# 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.

1. ### 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

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Jan 30, 2014
Corrected sorry
VCC is +30volts and VDD is -30 volts

3. ### Harald KappModeratorModerator

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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

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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. ### davennModerator

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Sep 5, 2009

some sharp and well lit pics would be great

Dave

6. ### Harald KappModeratorModerator

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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

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 KappModeratorModerator

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Nov 17, 2011

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

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 KappModeratorModerator

11,444
2,629
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. ### davennModerator

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Sep 5, 2009
Simmon
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

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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?