In RLC circuit, why can't we directly connect oscilloscope's probe between the capacitor's terminals

MrAl

Joined Jun 17, 2014
8,608
There are times when having a ground becomes a hazard. Probing around in SMPSs with a scope can cause some very unexpected results, for example. One hopes the protection circuitry in the scope will help prevent death-like results for cases where the ground becomes a path for more than you bargained for. It's not always a choice between the earth connection and the person. In general though, and overwhelmingly, proper grounding of test equipment is a better thing to do out of ignorance than removing a ground.
Hi,

Yeah agreed, and we cant do this with any of the PC scope that i know of so i guess that's a time when it's definitely out.
 

WBahn

Joined Mar 31, 2012
26,398
Hi,

Oh my apologies i misinterpreted your intent there. Really sorry. I understand your point now and i can assure you it is well taken.
No problem. I could have phrased it better to reduce the chance for misinterpretation, too.
 

WBahn

Joined Mar 31, 2012
26,398
There are times when having a ground becomes a hazard. Probing around in SMPSs with a scope can cause some very unexpected results, for example. One hopes the protection circuitry in the scope will help prevent death-like results for cases where the ground becomes a path for more than you bargained for. It's not always a choice between the earth connection and the person. In general though, and overwhelmingly, proper grounding of test equipment is a better thing to do out of ignorance than removing a ground.
Agreed. Before defeating the ground, the first step is to understand your equipment grounding "as is" and understand its effects on your measurements. There are several practical considerations you can take to improve (or make possible) your measurements without defeating the ground. Once you understand that, then you are in a much better position to make informed decisions about when and how to safely defeat the intrinsic grounding scheme to make otherwise impossible measurements (which are actually relatively few, but which certainly exist).
 

vanderghast

Joined Jun 14, 2018
57
Hello there,

Not every scope is like that. However, it's a better rule of thumb to believe that they ARE all like that than to believe that they are NOT.

For example, my CRT scope chassis is NOT connected to mains AC ground. (…) This allows the chassis to "float".

Even with a floating scope, and a floating circuit, the "ground" part of the probes are (generally) connected together through their connection to the scope itself (which can be check with a multimeter through a continuity check), so, the two probes having their "ground" at different places on the circuit , say at point A and point B, will, at best, bypass all the components of the circuits between A and B as if you were inserting a zero ohm resistance linking A and B.
 

MrAl

Joined Jun 17, 2014
8,608
Even with a floating scope, and a floating circuit, the "ground" part of the probes are (generally) connected together through their connection to the scope itself (which can be check with a multimeter through a continuity check), so, the two probes having their "ground" at different places on the circuit , say at point A and point B, will, at best, bypass all the components of the circuits between A and B as if you were inserting a zero ohm resistance linking A and B.
Hi,

Of course you can not place two grounds at two different points in the circuit when the two are connected together. You cant even do that sometimes when the circuit itself has a common ground for those two same points because then you bypass the dynamics of the ground circuit in the PC board or wiring.

A good example of this is when trying to measure the current in a capacitor (or other element) with two probes. Let's say we want to measure the current in two resistors that are connected in series, and we know the top resistor is 1 Ohm but dont know the other resistor value yet or want to make sure it is really say 10 Ohms. How do we connect the probes?
We can connect one probe across the 1 Ohm resistor and that givers us a voltage that is proportional to current. We can connect the other probe across the lower resistor and that gives us the voltage across that second resistor. So now we have the voltage and current in the bottom resistor. But we cant just connect the ground of one probe to the center tap and the other probe ground to the bottom of the lower resistor because that would short out the lower resistor. So what to do? We connect the ground of BOTH probes to the center tap of the two resistors, the common node for both, and then place the probe tips on the opposite ends of each resistor. That gives us the current but not the voltage because the voltage is inverted. So to remedy that we invert the 2nd channel. Now we see both current and voltage on the scope.
 
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