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oscilloscope probing on mains voltage through transformer

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bdunham7:

--- Quote from: Zero999 on May 24, 2022, 09:46:20 pm --- with a sharp resonant peak at around 100kHz.

--- End quote ---

Yeah, that's a detail not to be overlooked when trying to scope out the mains.  This will greatly exaggerate spikes and noise in some cases.  I tried using the 1:1 line-trigger transformer from an old Tek scope to make a sort of isolated mains probe and it really amplified certain diode switching noises.  I checked it with an AWG and between the transformer and the 18pF of probe capacitance, it had an apparent Q of 53--1V input for 53V ouput--at 204kHz.  The line trigger circuit in the original scope had a lot of filtering and I guess that's why.

Zero999:

--- Quote from: bdunham7 on May 24, 2022, 09:53:58 pm ---
--- Quote from: Zero999 on May 24, 2022, 09:46:20 pm --- with a sharp resonant peak at around 100kHz.

--- End quote ---

Yeah, that's a detail not to be overlooked when trying to scope out the mains.  This will greatly exaggerate spikes and noise in some cases.  I tried using the 1:1 line-trigger transformer from an old Tek scope to make a sort of isolated mains probe and it really amplified certain diode switching noises.  I checked it with an AWG and between the transformer and the 18pF of probe capacitance, it had an apparent Q of 53--1V input for 53V ouput--at 204kHz.  The line trigger circuit in the original scope had a lot of filtering and I guess that's why.

--- End quote ---
I believe it's because the transformer in itself has certain resonances, due to its physical construction. The higher voltage winding will probably have the lowest resonance mode because it's physically longer so will have a high inductance and parasitic capacitance. I expect larger transformers, will have lower resonant frequencies.

Kleinstein:

--- Quote from: Zero999 on May 25, 2022, 12:20:21 pm ---
--- Quote from: bdunham7 on May 24, 2022, 09:53:58 pm ---
--- Quote from: Zero999 on May 24, 2022, 09:46:20 pm --- with a sharp resonant peak at around 100kHz.

--- End quote ---

Yeah, that's a detail not to be overlooked when trying to scope out the mains.  This will greatly exaggerate spikes and noise in some cases.  I tried using the 1:1 line-trigger transformer from an old Tek scope to make a sort of isolated mains probe and it really amplified certain diode switching noises.  I checked it with an AWG and between the transformer and the 18pF of probe capacitance, it had an apparent Q of 53--1V input for 53V ouput--at 204kHz.  The line trigger circuit in the original scope had a lot of filtering and I guess that's why.

--- End quote ---
I believe it's because the transformer in itself has certain resonances, due to its physical construction. The higher voltage winding will probably have the lowest resonance mode because it's physically longer so will have a high inductance and parasitic capacitance. I expect larger transformers, will have lower resonant frequencies.

--- End quote ---

With a transformer there is not really a separate resonance for the primary and secondary. At least for the lower frequencies they are reasonable well coupled and there would be only a common resonance. A larger transformer would have more capacitance, but also less inductance. Chances are there is not much difference / general trend. This of cause does not apply to the mechanical resonance of the transformer core than can also be electrical visible. This one can be quite high Q, but mechanical damping from potting is possible.

OwaisAhmed:
i just want to use 230v to 15.2vac transformer rated for around 6 to 7ampere on secondary is it safe to do it or not.i don't know much about resonance and noise but since ii got oscilloscope im gonna find out what actually this is. and thanks for all the reply and help

mansaxel:
It probably is quite safe, providing the transformer is not an autotransformer. My hunch is that it is not, because the 220/15V ratio is favourable to full transformer constructions.

A practical test for this is:

Measure resistance between and on the windings. The primary side should measure perhaps 100Ω or so, the secondary much less. Several megaohms should be minimum reading between any primary and any secondary connections.  If these conditions are true, it is safe.

The caveats about distortion et c still apply of course.

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