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oscilloscope probe cable?
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tggzzz:
Compare and contrast the tone and knowledge displayed in the two adjacent posts copied below. My emphasis added for clarity.


--- Quote from: freda on October 19, 2022, 08:56:22 am ---
--- Quote ---Better to buy a 100x probe with known ratings for safetys sake than cobble something together that could expose you to lethal voltages.

--- End quote ---
:scared: there always has to be someone with the safety trolling......haven't you read the memo forum is for enthusiasts and professionals.

cobbling something out of what you have is the best of fun mostly :popcorn:

I'll give a plug for this utub https://youtu.be/Rl8I4PO66Uw
(Attachment Link)
although just 5x10M for the divider, I have alot more VR25 (1.6kV breakdown) types to use....

--- End quote ---

and


--- Quote from: freda on October 19, 2022, 09:03:25 am ---
--- Quote ---


And, as David Hess has to frequently point out, if you use those with AC coupling then the entire DC voltage appears across the input coupling capacitor. What could possibly go wrong?!

--- End quote ---
you mean....
wtf? maybe it is better to diy......

--- End quote ---

If the UUT and scope warrant it, I strongly suggest spending money on a *100 HV differential probe. It will be cheaper than either replacing a scope/UUT, or your relatives commuting to the hospital to see you.
nctnico:

--- Quote from: tggzzz on October 19, 2022, 07:56:59 am ---
--- Quote from: bdunham7 on October 19, 2022, 01:56:11 am ---
--- Quote from: freda on October 19, 2022, 12:05:46 am ---but i see you can buy 100:1 probe for cheap anyway, still i wonder where they put the divider?

--- End quote ---

There are different varieties, but the 'cheap' ones like the P2301C commonly found online simply use a 99M resistor close to the probe tip and the 1M scope input becomes the bottom of the divider.  So they are x100 and 100M total input impedance, plus capacitance of course.

--- End quote ---

And, as David Hess has to frequently point out, if you use those with AC coupling then the entire DC voltage appears across the input coupling capacitor. What could possibly go wrong?!

--- End quote ---
Not much. Nowadays those coupling capacitors are typically MLCCs which are extremely tolerant to overvoltages. There are some appnotes on that. If the capacitor arcs over, there still is the 99M Ohm resistor in series. Where things get iffy is at high frequencies where the series capacitance of the probe starts to become low impedance.

So for higher frequencies, getting a differential probe is a better choice indeed.
Mechatrommer:

--- Quote from: freda on October 19, 2022, 12:05:46 am ---hopefully in the probe and not at the box with the compensator at the BNC end.
hmm should i diy? have all the bits for making a divider (<1kV). might do both.

--- End quote ---
all probes i know have the divider at the probe tip.. diy is good experience to understand how its done and especially its tips impedance and effect on circuit under probed... you can try with normal (non-resistive) coax first and try to cobble things to get flatter respond, ymmv..

https://how-to.fandom.com/wiki/How_to_make_a_100X_oscilloscope_probe

bdunham7:

--- Quote from: tggzzz on October 19, 2022, 07:56:59 am ---And, as David Hess has to frequently point out, if you use those with AC coupling then the entire DC voltage appears across the input coupling capacitor. What could possibly go wrong?!

--- End quote ---

True for certain scope/probe combinations, but most expensive probes work the same way.  So either you get fairly rare x100/10M probes, use a scope that doesn't have the AC coupling up front or just remember to use DC coupling when measuring things with a large DC offset.  Many well-regarded bench DMMs have the exact same issue in that their AC-coupling input capacitors are only rated to 400V and thus will blow up under the same conditions.
bdunham7:

--- Quote from: nctnico on October 19, 2022, 10:53:36 am ---Where things get iffy is at high frequencies where the series capacitance of the probe starts to become low impedance.

So for higher frequencies, getting a differential probe is a better choice indeed.

--- End quote ---

I'm not sure why single-ended vs differential or even active vs passive really matters.  The total load, both capacitive and resistive, on the DUT would seem to me to be the primary issue.  And, as we're discussing here, if you have a small HF signal on a large DC bias, then 100M resistive seems better than the typical lower resistances of differential probes.  If you have high voltages at high frequencies, then your solution may be more expensive--and I wouldn't assume that inexpensive differential probes solve this problem as they have to be derated too.
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