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Why does a meter short out when measuring VCC and VDD at the same time

Discussion in 'General Electronics Discussion' started by Simmon, Feb 13, 2014.

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  1. Simmon

    Simmon

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    Jan 30, 2014
    Why does a meter short out when measuring VCC and VDD at the same time
    Why does a plug in bench 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?
     
  2. Simmon

    Simmon

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    Jan 30, 2014
    Corrected sorry
    VCC is +30volts and VDD is -30 volts
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not necessarily so.
    - Can you provide a sketch of your measurementg setup?
    - To what range was the meter set (voltage, current, magnitude)?
     
  4. Simmon

    Simmon

    76
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    Jan 30, 2014
    Voltage

    It's a Bench DVM meter that plugs into the wall

    Can this DVM meter being plug into the wall cause it to be grounded? causing a short from VCC +30 volts to -30 VDD ?

    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?
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    as asked show us your setup

    some sharp and well lit pics would be great

    Dave
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    How is your measurement set up?´How is the meter connected to the different voltages? How are the different voltage sources connected between each other?

    The reason for my questions:
    When you meter is set up for voltage measurement, it should not short the two voltages. Sounds like you put the meter into current measurement mode.
    Another possibility is if there is a ground loop somewhere in your circuit, e.g. caused by a grounded meter. This should not happen, but who knows. Check the manual, for any note reading ground connection of the meter. Use an Ohmmeter to verify the inputs of your dvm are not grounded.
     
  7. Simmon

    Simmon

    76
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    Jan 30, 2014
    How can I do a test like this? grounded to what? and where?

    Turn the DVM meter ON , to voltage mode and measure the ohms from the Red meter probe to what ground?

    I had my red dvm meter probe on VCC +30 volts and my black meter probe on VCC -30

    AC step down transformer , that it's secondary is floating , goes to rectified diodes, caps but has a Plus + 30 vdc and -30 vdc output

    How can a ground loop be caused by a grounded meter?
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Show us a diagram, please.

    Maybe a silly question to ask, but the test leads are put into the correct sockets of the meter? Most meters I know have separate connectors for current and voltage. Setting the meter to voltage mode doesn't help if the lead is set into the current connector.
     
  9. Simmon

    Simmon

    76
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    Jan 30, 2014
    NO power turns on
    Do I disconnect the plug and measure the ohms from the plug ground to the red and black probes?
    But where is my reference? is it the AC plug ground?

    Yes , set to voltage and pluged into the voltage sockets
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Measure the resistance between the dvm's inputs and the connectors of the two voltage sources. No other reference is required.
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    Simmon
    read carefully
    this is your last chance if you dont answer this before saying anything else
    I will lock the thread ... you have been asked multiple times

    show us your setup ... either drawings and or photos

    Dave
     
  12. tryppyr

    tryppyr

    228
    1
    Oct 22, 2013
    Probably an impertinent question... but what brand of meter is it? Wouldn't happen to have come from Harbor Freight, would it?
     
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