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Zero Potential is not ground, How do you tell the difference?

Discussion in 'General Electronics Chat' started by Billy Mayo, Dec 12, 2013.

  1. Billy Mayo

    Billy Mayo Member

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  2. Billy Mayo

    Billy Mayo Member

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  3. Billy Mayo

    Billy Mayo Member

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  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Sure. Probably easier than pulling chips.
     
  6. Billy Mayo

    Billy Mayo Member

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    Are you sure this Fluke 8842 can be used as a milliohm meter to track down shorts? it does have a 2 wire and a 4 wire button , but i'm just going to use the 2 wire

    Also the tech next to me uses his continunity checker to test semi-junctions , he said that the continunity checker outputs 9volts and a higher current on the Fluke 87 , is this true?

    True, My manager made me pull out 40 IC chips today to track down a short and it still didn't get us anywhere

    The Oscilloscope he uses must short out components or something, I think his oscilloscope ground needs to have an isolation transformer so you can't short out components right?
     
  7. ghostman11

    ghostman11 Well-Known Member

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    I think if your manager, uses an Oscilloscope in such a way, that he is frying boards. Then i cant honestly see the company being around long can you? So you have access to a bench meter, i mentioned this i think, but you are deciding to track down shorts, with 2 wire not 4 wire? why is this Billy?
    We have 4 bench meters here, all older but very capable machine, 2 are 6 1/2 digit 2 are 5 1/2 digit. On them, and many others like them there are 5 terminals for Ohms. We happen to have kelvin leads but obviously you dont have to. So forget the guard terminal for now, 2 terminals take the reading two terminals supply the constant I. The reading on auto Ohms range should with nice sharp probes, and a careful approach, get you to within less than 1/2 inch of a short no problem.
    So what reason can you give for using 2 wire? It cannot be speed, you are under no time pressure, you can spend all day and, sometimes more, trouble shooting one board.
    Let's take a look at that first. Your manager probes a (i assume non working board) with an Oscilloscope, why is he doing this? Is he meant to be trouble shooting? Because you know what Billy, that dose not add up to me, if he is probing the board then why pass it to you? He is aware he know's far more than you, afteral he has to tell you how to use a DMM. So why would that person then hand you a board they cant find a fault on?
    Why would this person, who apparently know's what he is doing, get you to pull 40 chips (thats alot of chip for a motor control board, let me guess...they are all FPGA's), when this person suspects you have a short and is aware there is access to a bench meter that will read sub milli ohm?
    Then you end with the classic Billy tactic............thread is dead so steer onto another topic floating scopes (sorry i mean floating the scope ground).
    I can answer that question for you. Search this site. There is a huge amount of information and professional opinion on floating scopes, as you are very much aware, that is a topic sure to fire up a debate and get polarized opinion. It has been done to death here Billy, try another path ;)
    And no i am not expecting you to answer a single question i have asked, how can you answer any of them? it would take you too long to come up with a logical answer to most of them ;).
     
    Last edited: Dec 17, 2013
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  8. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Billy;
    Please tell us what company you work for.
    What do they make?
     
  9. ghostman11

    ghostman11 Well-Known Member

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    Sorry Andy i dont mean to be picky, and i am the last person to mention spelling!!! but can you edit your spelling mistake please.
    in case you cant find it........... Break is spelt B R E A K not M A K E ;)
     
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  10. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I just want to be sure never to use any of there stuff.
     
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  11. ghostman11

    ghostman11 Well-Known Member

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    No fear of that! with there production rate i doubt if anything has actually left the building yet lol
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi guys,
    Please try to keep on topic.
    Eric
     
  13. ghostman11

    ghostman11 Well-Known Member

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    Sorry Eric, it's just the interlude, we are waiting for the official switch over, to talking about floating scope. Well at least i think scope floating is next??? But point taken we will wait in silence until the switch over happens
    Regards
    Jason
    P.S why you here??? i thought you would be in a slightly warmer SA, this time of year?
     
  14. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Billy check out EEV Blog, on there is a couple of video's of a guy called dave. who knows what he is doing, in a couple of them he shows you exactly how to trace a short with a multi meter than can read milli ohm's.
    Go watch it, it has pictures and words in the video. So it is easy to follow, rather than try to understand something that is written. In 40 minuets of looking at the video you will have all the answers.
    if you have trouble finding it, let me know and i will go find it for you.
     
  15. Billy Mayo

    Billy Mayo Member

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    You can use the 2 wire for milliohms, why do you need the 4 wire? the 2 wire is fine enough right?
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I did not read the manual.

    How can I tell, my crystal ball doesn't know what the other tech is using. There are times you need a higher open circuit voltage, e.g. HV rectifiers. Modern continuity testers don;t want to turn on a junction, so you can test in-circuit.

    So, is this on your manager? I'm not sure you have th skills to use a millivoltmeter, but I would use a 3-terminal mode, so you don't have to re-zero. Put two probes on the reference trace (ground) and make a spacial probe that connects the 4th wire to the tip of the probe.

    What I suggest you do, is to take two long wires (about 22 awg) and put them about 1" in parallel with each other. Solder a 0.1 ohm resistor or a short in the center, and put a bunch of 1K resistors about an inch apart, 5 on each side. Now practice finding the short. Put the reference wires (2) on an end.

    Most oscilloscope should basically be used without the ground clip if there is a connection to ground in the unit being tested. Differential mode should be considered in some case. The OSCILLOSCOPE should NEVER be put in an isolation transformer. Only the equipment should be isolated or the scope should be "inherently isolated". Many instruments have a LOW to ground specification, That spec may only be 30 V or so. Adhere to it.
     
  17. ghostman11

    ghostman11 Well-Known Member

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    Contentious topic, which is i suspect why the OP mentioned it, i know some the old hands float there scope (so to speak) on here, and lets be honest for the likes of Nigel this is fine, he know's what he is doing, but for most of us yes i fully agree KISS. As for the OP i get a little worried when he mentions picking up the multi meter ;)
     
  18. Billy Mayo

    Billy Mayo Member

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    If i don't use the O-scopes ground, it will make the signal/waveforms very noisey on the display and I can't trigger them too

    At some of my jobs , techs uses an isolation transformer for their oscilloscopes , why don't u?

    They use an isolation transformer to separate the grounds from the test fixture, power supply, variac, o-scope, etc.

    Each one has it's own isolation transformers , so there is no ground loops also or grounding problems

    They said that when using isolation transformers it doesn't LOAD down the wall outlets , is this true?
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The key is Is the system your debugging isolated or not? If it is not, no ground clip. If it is use a ground clip. Not 100% accurate, bit close.


    You can answer that.
     
  20. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Are they pulling your leg?
    Its still the same load transformer or not.
     
  21. Billy Mayo

    Billy Mayo Member

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    ya but they said when you plug in an external power supply, a test fixture , oscilloscope , it loads down the wall 120 volts and it creates grounding issues between the equipment when you're measuring it

    It's a loading problem and also a ground problem , is this true?
     

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