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

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Vanclair

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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?

- - - Updated - - -

Corrected sorry
VCC is +30volts and VDD is -30 volts
Click to expand...
 

How do you manage to measure two voltages at the same time with one DVM? Please describe your setup for this measurement, and the setting and specification for the DVM at this setting.
 

Gorgon said:
How do you manage to measure two voltages at the same time with one DVM?
Click to expand...
Maybe he just connected them both to the same DVM input at the same time. That would explain the burned PCB as well. A Y-splitter cable would be ideal for instigating this kind of ****-up.

Alternatively he might have hooked it up in such a way that a low value shunt resistor (normally used for measuring high currents) was connected directly between Vcc and 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?
 

Was the meter set to measure current in Amps instead of being set to measure voltages?
Does this bench DVM have its (-) input connected to earth and the power voltages you measured had voltages far from earth?
 

Is Your power supply 0V, Connected to power earth?
Could the same be for the plug in DVM? So when you supplied -30V to the minus lead, this was Connected to power earth too via the DVM? This could explain the burnout of the DVM.
 

Does this bench DVM have its (-) input connected to earth
Click to expand...

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?
 

In short. If both Your DVM and Power supply 0V/(-) are connected to Protective Earth(PE), for some reason. Connecting -30V to the DVM (-) would short that voltage through the DVM to PE, and possibly burn the PCB out.
Some of the mains based instruments I have seen, have had a link that makes it possible to Connect 0V to PE.

You should also know that we are only trying to make hypothetical solutions to Your problem, since we have not seen any pictures or schematics of Your setup or Your Equipment. You have all the answers, we can only ask questions.
 

Does the Bench plug in the wall type DVM meter probes get connected internally to earth ground or chassis ground?

Does the DVM meter + probe and - probe, get connect internally to earth ground or chassis ground of the DVM meter?
 

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