HV probe anomaly

Discussion in 'General Electronics Chat' started by strantor, Feb 22, 2014.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    Check out this 60 sec video description of my question. What gives? Here's an excerpt from the manual:
    [​IMG]


    Manual also says:
    But I don't see why it matters on a battery operated meter
     
  2. strantor

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    Nobody has a clue?
     
  3. bountyhunter

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    Sep 7, 2009
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    The only thing I can think of is maybe the hook up cable has a shield tied to the neg post. When connected correctly, the shield is attached to circuit ground in the meter. Flip it and the shield is at the top of the input resistive divider to the meter so it may be a noise issue. At 125 mV, noise could be a problem if the shield is floating.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    if you dont connect the probes ground lead, you will have a big problem with a "HOT" meter. and the difference in voltages was because of unballanced load for the voltage devider. the ac reading is different if the meter isnt on the ground side. dc would not have had this difference.
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    You should get the same difference by switching the hot and neutral on the other end.
    I expect a difference in the hot and neutral electrostatic radiation.

    The meter must have electrostatic shielded inside.

    Being 10M it works like the non-contact testers

    Try an isolated source.

    Or a faraday cage on the meter.
     
  6. tindel

    Active Member

    Sep 16, 2012
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    Thanks guys... I hope I don't confuse anyone, but I actually bought one of these from Strantor on fleabay and connected it to my scope figuring that 10Meg input impedance on a DMM isn't any different from the 10Meg input impedance of my scope.

    I tested a -3.3kV supply that I've been having problems with only to find that it's really -4.5kV on my scope! So I'm really just trying to understand if my supply is really at -4.5kV or not, and what strantor is finding is really adding to the confusion.

    The only difference I see is that I have a ground loop in my setup from the HV supply being referenced to earth and my scope also being referenced to earth. Because they are both referenced to earth I don't have the ability to swap the banana plug connector around because it will just short out the 75k resistor.

    You might be on to something bountyhunter. There's a lot to be said for measuring 125V with a /1000 probe. You are down in the noise. A higher voltage will result in a higher output voltage and will yield more accurate results.

    alfacliff - It should be pointed out that the high voltage probe is not floating... it is attached to ground (in strantor's case, neutral) so that there is no chance of more than 6V being on the output of the banana jack. I bet strantor would have been cussing in is video if that wasn't the case!
     
    Last edited: Feb 23, 2014
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Tindel,

    Can I assume you are using a banana plug to BNC adapter?

    Do you have the voltage regulation specification on that 3.3 kV supply?
     
  8. tindel

    Active Member

    Sep 16, 2012
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    I actually tested the HV supply with a fluke 87 tonight and found that either way the banana jack was oriented it produced 4.59kV... 1100 more volts than designed. I think I now know what my HV problems are... but that's for another thread.

    It also shows my theory of being able to plug this particular probe into an 10Meg oscilloscope probe is a valid theory.

    I think bountyhunter is right... I think it's just such a low voltage it's picking up noise. Do you have access to a HV probe source, strantor, to test this theory? I'd say probably 600V is the threshold since that's the range of most DMM and scope probes.
     
    Last edited: Feb 23, 2014
  9. tindel

    Active Member

    Sep 16, 2012
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    I'm using a 10x probe carefully wired to the banana jacks of the HV probe. I do plan on making a box with a banana jack input, a 9Meg signal resistor and a direct connect BNC cable. I think I need to buy a box of Altoids... don't you?

    The high voltage I'm measuring is out of a Tek 564: http://bama.edebris.com/download/tek/564/tek%20564.v6.pdf I'm actually measuring at the output of V822, the selinium (sp?) rectifier. The spec says -3.3kV +/- 100V.
     
    Last edited: Feb 23, 2014
  10. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    I have a hipot that I was planning to auction but haven't tested. If we suspect that there might be something wrong with these probes, then I'm not sure what the results of the hipot test would tell us. The hipot has not been tested with a known good HV tester. So if they don't read the same, then who's bad? If they do read the same, can we assume both are good?

    I will try to test this tomorrow.
     
  11. tindel

    Active Member

    Sep 16, 2012
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    Strantor - I think these probes are working just fine. I think your 120V test is just inadequate to show it working. One other variable is that I'm using a DC source and you're using an AC source.

    Note - I brought the voltage down on my HV supply using the provided trim pot and I saw some protection neons turn off. I need to recalibrate this scope, but I think the test was very successful and helped me get down to the problem I was having on this scope.

    Side note - the focus on these old 500 series scopes are unmatched by any scope I've ever seen. I wish I had this thing at work! I'm planning on putting it back in commission here at the house once I recalibrate it.
     
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