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backlit lcd stupid question

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large_ghostman

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i have some blue backlit lcd's but i dont know if the resistors on the back goto the led backlight or not as i cant trace them, but my main problem is i have a vague memory of once reading that lcd leds had very very small I requirements! i should have written it down because i cant now find the info, so my question is as a rule of thumb do they come with resistors already on?and if not or maybe anyway does anyone have a clue roughly what a normal hitachi type 2 line lcd need current wise for the back light, i was tempted to just try around 10ma but my memory is nagging me and i have a feeling thats actually too high but i dont know!
anyway thank you
 
Normally they are around 110mA some are higher, some are lower.. Have you not got the datasheet?

I have just put four small 16x2's in to service they went with 18R back light current resisters...The forward voltage is "Usually" around 4.2V... But the datasheet is the only place you'll get the correct figures.
 
no these were cheap Chinese ebay specials no datasheet! maybe my memory of a very strange figure relates to it being very high not low. 110mA seems alot for a led :D but at least i can go ahead and stick 10mA in and raise from there with no fear of burning it out :D
if i want to pwm the brightness tho i am going to have to use a transistor. thanks for the info. ebay is great price at times (these were mad cheap i got 10 units in a auction for £3 or 30p each :D), ive had them about 9 months as i normally use the ones without back light as they are normally a bit cheaper and cost matters on my budget :D
 
no these were cheap Chinese ebay specials no datasheet! maybe my memory of a very strange figure relates to it being very high not low. 110mA seems alot for a led :D but at least i can go ahead and stick 10mA in and raise from there with no fear of burning it out :D
if i want to pwm the brightness tho i am going to have to use a transistor. thanks for the info. ebay is great price at times (these were mad cheap i got 10 units in a auction for £3 or 30p each :D), ive had them about 9 months as i normally use the ones without back light as they are normally a bit cheaper and cost matters on my budget :D

110mA is a lot for a single LED. However it may not be a single LED illuminating a glass reflector. Some use multiple LEDs in an array to give even illumination in which case 110mA is not unreasonable.
 
its a funny looking led type unit so an array would seem to be about right, hard to tell as its a kind of glass thing set up i spose to shed the light evenly! anyway now i know its likely to be pretty high i can test at 10mA and slowly up it until it looks right.
 
Its the way I would go... I would put the anode and cathode on a variable power supply and whilst measuring the current, watch the voltage... Like I said before I always run my backlight at 3/4 of the rated current any way... They last longer....

I doubt it will be a single LED... having said that I used Displaytech LCD's and they used a single LED... I used an 82R resistor with a 5v rail.
 
I used one of those cheap blue LCD's recently for MrRB's small cap meter. I think I put a 1k resistor in series with the backlight on a 5v supply. You could use a preset to get just the brightness you require. You must set the contrast though or it will be unreadable no matter what the brightness!

Just remember that too much resistance won't hurt it, too little will.

Good luck with it!
 
I used one of those cheap blue LCD's recently for MrRB's small cap meter. I think I put a 1k resistor in series with the backlight on a 5v supply. You could use a preset to get just the brightness you require. You must set the contrast though or it will be unreadable no matter what the brightness!

Just remember that too much resistance won't hurt it, too little will.

Good luck with it!
The contrast is an entirely different thing... The backlight uses a lot more current!!
 
this didnt flag as having a response! sorry for not replying. i didnt get a chance to try it today as i have been busy on here and building the incubator! the contrast i normally set with a adjustable pcb pot/trimmer and leave it at that, but as i want a ultra cool hitech incubator i want the display to dim automatically at night :D, not decided which way to do that yet other than LDR but so many options for actually getting it to dim that i will probably wait untill i have the more important bits done then see if i have free pins or timers or whatever
 
I believe LED dimming is done using PWM these days - so an LDR would provide the controlling voltage for that. Be interesting to know how it goes!

Ian - I didn't confuse contrast and backlight - you mis-read my post!
 
Now I'm confused - I'm sure I used a 1K but you said you used 18R on yours so maybe my memory is faulty. I didn't want a bright display though, just enough light to read it. Unfortunately I heat-shrinked the resistor so can't really check.

"Mr Scottle" - Tee hee I like it :)
 
these blue LCD's you have, they wouldn't happen to be the blue LCD's that WERE in MY DRAW would they??? if so they are 65mA, but i assume these are some other LCD's because MINE were not cheap Chinese crap from ebay!! and they were also not in the lab but kept in MY DRAW where obviously you wouldnt dream of removing things from would you?o_O:meh:
 
kept in MY DRAW where obviously you wouldnt dream of removing things from would you?o_O:meh:

It's seems Scotland or the United States it's the same. My son owes me a few Tune ups and Oil Changes. Plus, he is doing my Plumbing today and my Daughters, if you get it my drift. He doesn't know electrical, he will eventually helping with an Upgrade on the house wiring, he just doesn't know it yet. I did my fathers around my son's age.
 
erm no not exactly those ones but maybe one that wasnt put away may have got put with my stuff in error :oops: you dont want it back anyway your draw is full! BTW are those MOSFETS the ones you didnt have when i asked?;)
 
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