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

SimpleKCLSimpleKVL

Joined Mar 3, 2019
11
In order to measure Vc, why can't we directly connect oscilloscope's probe between the capacitor's terminals ? As seen in the attachment, it is between the capacitor's first terminal and ground. What is the logic behind it ?

Attachments

• 29.2 KB Views: 95

pmd34

Joined Feb 22, 2014
520
If you have a mains operated oscilloscope, then the ground terminal of it is already connected to ground. Connecting it again would short out the capacitor. You would be OK if either your oscilloscope, or circuit was battery powered, with no direct ground connection.

SimpleKCLSimpleKVL

Joined Mar 3, 2019
11

spinnaker

Joined Oct 29, 2009
7,835
If you have a mains operated oscilloscope, then the ground terminal of it is already connected to ground. Connecting it again would short out the capacitor. You would be OK if either your oscilloscope, or circuit was battery powered, with no direct ground connection.
In other words you just answered a homework question for the OP. Good job.

SimpleKCLSimpleKVL

Joined Mar 3, 2019
11
In other words you just answered a homework question for the OP. Good job.
Yes, it was one of the post lab questions.

MrAl

Joined Jun 17, 2014
8,606
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. I can and have measured everything with that and also with other scopes like that long time ago. However, my PC scope has no way to eliminate that AC ground so i cant do with that one that i can do with my stand alone CRT scope.

Some CRT scopes you can either cut the ground pin off of the mains cord or use an adapter to eliminate the ground, which in turn removes the connection between the AC mains ground and the scope chassis. This allows the chassis to "float".
Again though which scopes you can do this with and which you cant brings up some question of what could go wrong if you get one that you can not do this with so you have to proceed with caution. If you use a scope impropely you could blow it out as well as your equipment. Someone i know blew out their PC scope doing this losing over a 100 dollars cost for that scope.

WBahn

Joined Mar 31, 2012
26,398
Some CRT scopes you can either cut the ground pin off of the mains cord or use an adapter to eliminate the ground, which in turn removes the connection between the AC mains ground and the scope chassis. This allows the chassis to "float".
Again though which scopes you can do this with and which you cant brings up some question of what could go wrong if you get one that you can not do this with so you have to proceed with caution. If you use a scope impropely you could blow it out as well as your equipment. Someone i know blew out their PC scope doing this losing over a 100 dollars cost for that scope.
And if you can blow out your $100 PC scope by defeating the safety ground connection, it is also possible to blow out the person operating the scope. Don't go working around ground connections until you know what the hell you are doing. Yaakov Joined Jan 27, 2019 3,955 And if you can blow out your$100 PC scope by defeating the safety ground connection, it is also possible to blow out the person operating the scope.

Don't go working around ground connections until you know what the hell you are doing.
And, my goodness, never "cut the ground pin off of the mains cord", just never do that. Never do that.

MrAl

Joined Jun 17, 2014
8,606
And if you can blow out your \$100 PC scope by defeating the safety ground connection, it is also possible to blow out the person operating the scope.

Don't go working around ground connections until you know what the hell you are doing.
Hi,

Dont go telling people who worked in the power industry for years what they should be and should not be doing. Actually, you'd be telling a lot of people not just one, who do exactly what i described. Just because you didnt do it does not mean that it is not possible or ok to do.
You dont know me, you dont know the techs that worked in the lab. You have no record of the 1000's of measurements made during that time and that was with just one company, yet i do. That's not just with silly single phase either, that's with three phase systems also that have other possible issues to think about.

MrAl

Joined Jun 17, 2014
8,606
And, my goodness, never "cut the ground pin off of the mains cord", just never do that. Never do that.
Hi,

Yeah well you should have worked in at least one company that actually did that and understand how it works.
Not for everybody but when you know what you are doing it works and it always works and it works well.
Now dont go and make me post screen shots of the scope views because they look the same as another other scope views

Yaakov

Joined Jan 27, 2019
3,955
Hi,

Dont go telling people who worked in the power industry for years what they should be and should not be doing. Actually, you'd be telling a lot of people not just one, who do exactly what i described. Just because you didnt do it does not mean that it is not possible or ok to do.
You dont know me, you dont know the techs that worked in the lab. You have no record of the 1000's of measurements made during that time and that was with just one company, yet i do. That's not just with silly single phase either, that's with three phase systems also that have other possible issues to think about.
To be fair, I think he was referring to the TS, not you. In other words, if you know what you are doing, you can act in ways that are too risky fo the ignorant. So, he was suggesting that without a mentor in the room it’s not a good idea to remove safety related things.

Yaakov

Joined Jan 27, 2019
3,955
Hi,

Yeah well you should have worked in at least one company that actually did that and understand how it works.
Not for everybody but when you know what you are doing it works and it always works and it works well.
Now dont go and make me post screen shots of the scope views because they look the same as another other scope views
The reason I said that is the same reason we don’t help with capacitive droppers here in this forum. I have made such cables, but I clearly mark them, and I don’t leave them to be mistakenly used by others.

Without such discipline, they are dangerous, and you already know that while the TS may not.

Using a “ground lifter” is another story. I like the bright orange ones.

Joined Mar 10, 2018
4,057
@Yaakov, I am curious, what are capacitive droppers, and ground lifters, bright orange.

I have not heard of those solutions before, my curiosity is up. I have clues as to what the
first is, but would prefer to hear it from an experienced user.

Regards, Dana.

MrAl

Joined Jun 17, 2014
8,606
The reason I said that is the same reason we don’t help with capacitive droppers here in this forum. I have made such cables, but I clearly mark them, and I don’t leave them to be mistakenly used by others.

Without such discipline, they are dangerous, and you already know that while the TS may not.

Using a “ground lifter” is another story. I like the bright orange ones.
Hi,

Oh ok that's very understandable, but i thought i made that clear. No worries though.

Oh yeah and just FYI and others, when i bought my CRT scope i bought it second hand from a long time tech guy and guess what?
Yeah, the ground pin was already cut off so i did not have to cut it
Kind of ironic right?

Sorry if i got a little huffy about this issue but i think it is just some misunderstanding about these kinds of issues, and yes same as with the "transformerless" power supplies which are used all the time but not recommended for beginners. I think as long as we stress the danger it is good. In fact, if we dont talk about this stuff they will never know and that's even worse 'cause they might not realize the dangers yet.
And then of course we cant always be grandmothers
On one site a while back i was talking about the dangers of those power supplies and someone called me an "old Jewish grandmother" ha ha.

Yaakov

Joined Jan 27, 2019
3,955
@Yaakov, I am curious, what are capacitive droppers, and ground lifters, bright orange.

I have not heard of those solutions before, my curiosity is up. I have clues as to what the
first is, but would prefer to hear it from an experienced user.
I
Capacitive droppers are power supplies that use a large capacitor directly on the mains to drop the voltage, followed by a rectifier to produce DC. They are used in cheap LED lamps, for example. They are dangerous because they are not isolated from the mains, and there is an AAC policy that no help is given concerning them. They are not relevant to the grounding issues, just to the safety question.

A "ground lifter" is actually an adapter used on American type AC mains plugs for cases where a receptacle does not have a three pin connector. It accepts a there pin (grounded) plug and provides two pins (live and neutral) and a ring terminal arranged so it can be fixed using the outlet's plate screw on the theory that the receptacle frame is grounded.

It gets the name "ground lifter" from sound techs who use them (with the ring terminal removed) to eliminate ground loops, and so AC hum, when plugging in interconnected amplifiers and sources on stage. It's not supposed to eliminate the ground, but in practice the ring terminal is rarely used even when there is not intention to "lift the ground".

I like the bright orange ones because they stand out and remind you that they are in use.

Yaakov

Joined Jan 27, 2019
3,955
Hi,

Oh ok that's very understandable, but i thought i made that clear. No worries though.

Oh yeah and just FYI and others, when i bought my CRT scope i bought it second hand from a long time tech guy and guess what?
Yeah, the ground pin was already cut off so i did not have to cut it
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.

WBahn

Joined Mar 31, 2012
26,398
Hi,

Dont go telling people who worked in the power industry for years what they should be and should not be doing. Actually, you'd be telling a lot of people not just one, who do exactly what i described. Just because you didnt do it does not mean that it is not possible or ok to do.
You dont know me, you dont know the techs that worked in the lab. You have no record of the 1000's of measurements made during that time and that was with just one company, yet i do. That's not just with silly single phase either, that's with three phase systems also that have other possible issues to think about.
Well, let's see here. I said don't do it unless you know what they hell you are doing. Do you know what the hell you are doing? If so, then that statement doesn't apply to you, now does it? But does the TS know what the hell they are doing?

I guess just because YOU knew what you were doing, that means that every newbie knows what they are doing and can just go cutting off the ground pins on cords without a thought in the world. Never mind the evidence that YOU yourself presented that doing so can lead to problems.

MrAl

Joined Jun 17, 2014
8,606
Well, let's see here. I said don't do it unless you know what they hell you are doing. Do you know what the hell you are doing? If so, then that statement doesn't apply to you, now does it? But does the TS know what the hell they are doing?

I guess just because YOU knew what you were doing, that means that every newbie knows what they are doing and can just go cutting off the ground pins on cords without a thought in the world. Never mind the evidence that YOU yourself presented that doing so can lead to problems.
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.