

    Floating and isolation are almost the same thing but not quite. Floating is as I described in the final example above. Isolation means that there is no DC path to the AC mains power supply.
     
    Last edited: Nov 7, 2011
  16. Wendy

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    Mar 24, 2008
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    What I was referring to in post #8 is a plug in power supply should be floating. If it isn't it limits what you can do with it, but in and of itself is not bad.

    Many circuits require a ± power supply. If the plug in power supply already has one side grounded then you can't do ± as easily.

    Old commercial power supplies like HP had (+) (GND) (-) banana posts, you could pick which side was grounded with a short jumper wire between the posts.
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    To eliminate your issue, put an isolation transformer before your ATX Power Supply.

    Then there will be no common current path between the scope and the circuit under test.

    1:1 Isolation transformers rated for 15A are somewhat expensive, though. Use a DMM to measure the voltage difference between your ground clip and the outputs to ensure the voltages are normal (should be if plugged into same outlet/power strip).

    You should run a ground wire back to the PSU for the circuit, rather than having the scope provide the return path, however.
     
  18. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    This is a concern of mine because I use negative potential of -5 and -12. So lets say I use -12 and 12 to make 24 volts... the scope ground reference would be at 0 and if test using the scope, it would be off by 12 volts. Am I right?
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    No. Your supply provides +5, -5, +12 and -12, all referenced to ground (0V).
    Make sure you connect the 0V wires from your supply to your circuit.

    The only place you can connect the GROUND clip of your scope is at the 0V point or any such common GROUND.

    Your scope will show +5, -5, +12, -12V. In this situation, you do NOT have a 24V source.

    If you connect, by accident or intentionally, the scope GROUND clip to anything else, you can have disastrous results!
     
  20. AcousticBruce

    Thread Starter Active Member

    Nov 17, 2008
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    When you say that I do NOT have a 24v source, do you mean that I shouldn't hook it up because its bad? Because I have successfully tested out with DMM and show -5 and 5 = 10 & -12 and 12 = 24 & -7 and 12 = 19 and so on.


    Basically I need to understand ground and need some education. None of my books go into detail here and I would like to understand home electronic bench safety. Where can I get this?

    BTW thanks for all the replies... you guys are amazing.
     
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