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| Better diff probe to avoid inaccurate scopes |
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| gf:
--- Quote from: Weston on March 21, 2023, 10:26:34 pm ---If the DUT is floating and has no ground reference the common mode voltage is poorly defined and could reach some high DC potential. Floating the scope would not necessary fix this, no would a better differential probe. You need to provide some sort of ground return for the DUT to limit the voltage it could float to. --- End quote --- The probe input impedance is "4 MΩ || 5.5 pF each side to ground", according to the datasheet. Isn't the resulting 4M+4M divider sufficient in order that the mean voltage between the two test points becomes the DUT's common mode floating voltage if the DUT itself is fully isolated (say 1GOhm) from ground? |
| Weston:
--- Quote from: gf on March 22, 2023, 09:01:24 am --- The probe input impedance is "4 MΩ || 5.5 pF each side to ground", according to the datasheet. Isn't the resulting 4M+4M divider sufficient in order that the mean voltage between the two test points becomes the DUT's common mode floating voltage if the DUT itself is fully isolated (say 1GOhm) from ground? --- End quote --- Just trying to make the most of Faringdon's babble. I can't think of any other reasons the diff probe would be not giving the expected waveforms, other than the power supply to it browning out. Actually, he should check that too. Is it battery powered? Batteries could be draining over the course of several minutes and distorting the waveforms. The DUT is DC isolated but there are capacitively coupled leakage currents. It's not uncommon to get a bit of a tingle from capacitively coupled voltages on laptop chargers if they use an ungrounded plug. The capacitively coupled voltage can also be rectified and cause a DC offset. I have killed a few devices due to the output of a lab power supply floating to some voltage above ground and then zapping the DUT when connecting a grounded cable. |
| Kean:
--- Quote from: Weston on March 22, 2023, 07:04:11 pm ---Just trying to make the most of Faringdon's babble. --- End quote --- Unfortunately this has proven to be a lost cause time and time again. |
| 2N3055:
--- Quote from: Kean on March 24, 2023, 01:42:19 pm --- --- Quote from: Weston on March 22, 2023, 07:04:11 pm ---Just trying to make the most of Faringdon's babble. --- End quote --- Unfortunately this has proven to be a lost cause time and time again. --- End quote --- Of course it has. I don't know why people even bother anymore.. If you carefully read his first two posts, he claims that passive 100x probe and isolation transformer and TA041 show same waveform.. And both must be wrong. Because he expected to see something and it showed something else... Instead of saying "oh I have oscillation here let me look in to it" he says it is not real. How does he know it is not real? Statement could have been "It shouldn't oscillate like this". Not "it's not real" how does he know "it is not real"? Because he measured twice using two different techniques and got same result... Hmmmm.... Yep, not real, I need better probe. Definition of better here being "one that will show same waveform like Ltspice predicted"..... So it's probe that is not real. LTSpice simulation is... Hence, waste of time... Because this is persistent pattern of behaviour, sadly... |
| tggzzz:
--- Quote from: 2N3055 on March 24, 2023, 02:57:39 pm --- --- Quote from: Kean on March 24, 2023, 01:42:19 pm --- --- Quote from: Weston on March 22, 2023, 07:04:11 pm ---Just trying to make the most of Faringdon's babble. --- End quote --- Unfortunately this has proven to be a lost cause time and time again. --- End quote --- Of course it has. I don't know why people even bother anymore.. If you carefully read his first two posts, he claims that passive 100x probe and isolation transformer and TA041 show same waveform.. And both must be wrong. Because he expected to see something and it showed something else... Instead of saying "oh I have oscillation here let me look in to it" he says it is not real. How does he know it is not real? Statement could have been "It shouldn't oscillate like this". Not "it's not real" how does he know "it is not real"? Because he measured twice using two different techniques and got same result... Hmmmm.... Yep, not real, I need better probe. Definition of better here being "one that will show same waveform like Ltspice predicted"..... So it's probe that is not real. LTSpice simulation is... Hence, waste of time... Because this is persistent pattern of behaviour, sadly... --- End quote --- I think people are getting the idea, and acting (IMHO) appropriately. Here are some of his recent threads with negligible responses... https://www.eevblog.com/forum/manufacture/smps-transformer-hipot-test-without-any-damage-whatsoever/msg4775711/#msg4775711 https://www.eevblog.com/forum/reviews/what-mains-harmonic-class-is-hair-removal-product/msg4773638/#msg4773638 https://www.eevblog.com/forum/renewable-energy/sense-resistor-in-switching-node/msg4772918/#msg4772918 https://www.eevblog.com/forum/renewable-energy/two-current-sense-transformers-reveal-the-current-best/msg4768889/#msg4768889 In order to give people the information that will help them choose whether or not to respond to his posts, I suspect it will be necessary to repeatedly point out his behaviour on new posts that he makes. It is a shame that is the case. |
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