Path: csiph.com!eternal-september.org!feeder.eternal-september.org!nntp.eternal-september.org!.POSTED!not-for-mail From: "Don" Newsgroups: sci.electronics.design Subject: Re: CoB LED filament analysis Date: Wed, 22 Apr 2026 01:44:12 -0000 (UTC) Organization: A noiseless patient Spider Lines: 82 Message-ID: <20260421d@crcomp.net> References: <20260411a@crcomp.net> <20260421a@crcomp.net> <3c6fuk16nn9tfhj24q3ulue347ab93jvh8@4ax.com> <20260421b@crcomp.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Wed, 22 Apr 2026 01:44:13 +0000 (UTC) Injection-Info: dont-email.me; posting-host="c7b8b181bc4b6a506454a32ce9f69a73"; logging-data="2037057"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+260zgfQv/kYNr+WGFJXmJ" Cancel-Lock: sha1:3H49D86FJGjSLbpjbchP5ScWfFU= Xref: csiph.com sci.electronics.design:743219 john larkin wrote: > Don wrote: >>john larkin wrote: >>> john larkin wrote: >>>> Don wrote: >>>>>Don wrote: >>>>>> Recent thread topic transitioned to a tentative opinion on a work-in- >>>>>> progress webpage: >>>>>> >>>>>> >>>>>> >>>>>> Radiography imparts interesting insight. Second opinions welcome! >>>>>> The current waveform flopped. Any advice on how to use a Fluke >>>>>> 80i-1000s connected to a Tek 2465B to display the 120 VAC current >>>>>> wave is appreciated in advance. Also, what does "P2 < P1" signify on >>>>>> the current probe? >>>>> >>>>>Alright you guys, a couple of current curve images are now available on >>>>>the webpage. For the present case, in regards to the probe's "P2 < P1" >>>>>orientation decal, P1 designates Line while P2 indicates Neutral. >>>>> The probe provides a sharper, cleaner curve compared to the >>>>>resistor. Both curves illustrate how the silicon filament primarily >>>>>presents itself as a capacitive load. >>>>> The filament's high voltage combined with its low amperage degrades >>>>>resistor curve quality. It took a 10 M ohm resistive current sensor for >>>>>the half-wave rectification effect to become visible. >>>>> The probe curve's mostly blue trace shows the peak-to-peak Line >>>>>voltage. It's used to properly trigger the scope. (Perhaps the scope's >>>>>line trigger accomplishes the same result with less effort?) The probe >>>>>curve's mostly green trace shows the current, with peaks and valleys >>>>>created by half-wave rectification at each end of the filament. >>>>> The current curve is set to 2mV per division. Does anyone know how >>>>>to transpose it to mA? >>>> >>>>It can't be capacitive. It has no mechanism to store energy. >>>> >>>>Unless one of the thingies on the end is a cap. A DC curve would >>>>resolve that. >>>> >>>>A thermal image would be interesting too. >>>> >>> No, it lights up with DC, so there's no series cap. >> >>ELI the ICE man. The trace of the current curve clearly leads the >>voltage trace, so the filament must present a capacitive load. >> A silicon device that presents a capactive load is different from >>a series capacitor: >> >> COB LEDs present capacitive loads through parasitic capacitances >> inherent to their densely packed chip-on-board structure. These >> capacitances arise between closely spaced LED dies, bonding >> wires, and the substrate, affecting driver circuits during >> switching. > > Since the current probe waveform is so different from the one that > uses a resistor, one must be wrong, or likely both. > > Stray capacitances are at least six orders of magnitude from causing > the phase shift that you are seeing. For what it's worth, my own expectations were subverted when the current curve indicated a non-resistive load. So, this thread tries to resolve the discrepancy. In my experience, the 320 Vp-p 1 mA combo crushes the current sensing resistor idea. It took a 10 M-ohm resistor in series with the CoB LED to see anything other than a ~320 Vp-p waveform across the so- called sensing resistor. Connecting a current sensing probe to an oscilloscope is even easier than inserting a resistor in series with a CoB LED. Yet given the unknown meaning of the "P2 > P1" decal on the probe, it's plausible for the probe to be connected backwards. This oscillogram depicts a probe reversal: Danke, -- 73, Don, WD7Q veritas _|_ liberabit | https://www.qsl.net/wd7q vos |