Questions about the oscilloscope

[fluidistic]

Hi,
I'm very stressed because I've a lab exam tomorrow. I wasn't aware I'd have to take it so I'm more than late when it comes to study for it. Furthermore I should be used to the oscilloscope while I had about 8 opportunities to "play" with it. Hence I have some questions :
I've dealt with a digital oscilloscope, a tektronics 1000B I believe. It has 2 channels.
Each probe have 2 crocodile clips. My first question is : Why when I use both probes (that I plug into both channels of the oscilloscope), I must not use the 4 crocodile clips? It could damage the oscilloscope, but how and why? I can use 3 crocodile clips with no problem, but my professor told us never to use the 4 of them.
My second question is :
Can someone explain me what is the connection to the ground? Why is it important? How can I do this with an oscilloscope? I never understood this part.
Thank you very much!

 

[mgb_phys]

Are these two croc clips in addition to the normal scope probe clip?
Or have they just replaced the normal scope probe tip with a clip to stop students breaking as many?

I've only ever seen this on a high voltage differential add-on

edit - assuming this is a normal scope setup. The clips on each probe are connected to the scope ground (because the measurement is made between the probe tip and ground) if you connect the ground clips from both channels to the circuit there are two possibilities.
Either you connect both of them to the ground in the circuit - in which case it's a waste of time, except for introducing some interference.
Or you accidentally connect the clips to different potentials in the circuit, which will cause a short an could lead to lots of current flowing into the scope.
This shouldn't damage the scope unless you are working on very high voltage circuits but might damage the clip.
It's likely to damage the circuit under test - or at it's power supply.

 

[fluidistic]

Thanks for the reply.
I'm still a bit confused about what is the ground. I've in mind AC circuits and never learned about what does the ground. I guess I should have learned this using the oscilloscope, but it didn't work with me.
About the probes : There was cables that fit into the channels 1 and 2 of the oscilloscope. On the other extremities of the cables there were those crocodile clips. Are these cables considered as probes? If not, then sorry for having said probes.
We clipped them into different positions in RLC circuits for example. So that we could see the phase difference between the resistor and the capacitor for example.

I don't even remember how to plug them into the circuit. It means we've plugged (in fact my friend, that's why I'm so lost now) only one clip of 1 probe, and 2 clips of the other probe, but I don't recall where and how.
Also, I have no idea when I have to invert the signal. I know that sometimes the signal comes reversed or so, I've no idea why. There's an option (I don't remember where) in the oscilloscope to reverse the signal and get rid of the problem.
I feel so lost!

 

[mgb_phys]

http://upload.wikimedia.org/wikipedia/en/thumb/1/11/Test_probe.jpg/250px-Test_probe.jpg
a scope probe

The croc clip connects to the ground on the scope - and ultimately back to the Earth pin on the plug.
The pointy tip is the signal. There are some very special setups that would have two croc clips as well as the pointy tip.

But I'm guessing for a teaching lab they have just make up some leads with a pair of croc clips, one ground and one signal to stop students breaking expensive scope probes.

http://t3.gstatic.com/images?q=tbn:WaSDbS-A-nMOOM:http://filedb.experts-exchange.com/incoming/2009/03_w10/t113363/Photo0029.jpg

 

[mgb_phys]

the tricky bit about scopes for beginners is that just like a multi-meter you are measuring a voltage between two points,
BUT one of the points is also earthed by the scope (except for some tricks with battery powered portables scopes) So you arent free to just connect the scope across any part of the circuit - because you would be grounding one point.
In a battery powered circuit this is Ok (or one driven from a floating PSU=a battery) you can chose any point you want to be zero volts.
But if the circuit is powered from a wall socket then it is likely to have an Earth point linked to the Earth in the power supply, which is ultimately linked to the Earth in the scope and so the croc clip - so you need to connect the Earth croc clip to the ground point on the circuit an make all measuremnts relative to that

The other tricky point with a dual channel scope (or even two separate scopes) is that both croc clips are grounded and so implicityl connected together, so even on a battery powered circuit you can't connect them to different points (ie points with a voltage difference between them) the safest thing to do is to only connect one clip to some point which is either zero, or you are declaring to be zero, an make measurements relative to this.

ps. don't worry if you are confused. I demonstrated this to one class even going so far as to take the cover off a scope to shw that the two probe grounds were connected and went to the Earth pin. The student went 'oh i see" - and replaced the dual scope with two separate scopes and plugged them into different power sockets on the bench - so the Earth's would no longer be connected.

 

[fluidistic]

Ok thank you very much for the detailed explanation!
I don't understand everything because English is not my mother tongue, but I get a vague idea I believe.

If I do bad on the tomorrow's exam, it won't affect my GPA, however the next one yes. I don't know why I'm so stressed. I think it's because I had 8 labs with the oscilloscope and I feel I didn't learn almost anything and my friend was doing almost all the job. Now I will make a fool of myself in front of the professor.

 

[mgb_phys]

They are a tool you need practice with to become comfortable.
But;
Then they go an change the position of all the controls on the new model.
Then they make digital ones were you have to hunt through a dozen menus to find a setting that used to be on a knob and you knew where it was.
Now they make scopes that are running windows - you have to wait for them to boot up and then they crash.

 

[waht]

youtube oscilloscope tutorial:

http://www.youtube.com/watch?v=hUIgAu3QQWQ&feature=related

 

[fluidistic]

Thanks for the tutorial. I've watched it today, although without sound. I've watched maybe 6 or 7, I'm still very stressed.

lol at windows on oscilloscopes.
Edit: Tomorrow I'll post how I did on the exam. I'm sure my head will get cut.

 

[fluidistic]

As I said, I post the result of my exam : I did well! Incredibly but true. I got extremely lucky thanks to a friend that passed it hours before. He explained all to me. Without him I was indeed lost. Wow.