Shorting out VCC to VDD when measuring with a meter

Discussion in 'General Electronics Chat' started by DexterMccoy, Feb 19, 2014.

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  1. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    Why does a plug in bench DVM meter ( 3 wire AC plug in DVM meter ) short out when measuring VCC and VDD at the same time?

    VCC is +30volts and VDD is -30 volts

    The Bench DVM meter shorts out the burns the circuit board, but why?

    How do you prevent this from happening?

    I had my red dvm meter probe on VCC +30 volts and my black meter probe on VCC -30 and it shorted out the power supply diodes and fried the board

    But why?

    Why would this create a short if the DVM meters negative input is connected to earth?

    -30 volts VDD and + 30 volts VCC = 60 volts potential difference

    Is the Bench DVM meter chassis ground connected to the meters probes?
    The meter probes would have to be internally inside the bench DVM meter connected to earth or chassis ground of the DVM meter

    If a circuit boards power supply earth or chassis is different then the DVM meters chassis ground or earth ground can this cause a ground loop or short when measuring from VCC to VDD rails?

    When can a SHORT occur when measuring from VCC to VDD supply rails?

    The grounds would have to be different how so?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    What is the make and model of your DVM?

    What do you mean by meter's negative input?
    Is this the same as the black probe?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Hopefully yours is not, even in a bench meter it is not customary.
    Regardless, if the + & - supply is isolated, it should not happen?
    Max.
     
  4. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    One is a Aglient 34401A
     
  5. rc3po

    Member

    Feb 12, 2014
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    Hi DexterMccoy,
    I'm no expert, but it sounds to me that although you think you are testing for voltage, you are actually inserting the DVM into the circuit and getting current instead. Next time, check for current and see what you get.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    Use a separate ohm meter and see if the negative lead of the bench DVM is connected to earth ground. (measure from the U pin of the power plug to the negative test lead) If the negative lead ties to earth ground AND the supply you are trying to measure has its zero voltage point tied to earth ground also, you are supplying a direct short to the negative 30 supply. Smoke comes out when you do that.
     
  7. BobTPH

    Active Member

    Jun 5, 2013
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    Hmmm. On mine it makes a loud click, starts beeping and flashing a light. I have to hit the reset button to turn it back on. No smoke.

    Bob
     
  8. dubbod

    New Member

    Oct 25, 2011
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    Looking at the attached circuit I do not see any +30 or -30 volt rails so I cannot see what the poster is about. If he is using a DVM acroos the power rails and it "shorts" out the board and "fries it" he has, I would suggest, set the DVM to measure current and that will cause a short circuit. I suggest he needs to be careful about his DVM setting.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    According to the user's manual for the Aglient 34401A, the unit can tolerate up to 500VDC between the chassis ground and the input negative lead.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    As has already been observed it is hard to imagine a voltage measuring device providing a low resistance path between the probes. A voltmeter (DVM) is normally in parallel with the voltage difference you are trying to measure. If it had a low resistance, the measurement would not make much sense because the current draw through would overwhelm the current in the load.
     
  11. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    What is it called when you measure the resistance from chassis ground to earth ground? or from the chassis ground or earth ground from the test fixture to the circuits chassis ground or earth ground? you're measuring the resistance from earth ground to another earth or chassis ground

    This test is to make sure the oscilloscope needs to have an isolated transformer

    If there is resistance between earth grounds or chassis grounds , then there will be a potential difference which will cause damage to the oscilloscope

    Do you guys know what this type of test is called or how to do this test?
     
  12. inwo

    Well-Known Member

    Nov 7, 2013
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    Hipot test comes to mind.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    "Ohmmeter test"?
     
  14. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    but when do you know when an oscilloscope needs an isolation transformer for measurements , so it doesn't damage the O-scope?

    The problem is that I don't know when to use an isolation transformer for a test fixture, circuit board or O-scope

    It it used when there is a ground potential difference between the O-scope and what you will be measuring
     
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    Most scopes have the chassis grounded to earth ground therefore the ground lead of the probe is also tied to earth ground. The isolation transformer would then best be used to power the unit under test. Be aware, however, that many isolation transformers only provide isolation to the power, ie the wire that would normally be tied to neutral is not tied to earth. The U pin on the input lead and the output socket of the isolation transformer are usually connected. Just think through the various possibilities for trouble when making the connections.
     
  16. profbuxton

    Member

    Feb 21, 2014
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    Dexter, I find your questions rather odd. Can you give me some background as to your qualifications and current occupation so any answers may suitably tailored to suit to current knowledge. Do you actually work in a electronics environment or do you just look over someones shoulder and then ask questions. I would be very interested to know just what it is that makes you so interested in pursuing some rather (to me), inane questions.
     
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  17. profbuxton

    Member

    Feb 21, 2014
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    What is is called when you measure resistance from earth ground to chassis ground? Its called (and I will say this slowly) AN EARTH CONTINUITY TEST!!. Why do we do this?
    To check EARTH CONTINUITY!!
    OK now go ahead and ask why we need to do this!!
     
  18. crutschow

    Expert

    Mar 14, 2008
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    Understand that Dexter or one of his aliases will likely continue to ask variations of the same question in different ways over and over until everyone gets tired of answering. :rolleyes: Then he moves on to another question.
     
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