Forums

Hi-temp LCD

Started by Don Y September 23, 2020
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?
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
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!)
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.
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
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
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)