Date: Wed, 06 Dec 2000 14:17:00 +0100 From: "Julian Stacey Jhs@jhs.muc.de" <jhs@jhs.muc.de> To: "Artem Koutchine" <matrix@ipform.ru> Cc: hardware@FreeBSD.ORG Subject: Re: LCD contrast control on extanded temperature lcds Message-ID: <200012061318.eB6DH1m04467@jhs.muc.de> In-Reply-To: Message from "Artem Koutchine" <matrix@ipform.ru> of "Sun, 26 Nov 2000 20:06:28 %2B0300." <000701c057cb$38b15fe0$0c00a8c0@ipform.ru>
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"Artem Koutchine" wrote: > I have managed to attach LCD (HD44780 compatible from > Powertip. PC2004-B model). However, the contrast is too bad and > i cannot even start developing software to control it the way it need. > (super quite home mp3 player, tired of working fans :)). > It is an extended tempretaure unit (-20C - 75C) and it needs > voltage from GND to -7V on Vo (contrast control pin). I cannot > figure out how to get -7V from pc power supply. > Your mail header asserted: Content-type: text/plain; charset="koi8-r" This is wrong, as you did not post in russian char set, but latin char set. The wrong charset="koi8-r" made your mail display in my EXMH in a small font, hard to read, so you're lucky I'm answering, (I normally delete unread, when fonts screw up). -------- There is no -7V supply on PC power supplies you'll have to use -5, or -12 & reduce it. What power supply are you using, PC-AT or ATX ? AFAIR they both have -5 & -12V supply, though I wont swear to it. For ATX my book shows pin 12 blue = -12, Pin 18 white = -5. My book doesnt show AT, but just check with a meter, that's what I'd do. How to reduce -12 to -7 Volt: A) go to electronics shop, tell them you problem, ask for a power regulator IC + heat sink to do the job, read leaflet on setting voltage, add maybe 2 resistors to set it, & a capacitor maybe (suggestion: 20 to 50uF, can't be precise without knowing more). B) Do it the crude way with a resistor, Use Ohms Law: Voltage = Current * Resistance V=I R Power Law: Watts = Voltage * Current P=V I ( so power also = Voltage * Voltage / Resistance P=V^2 /R = Current * Current * Resistance P=I^2 R ) The problem with (B) is one neads to know the current load, which may vary with contrast setting. > The other thing, is that LED backlight is heating badly. I thought > it has resistor built-in, but it seems as it does not. What resistor > should i use there for +5V line (typical current for the LED is 260mA). I guess the back lighting is not an LED (light emitting diode, a non linear device) but a resistive material (which may make it easier to calculate). I guess the pixels are LCD Liquid Crystal Display. I don't know what the backlighting should consume (maybe it says on the device ? or specification sheets on the web), but If I'm understanding you right, you'r feeding it 5V & it's drawing 260mA ? Then taking Ohms law: The back lighting resistance is effectively ... V = IR so V/I = R so R = 5 / 0.26 = 20 ohms If you put a 10 Ohm in series increasing total resistance to 30 ohms, that will drop current to 20/30 = 2/3 of original 260mA, ie 173mA Now decide if resistor will overheat: P = I^2 * R = .173 * .173 * 10 = 0.29 Watts `Normal' resistors are often only rated up to 0.25 Watts, so thats just beyond the limit, but OK, But resistors come smaller & larger. Ask a friend or shop or look in catalogue. Resistors are available that will do several watts. If the backlighting is some kind of non linear semiconductor, then the resistor wont have quite the numeric effect I indicated, but something like it, 2nd example: If you want to halve the backlight current: 130mA: add 20 ohms, if you make that out of 2 * 10 ohm resistors in series, then each gets P = I^2 * R Watts = .13 * .13 * 10 = .16 Watts ... well within the 1/4 watt limit for normal resistors. Television shops are more likely to have this stuff than computer shops. If I've made a mistake, I'm sure a fellow list reader will correct me (EG I dont know if backlighting technology is linear or non linear, though LCD (Re. contrast) is known to be semiconductor thus non-linear) Julian - Julian Stacey Independent Munich Unix Consultant http://bim.bsn.com/~jhs/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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