# Measuring AC waveform using oscilloscope

#### mac72

Joined Mar 23, 2013
17
Hi
Sorry for the beginner's level question, but I would be grateful if someone could explain.

I am playing around with RC circuits. I am using an oscilloscope to display the AC waveform across the resistor/capacitor in series. I do not understand why it will only display a waveform when I attach the probe and earth one way, i.e. when i swap the probe and earth lead around no voltage is shown. I expected to see a voltage no whatever way I connected the probe and earth across the circuit because it an AC waveform.

Can someone please explain this basic concept? I really appreciate your help.

Many Thanks
Mac

Joined Apr 16, 2011
581
Sounds like the circuit under test is already earthed - or the power supply to it.

#### Deleted member 115935

Joined Dec 31, 1969
0
The earth probe on the scope is just that, its connected to earth.
when you connect the earth from the scope to your circuit,
you are connecting that part to ground,

Even if your circuit is floating,
ground is a very low impedance path,
which your circuit is not able to drive

#### MrChips

Joined Oct 2, 2009
30,432
You cannot connect the GND clip of the scope probe anywhere you please.
Your scope is at ground potential and so is the GND clip.
Connect the clip in your circuit at a node that you are certain is at ground.

If your circuit is floating then you can connect the clip at any point you choose to be your 0V reference.
Remember, GND is your 0V reference voltage as far as the scope is concerned.

#### mac72

Joined Mar 23, 2013
17
Sounds like the circuit under test is already earthed - or the power supply to it.

Thank you

#### mac72

Joined Mar 23, 2013
17
The earth probe on the scope is just that, its connected to earth.
when you connect the earth from the scope to your circuit,
you are connecting that part to ground,

Even if your circuit is floating,
ground is a very low impedance path,
which your circuit is not able to drive
Thank you

#### mac72

Joined Mar 23, 2013
17
You cannot connect the GND clip of the scope probe anywhere you please.
Your scope is at ground potential and so is the GND clip.
Connect the clip in your circuit at a node that you are certain is at ground.

If your circuit is floating then you can connect the clip at any point you choose to be your 0V reference.
Remember, GND is your 0V reference voltage as far as the scope is concerned.
Ok thank you