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'[EE]: Less interesting LCD observation...'
2002\08\12@172553 by Barry Gershenfeld

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>Left it on the car dash once and when I came back, the unit
>was really hot, and the display was completely black.
>it came back after it cooled off.  And this happened many times

I had a LCD clock that I left on the dash that did that.  It
worked after it cooled.

Now for my question:  I've had a few LCD clocks that refused
to work after I replaced the battery.  My guess here is that
when the battery goes dead the internal circuits stop
oscillating, putting DC on the display, ruining
it for the next go.   Believable?

Barry

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2002\08\13@123850 by Peter L. Peres

picon face
On Mon, 12 Aug 2002, Barry Gershenfeld wrote:

>>Left it on the car dash once and when I came back, the unit
>>was really hot, and the display was completely black.
>>it came back after it cooled off.  And this happened many times
>
>I had a LCD clock that I left on the dash that did that.  It
>worked after it cooled.
>
>Now for my question:  I've had a few LCD clocks that refused
>to work after I replaced the battery.  My guess here is that
>when the battery goes dead the internal circuits stop
>oscillating, putting DC on the display, ruining
>it for the next go.   Believable?

No, the LCD runs on a charge pump driven by the battery (if 1.5V at
least). No clock = no LCD voltage. Much more likely that there is gunk and
dirt in the main board and the oscillator's self bias resistor is not
enough to get it out of its stuck state.

Also some early chips were bad enough that they self destructed in time
(20+ years). I have a freind who collects stuff and I've seen a couple of
strange items.  If unsure scope one of the xtal pins.

Peter

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2002\08\13@131113 by Barry Gershenfeld

face picon face
>>Now for my question:  I've had a few LCD clocks that refused
>>to work after I replaced the battery.  My guess here is that
>>when the battery goes dead the internal circuits stop
>>oscillating, putting DC on the display, ruining
>>it for the next go.   Believable?
>
>No, the LCD runs on a charge pump driven by the battery (if 1.5V at
>least). No clock = no LCD voltage. Much more likely that there is gunk and
>dirt in the main board and the oscillator's self bias resistor is not
>enough to get it out of its stuck state.
>
>Also some early chips were bad enough that they self destructed in time
>(20+ years). I have a freind who collects stuff and I've seen a couple of
>strange items.  If unsure scope one of the xtal pins.
>
>Peter

I could believe that, because they are older, and they have the
chip-on-board (under the expoxy blob) so reliability probably
wasn't at the top of the design list.    Thanks for the
suggestion about looking at the crystal, though.

Barry

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2002\08\14@090522 by Bob Barr

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On Tue, 13 Aug 2002 09:57:45 -0700, Barry Gershenfeld wrote:


>I could believe that, because they are older, and they have the
>chip-on-board (under the expoxy blob) so reliability probably
>wasn't at the top of the design list.

I'm confused by that statement.
My (admittedly vague) understanding is that chip-on-board's biggest
advantage is cost savings for large-volume manufacturing. I also
thought, though, that long-term reliability for COB was quite good as
well. Am I missing part of the picture?


Regards, Bob

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2002\08\14@134735 by Peter L. Peres

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On Wed, 14 Aug 2002, Bob Barr wrote:

>On Tue, 13 Aug 2002 09:57:45 -0700, Barry Gershenfeld wrote:
>
>
>>I could believe that, because they are older, and they have the
>>chip-on-board (under the expoxy blob) so reliability probably
>>wasn't at the top of the design list.
>
>I'm confused by that statement.
>
>My (admittedly vague) understanding is that chip-on-board's biggest
>advantage is cost savings for large-volume manufacturing. I also
>thought, though, that long-term reliability for COB was quite good as
>well. Am I missing part of the picture?

Low cost cob can use lower grade resins and materials and this can lead to
creep or weight loss or dimensional instability over time. The result is a
cracked chip or bonding wires ripped off after a minor shock or all by
itself. The long term reliability should be good if the mounting materials
are compatible (thermal expansion coeff., low dimensioanl creep) and of
compatible modulus (chip on glass lasts almost forever afaik). I suppose
that the fact that no plastic buffer layer is used on low cost cob is
known ...

Peter

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