Hi-temp LCD...

[Don Y]

I\'ve only once spec\'d an LCD display in a product. I recall lots of
different issues, criteria (transflexive, transmissive, high temperature,
etc.). But, have largely forgotten most of those issues (IIRC,
transmissive/transflexive/reflexive have to do with the treatment of
the back of the display)

I recall response time was sensitive to excitation voltage and temperature.
I don\'t recall frequency being particularly critical. And, I know that
duty cycle isn\'t, either (there are hacks you can exploit by using different
duty cycles on different segments)

Today, I had to troubleshoot an electrical problem on a friend\'s vehicle
so took an el-cheapo DMM along with me to probe voltages, continuity, etc.
Leaving it in the back of her car was an oversight as when I went to
retrieve it, the display was uniformly \"black\" (not just the segment areas).

I\'ve experienced this before and know that it \"recovers\" once cooled. But,
it got me wondering what is actually happening (physically) inside the glass
when this condition manifests?

 

[Martin Brown]

On 23/09/2020 21:51, Don Y wrote:

I\'ve only once spec\'d an LCD display in a product.  I recall lots of
different issues, criteria (transflexive, transmissive, high temperature,
etc.).  But, have largely forgotten most of those issues (IIRC,
transmissive/transflexive/reflexive have to do with the treatment of
the back of the display)

I recall response time was sensitive to excitation voltage and temperature.
I don\'t recall frequency being particularly critical.  And, I know that
duty cycle isn\'t, either (there are hacks you can exploit by using
different
duty cycles on different segments)

ISTR it is bad for their ultimate longevity to have a waveform duty
cycle that isn\'t roughly 50:50 duty cycle in the long term.

Today, I had to troubleshoot an electrical problem on a friend\'s vehicle
so took an el-cheapo DMM along with me to probe voltages, continuity, etc.
Leaving it in the back of her car was an oversight as when I went to
retrieve it, the display was uniformly \"black\" (not just the segment
areas).

I\'ve experienced this before and know that it \"recovers\" once cooled.  But,
it got me wondering what is actually happening (physically) inside the
glass
when this condition manifests?

The liquid crystals have gone into the homogeneous state where they no
longer change the polarisation of the light passing through them.

https://support.lumerical.com/hc/en-us/articles/360042216134-Polarization-properties-of-a-helically-twisted-liquid-crystal

Isn\'t a bad explanation. All bets are off if it gets so warm that the
seal on the display fails due to differential expansion. The LCD will
recover when it cools down and realigns.


--
Regards,
Martin Brown

 

[Don Y]

On 9/23/2020 2:19 PM, Martin Brown wrote:

On 23/09/2020 21:51, Don Y wrote:
I\'ve only once spec\'d an LCD display in a product. I recall lots of
different issues, criteria (transflexive, transmissive, high temperature,
etc.). But, have largely forgotten most of those issues (IIRC,
transmissive/transflexive/reflexive have to do with the treatment of the
back of the display)

I recall response time was sensitive to excitation voltage and
temperature. I don\'t recall frequency being particularly critical. And, I
know that duty cycle isn\'t, either (there are hacks you can exploit by
using different duty cycles on different segments)

ISTR it is bad for their ultimate longevity to have a waveform duty cycle
that isn\'t roughly 50:50 duty cycle in the long term.

Yes, they don\'t LIKE \"dc\" -- which is effectively what tweeking the duty
cycle produces. But, they don\'t SCREAM and SHOUT when presented with \"dc\",
either. You have to know how much you can \"push things\" before you pay
a recognizable cost.

Today, I had to troubleshoot an electrical problem on a friend\'s vehicle
so took an el-cheapo DMM along with me to probe voltages, continuity,
etc. Leaving it in the back of her car was an oversight as when I went to
retrieve it, the display was uniformly \"black\" (not just the segment
areas).

I\'ve experienced this before and know that it \"recovers\" once cooled.
But, it got me wondering what is actually happening (physically) inside
the glass when this condition manifests?

The liquid crystals have gone into the homogeneous state where they no
longer change the polarisation of the light passing through them.

But, why \"black\" instead of \"clear\"?

> https://support.lumerical.com/hc/en-us/articles/360042216134-Polarization-properties-of-a-helically-twisted-liquid-crystal

Thanks, I will read (after my nap)

Isn\'t a bad explanation. All bets are off if it gets so warm that the seal
on the display fails due to differential expansion. The LCD will recover
when it cools down and realigns.

Yes. My understanding of the hi-temp devices was fluid and seal upgrades.
(after all, there\'s little else involved!)

 

[Sjouke Burry]

On 23.09.20 22:51, Don Y wrote:

I\'ve only once spec\'d an LCD display in a product. I recall lots of
different issues, criteria (transflexive, transmissive, high temperature,
etc.). But, have largely forgotten most of those issues (IIRC,
transmissive/transflexive/reflexive have to do with the treatment of
the back of the display)

I recall response time was sensitive to excitation voltage and temperature.
I don\'t recall frequency being particularly critical. And, I know that
duty cycle isn\'t, either (there are hacks you can exploit by using different
duty cycles on different segments)

Today, I had to troubleshoot an electrical problem on a friend\'s vehicle
so took an el-cheapo DMM along with me to probe voltages, continuity, etc.
Leaving it in the back of her car was an oversight as when I went to
retrieve it, the display was uniformly \"black\" (not just the segment areas).

I\'ve experienced this before and know that it \"recovers\" once cooled. But,
it got me wondering what is actually happening (physically) inside the glass
when this condition manifests?

A liquid crystal is something in between a fluid and a paste.
Heat it , and it turns into a fluid entirley, and stops working.
cooling restores the long chain molecules to the proper state(more or less).
Those molecules stand like a cats hair normally on the glass surface.
But not, if they get heated to much.

 

[Jeff Layman]

On 23/09/2020 21:51, Don Y wrote:

Today, I had to troubleshoot an electrical problem on a friend\'s vehicle
so took an el-cheapo DMM along with me to probe voltages, continuity, etc.
Leaving it in the back of her car was an oversight as when I went to
retrieve it, the display was uniformly \"black\" (not just the segment areas).

I\'ve experienced this before and know that it \"recovers\" once cooled. But,
it got me wondering what is actually happening (physically) inside the glass
when this condition manifests?

I used to leave a small LCD thermometer in my car before such things
were always included in the dash display. On returning to the car one
blisteringly hot day in summer 2003 I found the display completely
black. It returned when the device had cooled down. Out of interest I
pressed the min/max button to recall the highest temp that day. I was
amazed to find it was 61.3 deg C! Of course, it could have been higher
but the thermometer might not have stored it once it reached that temp.

--

Jeff

 

[Martin Brown]

On 23/09/2020 22:50, Don Y wrote:

On 9/23/2020 2:19 PM, Martin Brown wrote:

I\'ve experienced this before and know that it \"recovers\" once cooled.
But, it got me wondering what is actually happening (physically) inside
the glass when this condition manifests?

The liquid crystals have gone into the homogeneous state where they no
longer change the polarisation of the light passing through them.

But, why \"black\" instead of \"clear\"?

The LCD is between crossed polarisers and in its cool ordered liquid
crystal state rotates polarised light so that it is transmitted.

By applying a transient voltage you randomise the orientation of the
molecules so that they no longer have any systematic effect on
polarisation and the crossed polarisers then snuff out the light. That
is why you have to apply an ac drive to make the segments stay lit.

Liquid crystals are only just ordered and you can make them go
disordered with temperature as well as transient electric fields.

I have done the odd low power bare metal driver for LCDs in combination
with PICs and other micropower devices in the past. The 16F877 had just
enough outputs to drive 4 seven segment displays and do something.

https://support.lumerical.com/hc/en-us/articles/360042216134-Polarization-properties-of-a-helically-twisted-liquid-crystal


Thanks, I will read (after my nap)

Isn\'t a bad explanation. All bets are off if it gets so warm that the
seal
on the display fails due to differential expansion. The LCD will recover
when it cools down and realigns.

Yes.  My understanding of the hi-temp devices was fluid and seal upgrades.
(after all, there\'s little else involved!)

Get it too warm and the chemistry might also fail too.

--
Regards,
Martin Brown

 

[Don Y]

On 9/24/2020 2:39 AM, Martin Brown wrote:

On 23/09/2020 22:50, Don Y wrote:
On 9/23/2020 2:19 PM, Martin Brown wrote:

I\'ve experienced this before and know that it \"recovers\" once cooled.
But, it got me wondering what is actually happening (physically) inside
the glass when this condition manifests?

The liquid crystals have gone into the homogeneous state where they no
longer change the polarisation of the light passing through them.

But, why \"black\" instead of \"clear\"?

The LCD is between crossed polarisers and in its cool ordered liquid crystal
state rotates polarised light so that it is transmitted.

Yes, I realized that as soon as I thought about the transflexive, etc. issues.
One tends to think of displays involving motion as moving TO a displayed state
from a rest state. The fact that the rest state can be anything other than
what you\'d intuitively consider as \"relaxed\" is the metal adjustment needed.

By applying a transient voltage you randomise the orientation of the molecules
so that they no longer have any systematic effect on polarisation and the
crossed polarisers then snuff out the light. That is why you have to apply an
ac drive to make the segments stay lit.

Liquid crystals are only just ordered and you can make them go disordered with
temperature as well as transient electric fields.

I have done the odd low power bare metal driver for LCDs in combination with
PICs and other micropower devices in the past. The 16F877 had just enough
outputs to drive 4 seven segment displays and do something.

Yes. I prototyped a numeric \"remote 7seg display\" many years ago with some
CMOS MSI decoder/drivers (I forget the P/N). My most memorable takeaway
from the experience was that the \"power\" switch I added to the display was
completely ineffective, as originally designed (there\'s enough power in the
signal lines to the unit to drive ALL of the display electronics!)

Other than that, I\'ve used a 7x95 multiplexed LCD (driven by some devices
from Harris?). But, that was sited in a temperature controlled medical lab
so environmental issues weren\'t significant.

I\'ve some small graphic glass that I\'ve considered installing in my HVAC
controller. But, that is located adjacent to the furnace and I wouldn\'t
want to go to the effort of adding it to the design -- only to discover
it is unusable, in practice (so, rely on the remote displays, instead)