From: Justin Richards <justin.richards@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sun, 7 Nov 2021 22:52:43 -0800
Subject: [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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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 tear
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. The
system continues to work fine, the compliance voltage simply drops by 3v
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 strips.
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 when
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 its
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 around
540mA.taking measurements while increasing a  variable resistor in series
with the LEDs.

Justin
--=20
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