oscilloscope probing on mains voltage through transformer - Page 1
Forum section: electronics / general

author: OwaisAhmed
author_rank: Contributor
timestamp: May 23, 2022, 10:16:53 pm
content: can i probe secondary side of transformer on oscilloscope. or does it still count as short through earth to neutral

author: mansaxel
author_rank: Super Contributor
timestamp: May 23, 2022, 10:24:56 pm
content: If it is a full transformer, no problem.The waveform might not be 100% accurate; a transformer has distortion.Do note that a typical Variac and all forms ofAutotransformersnot qualify as safe to do this on; since they're not galvanically isolated.When in doubt, measure resistance (preferably with an insulation tester so test voltage will be closer to working voltage) betweeen primary and secondary. On a transformer not connected to the mains of course.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 24, 2022, 01:16:25 pm
content: One thing you can do is to use a transformer rated for a higher primary voltage (for example, 240V transformer on 120V mains or 380V transformer on 240V mains) so that it would be a lot more linear.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 24, 2022, 05:59:46 pm
content: Especially small transformers (< 5 VA) often rund quite close to saturation and may show quite some distortion. Instead of 1 transformer for a higher voltage one could also use 2 transformers with a similar secondary voltage: primary in series and secondary in parallel.The nominal transformer voltage is with nominal load. The no load output is a bit higher than the nominal voltage ratio.Another alternative way is using the transformer as a current transformer with a relatively large resistor in series to convert mains voltage to current and than back to voltage on the secondary. So something like 100 K in series to the primary and than a rather low resitor (e.g. 10 or 100 Ohms - depends on the transformer ratio, aim for a low output voltage) at the secondary. I have not tested this, but it could work resonable well even with a relatively small transformer.

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2022, 09:46:20 pm
content: Yes, it will work. A step-down transformer is ideal, for low frequencies.The secondary voltage is normally a little higher than the rating. Connect it to the mains, measure the actually primary and secondary voltages and calculate the scaling ratio.Over what bandwidth? I've found small mains transformers to be fairly linear up to a few tens of kHz, with a sharp resonant peak at around 100kHz.

author: bdunham7
author_rank: Super Contributor
timestamp: May 24, 2022, 09:53:58 pm
content: Quote from: Zero999 on May 24, 2022, 09:46:20 pmYeah, 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.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2022, 12:20:21 pm
content: Quote from: bdunham7 on May 24, 2022, 09:53:58 pmI 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2022, 03:57:53 pm
content: Quote from: Zero999 on May 25, 2022, 12:20:21 pmWith 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.

author: OwaisAhmed
author_rank: Contributor
timestamp: May 25, 2022, 04:20:43 pm
content: 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

author: mansaxel
author_rank: Super Contributor
timestamp: May 25, 2022, 04:35:58 pm
content: 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.