Electronics > Beginners
Nead an insulated probe for oscilloscope
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FriedMule:

--- Quote from: tautech on June 06, 2019, 12:54:14 am ---
--- Quote from: FriedMule on June 06, 2019, 12:30:33 am ---great replyes! :-)

Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?
How would you do that?

--- End quote ---
Why ?
What could you possibly learn from looking at the LV AC ripple from the transformer. We all know it will be at mains frequency.
What matters is the ripple at the PSU output under various states of load and for which you'd just connect across the output terminals (after checking 0V doesn't create ground loop issues with a DMM) and use a 1x probe for the additional sensitivity it offers.
For a low impedance source like a PSU rail the capacitive loading of a 1x probe is never an issue.

--- End quote ---
As Electro Detective says, I would like to have the ability to measure two places in a circuit and then tweak, tinkerer and play. Why, because I think that will be a great way for me to learn what happens in a circuit, before and after, a way to follow the power, from input to output, trough an opamp, logic circuit or what not. :-)
FriedMule:

--- Quote from: David Hess on June 06, 2019, 03:41:36 am ---
--- Quote from: FriedMule on June 05, 2019, 11:48:01 am ---Am I understanding right if I am saying that I can either build a crappy probe that will be of no use anyway, or dig deep in my pocket once for all and get a decent one?
--- End quote ---

That pretty much sums it up.

The old way was to use an isolation amplifier between the probe and oscilloscope:

http://w140.com/tekwiki/wiki/A6902


--- Quote ---But what about that 500X attenuation, would that not eliminate the 2uV noise I may look for in an circuit?
--- End quote ---

A high voltage differential probe will not be useful in that case but there may be other ways if that is your requirement.  An isolated probe will not measure 2 microvolts noise either.

An active differential probe might be able to resolve 2 microvolts under the right circumstances.


--- Quote from: AndersJ on June 05, 2019, 01:01:18 pm ---You will most likely never see 2 uV signals on a oscilloscope, regardless of which probe you use.
--- End quote ---

A Tektronix 7A22 differential amplifier can resolve 2 microvolts over a range of +/-1 volt but only at a bandwidth significantly lower than its rated 1 MHz.  Doing better than this even 40 years later would be a significant challenge but it could be done.

--- End quote ---
TThe 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.
Simply a way to poke around.
David Hess:

--- Quote from: FriedMule on June 06, 2019, 03:49:28 am ---The 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.
--- End quote ---

High voltage probes are available with lower attenuation factors yielding lower noise and that is the modern solution but not particularly inexpensive.  Even a 500x attenuation probe might be acceptable for making a floating measurement of everything interesting across a bridge rectifier.  Dave's probe is x10 and x100 and will certainly work.

Either a Tektronix 7A13 (100 MHz) or 7A22 (1 MHz) could make that measurement easily and I have used them to do it.  But they are not an inexpensive method either.

An isolated probe would also work great for this but today that pretty much means buying an oscilloscope which includes isolated inputs as a feature.

Bob91343 mentioned combining two oscilloscope channels by subtracting one from another and that will work well in this case.  Some oscilloscopes are better at this than others.
FriedMule:

--- Quote from: David Hess on June 06, 2019, 04:08:42 am ---
--- Quote from: FriedMule on June 06, 2019, 03:49:28 am ---The 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.
--- End quote ---

High voltage probes are available with lower attenuation factors yielding lower noise and that is the modern solution but not particularly inexpensive.  Even a 500x attenuation probe might be acceptable for making a floating measurement of everything interesting across a bridge rectifier.  Dave's probe is x10 and x100 and will certainly work.

Either a Tektronix 7A13 (100 MHz) or 7A22 (1 MHz) could make that measurement easily and I have used them to do it.  But they are not an inexpensive method either.

An isolated probe would also work great for this but today that pretty much means buying an oscilloscope which includes isolated inputs as a feature.

Bob91343 mentioned combining two oscilloscope channels by subtracting one from another and that will work well in this case.  Some oscilloscopes are better at this than others.

--- End quote ---
What is bothering me is that I may have to pay more then a beginner scope costs:-)
I my naivity, I had hoped for a simpl-ish solution on a circuit, that could isulate my 1x/10x pasive probes for my noob usage.
I thought that a few mega Hertz and voltage under what the scope is build fore, could be handled by some sort of transformer, optocoupler or something like that, could handle it:-)
David Hess:

--- Quote from: FriedMule on June 06, 2019, 04:52:54 am ---What is bothering me is that I may have to pay more then a beginner scope costs:-)
I my naivity, I had hoped for a simpl-ish solution on a circuit, that could isulate my 1x/10x pasive probes for my noob usage.
I thought that a few mega Hertz and voltage under what the scope is build fore, could be handled by some sort of transformer, optocoupler or something like that, could handle it:-)
--- End quote ---

That is exactly how this works:

http://w140.com/tekwiki/wiki/A6902
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