Probe oscilloscope ground

[ivenzar]

Hello,

I have a doubt with NO differential probes.

When you plug in an oscilloscope, it is referenced to the "building ground" (50Hz sinusoidal ref).
If I measure the output of a DC galvanic isolated convert (example: Flyback), I must connect the GND of the probe to a different potential (It is not the same ground as the building ground).
The same event occurs when you measure a battery. Battery ground is different from the oscilloscope ground, which is connected to the building net.
When I connect the ground of the probe to a different reference ground I see an small increase of the current comsumption .
Is there a rupture limit that an oscilloscope can support with diferent potential grounds?
Why happends this?

Greetings!

 

[FvM]

When I connect the ground of the probe to a different reference ground I see an small increase of the current consumption.

Which current consumption?

There should be no problem if the probed circuit is actually isolated. A SMPS has usually "Y" capacitors between primary and secondary, a 50 Hz and possible switch frequent current can flow through the probe ground.

Isolation failure or mistaken probe connection can however damage the oscilloscope + device under test. Safety rules might impose the usage of an additional safety transformer for the mains connection. I would also prefer a differential high voltage probe in similar situations.

 

[AJBotha]

Hi

I am not sure if I understand your question 100%, but yes, Ground potentials differ.

You always connect the Ground Crocodile clamp of your oscilloscope probe to the same circuit where you want to measure signals.

If you want to measure the voltage of a battery, you would connect the GND clip to the negative terminal.
If you want to measure something that is powered by a bench supply, you connect the GND clip to the negative terminal eg.

If you have a dual channel supply, the GND potentials may also differ,
so tie them together if you want to measure signals connected to both of the outputs.

The same with interfacing boards, if two boards are powered from two different supplies,
tie the GND connections if you want to measure signals between them.

Hope it helps?
AJBotha