From: Justin Richards <justin.richards@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 8 Nov 2021 16:43:29 -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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Sean,

Interesting results from your experiment,  It adds a little weight to my
suspicions particularly that they heal but not permanently. The failure
within hours of each other is also curious.

Its $50 to find out and in the meantime I am going to setup a bench PSU to
continuously cycle on/off individual strips and monitor for failures. I
know the current the TV drives them at, but I am curious what rating the
manufacturer has given them.

For what could be considered a simple two legged device they do have a
spectrum of failure modes.  At work we have several that fail flashing.
They were ordained as suitable low RF noise incandescent replacements but I
suspect they might not pass if they are in one of their many and varied
failure modes.

Justin


On Tue, Nov 9, 2021 at 3:16 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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