How to verify that PSU is not grounded (to protect my scope from short)?

Discussion in 'General Electronics Chat' started by Andruxa, Sep 14, 2013.

  1. Andruxa

    Thread Starter New Member

    Sep 14, 2013
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    I use a modified PSU for my breadboard. Now I need to use an oscilloscope. But I want to know how to verify that my PSU is not grounded, because I want to avoid damaging my scope. So, question #1 is, how to check whether my PSU is not grounded. Question #2 would be, if mains use only neutral (neutral is merged with ground) how can I check that and what does that mean for my scope?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Normally you can either isolate the scope or the board.
    If it is a switching supply, it is most likely isolated from earth gnd.
    The way to test take both a resistance reading and a voltage reading from the P.S. output to earth GND with power off, it should read high resistance, power on you may read a voltage due to stray inductive/capacitive currents, while measuring, place a <1k resistor across the measured points and the reading should fall to zero.
    If the voltage remains or the resistor gets warm, it means there is a potential from the measured point to earth gnd.
    It is normal for most 'scopes to have the probe common connected to earth ground.
    Max.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Another way is to use the tip of your scope probe without connecting the ground clip of the probe.
    While watching the scope, touch the scope probe to all parts of the power supply, +ve, -ve output, If the power supply is floating you should be seeing 60Hz AC pickup on the scope.
     
  4. Andruxa

    Thread Starter New Member

    Sep 14, 2013
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    I think I may have used an incorrect expression. I am using an "ac/dc(adapter)" which is made out of plastic. Input is 220V and output is 12V.

    Anyway I discovered by continuity test that it's not grounded, so this means it's safe to use with my oscilloscope. The oscilloscope however I also tested using continuity test and is grounded. MaxHeadRoom, is there any reason that I should use resistance and voltage instead of continuity check?

    MrChips, I have an AC/DC converter so that wont work if I understand your method correctly right? It's my fault for using term PSU instead of AC/DC converter, I'm sorry.
     
  5. #12

    Expert

    Nov 30, 2010
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    I realize it's scary when you try to connect a ground clip and something explodes, but it's not all that complicated to use a VOM to measure for continuity to ground from the power supply's outputs or to measure for an AC voltage from the power supply ground to the 'scope ground. The other stuff is just different ways to do the same thing. It only depends on how you think that will determine which method you will use.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Resistance continuity check, same thing. Using a resistor is just to ensure any measured stray voltage actually collapses and does not support current that would indicate a potential reference between one and the other.
    Max.
     
  7. Andruxa

    Thread Starter New Member

    Sep 14, 2013
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    Very scary yes. I got shocked in an old house once and due to an awkward position of my body the shock lasted a few seconds until I got kicked away. Since that day I am completely terrified of mains electricity. Even when I know I can't theoretically get hurt I get paranoid, my heart starts racing and my subconsciousness is telling me it's going to end badly. So I want to double, triple or even quadruple check everything :)
     
  8. #12

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    Nov 30, 2010
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    I'm with you. My first job was 30,000 volts, and you never get over it. 40 years later, and I actually break out in a sweat if I don't have 3 layers of safety between me and the job.
     
  9. vk6zgo

    Active Member

    Jul 21, 2012
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    If you are not confused about the possibility of contact with the Mains,you may be worried that you may short out the output of the "AC/DC Adaptor".(this is what is commonly called a "Plugpack" in Australia,& a "Wallwart" in the USA)

    Having one side of the output connected to Mains Earth only means that if you inadvertently connect your 'scope probe backwards,you will short out the Plugpack output.
    This would make a bit of a "zap" but would be incapable of supplying enough current to do much damage.

    A full size Bench supply could supply such current,but is usually fitted with overcurrent protection which would again prevent damage.

    The usual reason warnings are given about the possibility of damaging your probe,Oscilloscope ,and/or yourself are when
    the voltage source you are looking at is capable of providing very large amounts of current ,such as very big internal supplies in equipment,large storage batteries,& most importantly The AC Mains or anywhere around the unprotected side of Switch mode supplies,operated off the Mains.

    The Mains can supply extremely high currents until the fuse in the house Meter Box blows,or if you have one,the RCD trips.

    By the way,don't let ANYONE talk you into removing the protective Earth connection from your 'scope,because if you do that & think you can now connect your probes to the Mains,you will probably become an Electrocution statistic.

    Without the protective Earth,you won't get a big BANG when you inadvertently connect your earth clip to the Active line,but you will elevate every metal part of the 'scope to mains active.
    It is then just a matter of time till you join your ancestors!
     
    Last edited: Sep 14, 2013
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are cases where auxiliary supplies are referenced directly to Earth ground, including your desk top computer SMPS 5v/12v, the common of which is directly connected to earthed neutral by way of the ground conductor.
    Max.
     
  11. sheldons

    Active Member

    Oct 26, 2011
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    post a picture of your power supply and let us see it-chances are its a double insulated transformer based supply in which case when you connect it to whatever it is being used with and you connect the ground lead to the supplys ground or negative side on the power supply output you will not have any bangs or magic smoke....if the supply is a switch mode type if it isnt mains isolated there will be a hefty bang when you connect the scope ground if you dont use an isolation transformer....
     
  12. Andruxa

    Thread Starter New Member

    Sep 14, 2013
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    vk6zgo, if understand you correctly, when you said "you may short out the output of the AC/DC Adaptor", you meant that my "wallwart" would probably short out before my oscilloscope (which is entry level Rigol DS1102e btw) or probe would get damaged by high current passing through it?

    Sheldons, here are the pictures of my adapters. I use:
    - AC/DC 220V to 3V-12V adaptor
    [​IMG]

    - AC/AC 220V to 12V adaptor
    [​IMG]
     
  13. sheldons

    Active Member

    Oct 26, 2011
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    ok not the best supply in the world the markings shown on the first pic show it is a double insulated supply,it is transformer based and the adjustable side to it will be an LM317T adjustable reg type of design....rated at 300 ma....the second pic shows a supply which again is double insulated at 230v ac input and 12v ac output at 1000 ma....so you need to convert the ac output to dc before you can use it with whatever you have if it needs a dc supply.....
     
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  14. #12

    Expert

    Nov 30, 2010
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    sheldons says they are safe.

    Oh look! There's my QC stamp!:p
     
  15. vk6zgo

    Active Member

    Jul 21, 2012
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    This used to be common practice with many linear supplies.

    There is nothing particularly scary about the the Mains Earth.
    After all,it was often connected to your water system.
     
  16. vk6zgo

    Active Member

    Jul 21, 2012
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    If a switchmode supply is correctly designed*,the output circuit is isolated from both Mains Active & Neutral by the transformer used in the switching circuit.
    (OK, switchmode people call it a multiwinding choke,but it is a transformer in all but name)

    If one side of the output is connected to the Mains Earth,it is probably a distinct connection,not passing through the "guts" of the SMPS.

    Modern, (& seriously dumb practice in my opinion),is to connect a special mains rated capacitor from the supply output back to the Mains,as part of an EMC protection circuit.
    The reason I say "dumb" is because it freaks people out,so they think they are going to be electrocuted.


    With this circuit inclusion, & the output isolated from Earth,you may get a miniscule "bite" from the SMPS output if you go between it & Mains Earth,but it can't hurt you or your Oscilloscope.

    *NOTE:- I said "If a switchmode supply is correctly designed"!
    There are a few ones around which may not merit that description.
     
  17. t06afre

    AAC Fanatic!

    May 11, 2009
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    They both have the class II or double insulated electrical appliance symbol. So it should be safe do measurement with your scope.
     
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