Voltage meters and Oscilloscopes

[elnaldo]

Be careful when you connect an oscilloscope. The "ground" end of the probe is sometimes connected to the ground plug of the main AC plug, and in my dual trace scope, both "ground" are interconnected, so you can't attach the ground clip to any point. A volt meter is different, you can measure voltages across any points. But the scope ground shouldn't be connected to any point.

 

[slateef]

elnaldo, that's good info to know. So, to clarify, let's say for instance I'm wanting to send a sine wave to the aux input and then look at the output on both the right and left channel. I'd hook up the hook clips of the oscilloscope to the right and left speaker outputs and then just leave the ground clips alone? Could you please give me a real-world example, using just one oscilloscope probe and then with both? Thanks!

 

[elnaldo]

The ground clip can go to amp chassis. But, for example, you shouldn't connect the ground clip to the positive speaker terminal and the tip to the negative. Or, you shouldn't connect the ground clip to a +50v point without some extra caution (If you connect one ground clip to a high voltage and the 2nd probe of a dual scope is grounded to chassis, you will short the voltage to chassis through the 2nd probe, at least with my scope) A voltmeter, instead, can be connected any way.

Connecting the ground clip to chassis or ground potential is OK most of the time. In some circuits with separate grounds you could be interconnecting 2 grounds that should be disconnected.

I'd say you must get familiar with your scope and read the manual.

 

[NAD80]

Slateef, you could "float" the scope in relation to the AC line. By using a 2 pin AC adapter designed for old 2 wire wall sockets (wire type). Remove the ground connection ( green wire) from the plug adapter. This way the scope is floating and no ground connection is made. Then you can't connect one side of the AC line to ground. Sometimes it is the only way to use the scope.

 

[elnaldo]

+1, I use my scope with a 2 pin adapter.

Anyway you need to understand that both probes ground clips have continuity. At least in my unit.

 

[NAD80]

Have to float the meters and scopes at work. Non isolated AC controllers are part of what I test at work.

 

[I LIKE MUSIC]

Post #8 in this thread is a good example.

If you have a dual channel oscilloscope with channel add and invert you can use both scope probes (the tips) ungrounded as a form of differential input. This is not necessarily the best, but if you need or want to look a signal that is not referenced to ground this may work okay.

Remember if you defeat the safety ground on an oscilloscope that may, depending on the application, cause any of the exposed metal parts of the oscilloscope to have potentially dangers voltages on them.

Depending on the application, proper grounding may be important to reduce unwanted noise in the measured signal.

 

[elnaldo]

Post #8 in this thread is a good example.

Yes, the guy at the video covered and explained all the possible dangers and how to connect the ground probe safely.
I wish I had seen that video before...