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Old 02-13-2021, 06:21 PM   #1
EasyGoing1
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Default General question about oscilliscopes

What is it about scopes, that make them unique from multimeters? I ask that for two reasons based on my own experience with the last scope I had.

1) I learned that getting a scope to read current is apparently not a trivial thing and you need some fairly expensive probes to be able to do it.

2) When I tried to use the ground clip on the scope probe to read the signal over a single resistor by putting the ground lead on one side then the probe on the other side of the resistor, it immediately gave me some kind of a warning about over voltage I think ... I didn't leave the message up long enough I just saw the red and disconnected immediately... but I figured that the problem was that the ground lead has to be connected to a point on the circuit where there is no voltage at all and if you don't then it's all bad, but I don't understand why?
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Old 02-13-2021, 07:05 PM   #2
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Default Re: General question about oscilliscopes

the scope is grounded,
if what your testing is also grounded then you will get a voltage difference.

a questionable trick is to forget the ground clips and put one probe on each side of the resistor.
then use the math function to display A-B

btw, always keep the probes on the x10 setting.

to answer the first question, a meter just shows voltage or current.
a scope shows it over time - like going from 2d to 3d
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Old 02-13-2021, 10:47 PM   #3
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Default Re: General question about oscilliscopes

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Originally Posted by stj View Post
the scope is grounded,
if what your testing is also grounded then you will get a voltage difference.

a questionable trick is to forget the ground clips and put one probe on each side of the resistor.
then use the math function to display A-B
I'm assuming you mean that you leave the ground clips connected to ground, then use two channels of the scope, one on each side of the resistor, then let the math function tell you the difference between the two nodes?

Why is this trick "questionable"? is it somehow potentially unsafe?

Quote:
Originally Posted by stj View Post
btw, always keep the probes on the x10 setting.

to answer the first question, a meter just shows voltage or current.
a scope shows it over time - like going from 2d to 3d
Right, but it seems that getting a scope to inject itself into a circuit as you can do with a multimeter .. is not something that scopes are really designed to do where as with a multimeter, you switch the hot lead over to the current port and then the meter is inserted into the circuit ... not something you can do - apparently - with a scope ... for some reason ...
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Old 02-14-2021, 12:58 AM   #4
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Default Re: General question about oscilliscopes

Quote:
Originally Posted by EasyGoing1 View Post
I'm assuming you mean that you leave the ground clips connected to ground, then use two channels of the scope, one on each side of the resistor, then let the math function tell you the difference between the two nodes?

Why is this trick "questionable"? is it somehow potentially unsafe?
No you don't connect the ground leads at all, it is not really questionable.
But there are still voltage ratings that must be respected in this case too.
A good primer on this is the following EEVBlog video:
EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

Quote:
Originally Posted by EasyGoing1 View Post
Right, but it seems that getting a scope to inject itself into a circuit as you can do with a multimeter .. is not something that scopes are really designed to do where as with a multimeter, you switch the hot lead over to the current port and then the meter is inserted into the circuit ... not something you can do - apparently - with a scope ... for some reason ...
Yes this is true, it is due to the way a scope works, it is very very different to a multimeter, they have different uses.
The following video might give you an idea about the probes of a scope and what they really are:
EEVblog #453 - Mysteries of x1 Oscilloscope Probes Revealed
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Old 02-14-2021, 04:21 AM   #5
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Default Re: General question about oscilliscopes

some usefull shit is here:
https://www.youtube.com/user/Keysigh...loscope/videos

https://www.youtube.com/user/RigolTech/videos
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Old 02-14-2021, 03:18 PM   #6
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Default Re: General question about oscilliscopes

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Well that's 30 minutes of my life I'll never get back...

I normally love that guy's videos but he really drug this one out.

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Old 02-14-2021, 03:32 PM   #7
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Default Re: General question about oscilliscopes

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Originally Posted by Per Hansson View Post
A good primer on this is the following EEVBlog video:
EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
YES, that video explained it perfectly. I had no idea that the ground lead on a scope was connected to earth ground. Of course in the case of a USB based software use only scope, it would be connected to the DC ground of the computer and not the earth but that being the case, connecting it anywhere else in the circuit that has voltage means you're shorting the computers ground with that voltage ... bad juju
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Old 02-14-2021, 04:09 PM   #8
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Default Re: General question about oscilliscopes

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Of course in the case of a USB based software use only scope, it would be connected to the DC ground of the computer and not the earth
wrong.
unless your using a laptop with no charger or other connection to a seperate mains powered device.

laptop chargers capacitivly couple the output earth to the neutral input and ground - if it has one.
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Old 02-14-2021, 05:04 PM   #9
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Default Re: General question about oscilliscopes

OK, so if you were going to use a scope to look at the AC signal coming from an outlet, would you just leave the ground clip floating?
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Old 02-14-2021, 05:48 PM   #10
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Default Re: General question about oscilliscopes

yes,
the scope is already connected to mains ground.

that's dangerous btw incase of a surge.
if you want to monitor mains you can use an old transformer to drop the voltage and still see the waveform.

infact to see how to use a scope on risky voltages - go watch some electroboom video's
you can also watch AC current without an electrical connection by wraping the ground clip around a wire a couple of turns and then clipping it to the probe tip - to create and inductive coupling.

Last edited by stj; 02-14-2021 at 05:52 PM..
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Old 02-14-2021, 11:03 PM   #11
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Default Re: General question about oscilliscopes

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you can also watch AC current without an electrical connection by wraping the ground clip around a wire a couple of turns and then clipping it to the probe tip - to create and inductive coupling.
... sounds kind-a Mc Guiverie .... ish ....
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Old 02-15-2021, 03:46 AM   #12
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Default Re: General question about oscilliscopes

it works
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Old 02-15-2021, 01:48 PM   #13
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Default Re: General question about oscilliscopes

Quote:
Originally Posted by EasyGoing1 View Post
Well that's 30 minutes of my life I'll never get back...

I normally love that guy's videos but he really drug this one out.

Agreed, but this one and the other is very useful to keep in the back of your mind when you work with a scope.
Both of these can otherwise make you come-a-gutsa
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