From: graham foulkes <bespoken1@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 8 Nov 2021 11:57:09 -0800
Subject: Re: [EE]: Sony LED TV repair update KD-60X6700E. 50 backlight LEDs
 in	series.
Thread-Topic: [EE]: Sony LED TV repair update KD-60X6700E. 50 backlight LEDs
 in	series.
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Hi
Long long ago, in the days when incandescent light bulbs  and fluorescent
tubes were manufactured in quantity, all products were tested for
functionality at some point in the manufacturing process. As this was
automated and was a minor addition in manufacturing cost compared to the
total manufacturing cost, it made sense to do it.
  Now, with individual LED lamps costing so little and production
quantities so large, batch testing is probably the way it's done today.
Production variables and multiple manufacturing facilities may account for
the spread in lifetime and failure modes seen in your tests.
Just my two penno'th.
Graham

On Mon, Nov 8, 2021, 9:16 AM Sean Breheny <sean.breheny@gmail.com> wrote:

> Interesting problem!
>
> I suspect you are correct about capacitance causing the flash.
>
> I once worked on a product which used blue LEDs as indicators and it was
> important that they had a consistent brightness (so that they didn't look
> different from each other when all were on, which was the normal state -
> off indicated a problem). We had a problem with these LEDs either failing
> entirely or going dim after some time in service. In the end it turned ou=
t
> to be an ESD problem during assembly but before we knew that, we had a
> question about whether the problem could have been caused by temporary
> overcurrent on the LEDs because we had a problem early-on where we had be=
en
> running them at too high a current before we swapped out the driver board=
 -
> and this had affected hundreds of units in the field so we couldn't just
> preemptively replace the LEDs themselves.
>
> To answer whether the observed misbehavior could have been caused by
> overcurrent, we set up an experiment where we ran 10 LEDs each at 5
> different current levels (from my memory): 100% nominal, 150% nominal, 20=
0%
> nominal, 300% nominal, and 400% nominal. Nominal was the maximum
> recommended continuous current. We continued to run the experiment for
> about a year - even after we had solved the fundamental problem with ESD
> precautions. 100% never failed. 400% all failed completely within hours o=
f
> each other after about 3 or 4 days. But the really interesting thing was
> the tremendous variation in the fate of the intermediate units. Some even
> up to 200% never failed. But some failed open. Some stopped producing lig=
ht
> but still drew current (I don't recall what the voltage drop was). Some
> became dim while still drawing the same current. Some became intermittent
> where wiggling the leads would cause them to flicker (surprising that
> stress could be so easily transmitted through the epoxy body). Some were
> intermittent where no external influence seemed to start or stop the
> intermittency - it was something purely internal.
>
> I will say that I don't think we ever saw one with any type of failure
> which "fixed" itself permanently. Some stopped producing light and then
> started making light again, but there was always intermittency later, too=
..
>
> Sean
>
>
> On Mon, Nov 8, 2021 at 1:55 AM Justin Richards <justin.richards@gmail.com=
>
> wrote:
>
> > Thought this may be interesting or someone may have experienced
> > similar with self repairing LED's
> >
> > Has anyone experienced strange failure modes of LED's where they
> > randomly open circuit but then randomly behave like normal LEDs after
> > perhaps welding the bond wires short again.
> >
> > I initially expected to find shorted LEDs but after doing a complete te=
ar
> > down all the LED's are now operating normally with volt drops ranging
> from
> > 2.98v - 3.45v.  And purposely shorting out LEDs  has no effect.  i.e. T=
he
> > system continues to work fine, the compliance voltage simply drops by 3=
v
> > each time implying the PSU is doing a good job as a const curr source.
> >
> > I think there is a LED or two that has failed open but when hit with a
> > sudden voltage rise from 0 - ~200v welds itself shut then after some
> > heating/cooling cycles it or another one fails.
> >
> > Is that plausible?  Many videos doing similar repair only ever seem to
> find
> > failed LED strips and works fine once they have been replaced.
> >
> > Another interesting observation is after disconnecting one of the strip=
s.
> > As mentioned they are all in series, however, in this mode they briefly
> > flash when powered on which should not be possible.  I am guessing the
> > capacitive effect of the cabling provides for a small current to flow
> with
> > such a high dv/dt.  This brief flash looks very similar to the flash wh=
en
> > the TV fails to power on and hence the reason for my crazy theory.
> >
> > Other info ..
> >
> > The TV worked for a short time with a NTC thermistor in series with the
> > LEDs.
> >
> > As it failed again I have dis-ssembled expecting to find some failed
> LEDs.
> > However, during testing it randomly powered on and has since been rock
> > solid.
> >
> > I have taken many measurements but here are the key parameters while it=
s
> > running ok
> >
> > 50 LEDs all in series with a compliance voltage of 165VDC and a current
> of
> > 540mA.
> >
> > The compliance voltage increases and current is roughly stable at aroun=
d
> > 540mA.taking measurements while increasing a  variable resistor in seri=
es
> > with the LEDs.
> >
> > Justin
> > --
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