CoB LED filament analysis

The question is, what do you think you see when you look at:

Danke,

Forget about all the above stupid questions and be honest. What did you expect a current curve to reveal about a device that draws mA from 320 ACVp-p?

It takes a fool to follow foolish advice. Fool me once, shame on you. Fool me twice, shame on me.

Danke,

What's inside?

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Very bright flash?

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Fair enough. Given a rock solid resitive load with a phase shift of zero, then there's something wrong with the phase shown by the current probe. It's my hunch that the waveform itself shows something useful.

I'll repeat the experiment with a 100 ohm current sense series resister in case something was overlooked the first time.

Danke,

There's something wrong with your measurements. I'd like to see a schematic of your setup. And what's the load impedance on your current probe? Is its frequency response even good down to 50 Hz?

The voltage across a 100 Ohm current sense resistor passing a mA should be 100 mV, simple Ohm's law. If you see 320V, there's something wrong.

Jeroen Belleman

There's not a lot of intellectual brilliance on show in this thread.

This thread turned too treacherous. It's time for a brain teaser to get things back on track. Put on your quantitative instincts hat and take a good long look at all of the images shown at:

Now put on your thinking cap. There's something glaringly wrong (so to speak) with the experiment. It may explain the phase difference discrepancy. Stay tuned.

Danke,

Alright you guys, this thread's fun again! There's yet another intuitively obvious source of error for those with quantitative instinctive eyes to see. And it has nothing whatsoever to do with my

Danke,

Nailed it! The expected curves now appear. Yet, my original current curves are flawless. Filament faffing finally pays off. Uninhibited brainstorming, no matter how insane it is in the interim, imparts insight. Next clue: Bob Pease.

Danke,

You're wrong, no doubt about it. Stray capacitance caused my problem. And your sweeping generality at this juncture led me to question my correct quantitative instincts.

"ELI the ICE man. The trace of the current curve clearly leads the voltage trace" leads to the correct solution.

Hint: stray capacitance originates from more than one source.

Danke,

Probably the biggest capacitance is the LEDs themselves.

If you used a sensible value current shunt resistor - ohms and not megohms - the effects of the capacitance would be invisible with 60 Hz excitation.

Measure the capacitance and do the math. Or Spice it.

"ELI the ICE man" is not quantitative.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Or the rectifier diodes. But still not much c.

Measure it.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

OK, We'll do it your quantitative way. Stray capacitance is at least six orders of magnitude higher than what you imagine. Because the biggest capacitance does not originate with the filament.

There's no need to goof with a shunt resistor when an accurate current probe measurement is already available:

It shows you everything you need to do your own math. Show me your math and I'll included on my webpage, with your permission.

Hint for people who still don't see the light (so to speak): Bob Pease said, "My favorite programming language is solder." On a related note, Pease passionately hated one piece in particular, amid the equipment found in an electronic lab.

Danke,

The LED string does not begin to conduct until the peak voltage has been reached. Then the current increases abruptly and the voltage collapses due to a relatively high source impedance. The string continues to conduct even as the voltage is collapsing. The voltage collapse is not immediate because there is a significant capacitance across the LED string after the source resistance (which may be the current sense resistor).

John

Umm, no.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Larkin's correct about one thing. The filament's intrinsic capacitance does not play a role in the observed phase shift. When the last paragraph says "expected curves" it means current curves with zero phase shift relative to voltage. The voltage and current relationship is totally resistive, as expected.

While perusing TROUBLESHOOTING ANALOG CIRCUITS by Bob Pease for my next hint, it occured to me how Larkin is the anti-Pease. And it goes a long way in explaining Larkin's frequent battles with the late, great Jim Thompson.

Danke,

My "battles" with JT were sporting.

Pease lived near me, on Miramar Av. I met him several times. He was friendly and sold his and Jim Williams' books out of his house.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Jim, may God hold him in memory eternal, repeatedly impugned the moral character of John’s delightful wife, as well as John’s own.

Something about having been in a sorority at Boston University, as I recall. (I guess in JT’s era they wouldn’t date MIT guys, or something.)

Jim occasionally tried to pull me into it, because John and I are friends. A fine engineer, a flawed man. So many of us are like that.

Another time, iirc, John refused a job to Bill Sloman.

The echoes are still being heard.

Cheers

Phil Hobbs