LCD screen slow to lit up - Page 1
Forum section: electronics / beginners

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 03:11:15 am
content: I have an LCD-based digital readout (black and white) and when I turn it on, it takes about 5 min. for the contrast to come up to normal parameters. If I turn it off and then on again within a few min. - it's fine, but any more than that and the cycle repeats. Am I right to suspect a bad capacitor?

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 22, 2013, 03:34:51 am
content: How are you powering it? With a potato battery? Lemon battery? Lemon Party?Golden rule of troubleshooting "thou shalt test voltages"

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 03:56:17 am
content: Quote from: Legit-Design on December 22, 2013, 03:34:51 amThe device is a full factory form, mains powered. It's an Acurite 200s dro.To be more specific, it does lit up white right from the beginning - although with a shade of blue. It's the black pixels that are faded and takes about 5 min. to develop contrast, to become black.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 22, 2013, 04:19:35 am
content: http://www.acu-rite.com/200s.htmlproduct infohttp://www.acu-rite.com/pdf/200S_Sept09.pdfManualhttp://www.acu-rite.com/pdf/manuals/200S_Man_en_532823-24_Ve02.pdf5.7” Transmissive LCDCrystal clear5.7" TransmissiveLCD ScreenQuoteDo brightness and contrast adjustments have any effect how quickly it becomes fully operational again? For example trying the extremes of adjustments for testing purposes.If it just needs time to warm up then LCD shouldn't be faded.Can you take more pictures of the boards, component side maybe?I must admit I have no experience with that kind of LCD displays or how they might behave, but I'm sure someone here must have. For example simple small solar solar/light powered calculator will start to fade it's display when voltage gets too low. Might be that voltage for this one isn't high enough and needs time to charge up because of faulty capacitor, or it also needs time to warm up so things start to work.

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 05:27:01 am
content: Good idea about bumping up the contrast at start up, it actually works and you can bring it around where it looks about right. However, as it warms up it becomes over-contrasted (black pixels start to fade into blue). So to keep it right I need to reduce the contrast progressively as it warms up. I didn't realised that before.Hard to take picture of these nuances under artificial light though.There are all those capacitors on the power supply side, the big one is ok so maybe one of the smaller ones is gone?

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 22, 2013, 05:39:58 am
content: Quote from: Leuven on December 22, 2013, 05:27:01 amMy guess would be that the actual LCD wont take hardly any current. Those capacitors are supplying the whole board, can you disassemble it and get closer to the actual LCD? There must be smaller surface mount capacitor/s that is actually decoupling the lcd supply. If you can measure voltage across this capacitor from cold boot, then it could determine if it is the voltage slowly rising and fixing LCD contrast and/or it is actually something warming up and making contrast normal again. Also more detailed pictures about inner workings of this device might prove helpful.

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 06:23:54 am
content: Quote from: Legit-Design on December 22, 2013, 05:39:58 amSure, here is a picture of the board. I could power the thing up as is and measure voltage over capacitors, if I knew which ones.Larger picture here if it helps:http://www.hotpen.co.uk/others/layout2.jpgBack of this board here:http://www.hotpen.co.uk/others/layout3.jpg

author: WarSim
author_rank: Frequent Contributor
timestamp: December 22, 2013, 07:09:42 am
content: The only thing that it sounds like to me is a Ve generator problem.I see a 5v for Vcc and a backlight inverter but no Ve inverter input it may be on the LCD.The Ve inverter seems to be either.Defective.Under powered therefore voltage is too low and will cause your symptoms.Contrast pot is faulty or replaced with a non-compatible pot value and dragging Ve down.The turn on timings are being violated.Yes some of these a low probability but included anyway.Some older LCDs require an involved startup sequence. Where the blank screen idling is required and is dependant on the LCDs startup sequence. I have seen so many different ones I dare not guess.Sent from my iPad usingTapatalk

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 22, 2013, 07:28:01 am
content: Quote from: WarSim on December 22, 2013, 07:09:42 amI think the lcd contrast is driven by those op amps (U19, U15, U1?), how else could the buttons adjust the contrast by software control.Power supply in, does it actually show +5V with multimeter? What about other side, it says D13 +5.1V and D14 +3.3V those look like LEDs, are they light when you power up the device? My guess is those indicate if the named voltages are ok.For the actual LCD contrast problem U19, U15, U1?(looks like same IC as U15), those look like OP AMPS. They actually drive different voltage for the LCD to adjust contrast, since it has contrast adjustment by software. Above U19 is RT2 is that some kind of thermistor? Why would they put thermistor there? Thermistor would explain why it starts to work when warmed up.Those thicker traces coming from U15 and the U1? going to LCD connector can you measure those relative to GND? When you adjust the lcd contrast yourself you should see voltages rise and fall on those relative to what you are pressing.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 22, 2013, 07:48:58 am
content: Manufacturer claimedOperating temperature 0° C to 45° Cso they need temperature compensation for the LCD so it will work steadily at any temperature?I guess it is somekind of "LCD CONTRAST ADJUSTMENT WITH TEMP COMPENSATION"http://www.pacificdisplay.com/lcd_cont_adj.htmMust admit I had no idea LCD would require such circuitry, makes sense based on what I've seen. Most projects I've had only had adjustable backlight and single trimmer for contrast adjustment. In school it was common prank to misadjust the trimmers so others would think their microcontroller code is not working.Now that circuit is just not working correctly.

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 08:04:20 am
content: Quote from: Legit-Design on December 22, 2013, 07:28:01 amOk here are my findings:Power supply has 5.1V on all 3 pins, so that's ok.LEDs are both lit up at D13 and D14.U13 is indeed driving the contrast - 19V at lowest contrast setting and 14.3V at highest (counter-intuitive?)Also, if it is of any importance, the meter shows negative voltage.U15 is not the brightness though, must be driving some other setting.(U13 is in fact "U1?", top one)

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 08:10:22 am
content: I am leaving the display on and took a note of that contrast voltage at 17.4V. Will measure again in 15-20 min to see if it changes.Edit: no need to wait, it's at 17V after a few minutes and dropping slow but steady. So that's where my unwanted contrast boost is coming from!

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 22, 2013, 08:34:35 am
content: It has CCFL (cold cathode fluorescent) backlighting? Probably since it has high voltage backlight inverter. I think that is "brightness". Or does the display have ability to adjust it's background for different brightness levels?If that circuitry is working anywhere like I think it is, can you try blasting RT2 above U19 with cold spray. Or one of those compressed air in a spray can but just turn it upside down -> cold spray.I think U19 with RT2 thermistor is working as something similar to LCD CONTRAST ADJUSTMENT WITH TEMP COMPENSATION (http://www.pacificdisplay.com/lcd_cont_adj.htm). If contrast voltage changes with temperature like when it's started cold then that could be the problem. What kind of environment are you using it? Does it get cold or is it normal room temperature?

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 09:27:11 am
content: Quote from: Legit-Design on December 22, 2013, 08:34:35 amYes, display is definitely CFL. I'm in the workshop and it's quite cold, I'd guess about 10-12ºC. I just got this today so haven't got the chance to test it under different room temperatures.The idea is to chill that thermistor and take another reading, right? Too bad I don't have any cold spray or air can.Correct me if I'm wrong, but isn't this temp-based contrast adjustment working in the opposite direction for me? The text says:<< an under voltage condition with diminished display contrast will result at low temperatures, and a "ghosting" or overdrive condition will occur at high temperatures>>Isn't the display supposed to start cold with boosted contrast, and diminish as it warms up? In my case it's the other way around. And if I understand the text correctly, the problem with lower temperatures is a drop in voltage. The thyristor seem to be doing exactly that - boost voltage at lower temps, but the effect if reversed maybe because the LCD has the opposite response to voltage boosts (less contrast)? What am I missing here?

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 09:37:33 am
content: Yep, I warmed the thyristor a little bit with a heat gun and sure enough the contrast voltage dropped. This thing seems to be working as it should. Maybe I have a display problem?

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 09:53:43 am
content: Manual for the LCD display. I the diagram at chapter 6 "Optical characteristics" saying contrast should go up with voltage?http://www.farnell.com/datasheets/52636.pdf

author: Leuven
author_rank: Regular Contributor
timestamp: December 22, 2013, 10:15:35 am
content: … actually scratch that, manual contrast works fine which means the display is ok.

author: Leuven
author_rank: Regular Contributor
timestamp: December 23, 2013, 03:36:38 pm
content: Got it working by replacing that thermistor with a 10k resistor. Works perfectly now.Thank you very much Legit Design, you were right on the money!

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 27, 2013, 11:59:22 am
content: Quick googling I found this:http://www.navsea.navy.mil/nswc/crane/sd18/Public%20Documents/ProductArchive/Thermistors.pdfQuote from:http://www.navsea.navy.mil/nswc/crane/sd18/Public%20Documents/ProductArchive/Thermistors.pdfBold part is what I think might have happened. I guess thermistors failing is something that happens often (I learn something new everyday). Thermistor failing might be due to inproper design of the circuit, maybe they were loading the thermistor too much. One way to find out would be to trace the actual circuit (in DaveCAD ). Looking at the pictures now it might also be something as simple as cold joint on one of the thermistor legs, little too much resistance in one of the joints might cause unexpected behavior and when it heats up it might have started to work properly.

author: WarSim
author_rank: Frequent Contributor
timestamp: December 27, 2013, 05:41:53 pm
content: Wow, Temp comp was way down on my list of probabilities.Snag didn't sound like a temp comp snag. Catastrophic failures are so much more common, and take under 90s to express a symptom.Good find.The advantage of different people with different experience.Sent from my iPad usingTapatalk