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From: "Don" <g@crcomp.net>
Newsgroups: sci.electronics.design, sci.physics
Subject: Re: CoB LED filament analysis
Date: Tue, 19 May 2026 03:30:13 -0000 (UTC)
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Don wrote:
> Arie de Muijnck wrote:
>> Don wrote:
>
> <snip>
>
>>> My attempts to appease audience obsession with a series shunt resistor
>>> caused me to flip the scope's input impedance to "1M ohm AC" and
>>> inadvertently leave it at that incorrect current probe setting.
>>> After the scope's set to "1M ohm DC" a flat-line trace appears when
>>> the two half-wave rectifiers are "off:"
>>>
>>>      <https://crcomp.net/ledfilament/curveprobe4.png>
>>>
>>> If that's what Arie means by "required shunt load resistor," then the
>>> original current probe curve also shows a more-or-less flat-line trace:
>>>
>>>      <https://crcomp.net/ledfilament/curveprobe.png>
>>>
>>> And the anomaly again indicates capacitive current curve corruption
>>> caused by the solderless breadboard.
>>
>> No, the phaseshift was caused by the current transformer being unloaded.
>> The perfect load of a CT is a short (e.g. a transimpedance amplifier).
>> The breadboard would not cause a measuremable shift. The scope probe alone is
>> more capacitive.

<snip>

> In the end, I need to pay more attention to the response curves included
> in Fluke's manual. If I remember correctly, in TROUBLESHOOTING ANALOG
> CIRCUITS Bob Pease said he finds graphs more revealing than mathematical
> analysis.
> 
> It's time to end my participation in this thread. In closing, I hope
> this Chinese aphorism is correctly displayed:
>
>    不诱于誉，不恐于诽，率道而行，端然正己。
>    
>    Not tempted by praise, not intimidated by criticism; walking
>    the path of principle, one remains upright and true to oneself.

Now is the time to re-engage with this thread.

A cheap CoB LED filament probably uses a Graetz bridge, constructed
with aluminum wire bonding:

    COB technology is widely used in LED designs, providing 
    higher lumen density and improved thermal management.
    Wire bonding enables compact LED arrays with efficient 
    heat dissipation, leading to brighter, longer-lasting 
    lighting solutions in automotive, industrial, and 
    consumer applications. ...
        Wire bonding remains a crucial technology in modern
    electronics, offering flexibility and cost-efficiency in 
    a variety of applications, including 3D ICs, power 
    electronics, and COB LEDs. While material and 
    manufacturing costs can vary, especially for high-volume
    production, the cost advantages of wire bonding become
    evident as production scales. [1]

In regards to Arie's remarks, the Fluke i1000s is an AC current probe.
In other words, a capacitor couples its current transformer to its
transimpedance amplifier. Perhaps that capacitor is culpable?
    The Fluke i1000s 100 mV/A Users Manual [2] response curve shown in
Figure 2 for a 100 mV/A range indicates a large phase shift is
plausible. Yet all phase shift disappears when a CoB LED bulb is
substituted for a bare metal filament. The breadboard's parasitic
capacitance of 31 pF is negligible.

EUREKA by USA Poet Laureate Edgar Allan Poe, argues that scientific
inquiry begins with an intuitive leap of imagination and more-or-less
ends with measurement. In other words, a hypothesis provides a roadmap
to give direction to measurement.

Here's the first image shared by me in this thread:

    <https://crcomp.net/ledfilament/curveprobe.png>

It shows a color mismatch between the blue tinted voltage trace and the
green tinged current curve. That's a crucial clue.
    My mind mulled this clue during a bicycle spin up a nearby mountain
with Bach playing through the earbuds. Then an intuitive leap of
imagination took place.
    What if neither the breadboard nor the probe's intrinsic parasitic
capacitance causes the phase shift? Is it possible that an idling
transformer mounted near the breadboard causes the anomaly?
    Yes, the idling transformer creates the problem. And a power switch
added to the transformer to power it off when not in use restores the
current curve to zero phase shift.

Note.

[1] <https://resources.altium.com/p/wire-bonding-modern-applications-technology-trends-and-cost-considerations>

[2] <https://assets.fluke.com/manuals/i1000s__umeng0000.pdf>

Danke,

--
73, Don, WD7Q                                             veritas    _|_
                                                          liberabit   |
https://www.qsl.net/wd7q                                  vos         |