Thanks,
I will try the math method (but remember doing it some years ago and seem to remember it giving a noisy reading?)
Things are strange……
The method that I depict in the pdf of the top post, is a method which worked fine when we first started doing it, but then it started going more wrong gradually over the weeks..
When problems first started, the LED driver would not start up with the scope probe connected to it, but then would start up when we removed the BNC of the scope probe from the scope, then when we connected the scope probe back in, the led driver would stay working…..but now, whenever the scope probe is connected up, the LED driver badly malfunctions. … its input current waveform gets very distorted (we saw this with a TA009 clamp on current probe), and the power goes well above what it should be , (by 100% )
Recently, we replaced the 1W buck controller (in the LED driver product) with a LNK3202 (previously it was a LNK302)..and we wonder if maybe the LNK3202 has a much faster switching transition and is thus more noisy. Maybe this noise is manifesting itself through the coaxial probe in the manner described?
We would use the TA009 clamp on current probe instead, but it does not give as nice images as the DIY coaxial probe method shown in the diagram of the top post. (at least when the coaxial probe method was working)
TA009 current clamp:
https://www.picotech.com/download/manuals/DO086-TA009-CurrentClampUsersGuide.pdf
We also have a TA041 differential probe, but when we connect this across the sense resistors, we get alternate 10ms half sines of different amplitude due to the leakage current in the TA041 diff probe. Also, the TA041 diff probe waveform is more noisy than the DIY coaxial probe method (when that method worked)
TA041 Differential probe:
https://www.picotech.com/download/manuals/ta041-differential-probe-users-guide.pdf
We are thinking that the only way to get good current wavefroms in an offline power supply is to use a really expensive clamp on current probe. Do you agree?
I mean, we even put a heavy differential and common mode filter upstream of the LED driver and this didn’t help at all with the DIY coaxial probe method.
Another strange thing is, that when I scope the 14V internal rail in the LED driver with a x10 scope probe, that doesn’t cause the led driver to malfunction...even though it again involves earthing the part of the led driver circuit to which the scope probe ground connects to.