LED type and amount for LCD monitor lighting

[garrettp]

Hi again all,

As my last thread went, I give up on trying to get inverters working for my Apple iMac LCD displays. And one of the displays I am going to make external since I only have 1 iMac and two screen .

So, what I want to do is power the LCD's from the 12v source that fed the LCD inverter. There are 2-12v feeds and I think I read somewhere that they are good for 3 amps.

I have tried a few LED configurations using a PSU from a computer (I take bad PSU's and use them for 12v sources). And when I get close to the voltage rating of the LED's I start to get a whine from the PSU. That worries me. I want to make sure that I do not tax the PSU on the iMac.

Now, I have not done any research yet as to the new LED powered LCD's. So at this point I have no idea what type or how many they use or power consumption either. With that, if anyone has any info on this that would help me out, that would be great.

Thank You,
garrettp

 

[duffy]

I can tell you that white LED's are not as efficient as fluorescent lamps. I worked on a project to make LED lamp replacements for fluoro's, but the backer lost interest when I compared lumens to watts on both and told him he couldn't advertise it as "more efficient".

I've also done edge-lit panels to get a consistent brightness, and I can tell you you will have to change the dot screen pattern if you are going from a linear element to multiple point sources, and this is a pretty involved process.

 

[garrettp]

I figured I could use the SMD LED's and just run them in parallel. This would give me the small depth I need and also the most consistant type of lighting as I could run them tight together. There is not much room where the CCFL's are and this would be about my only choice. I just need to figure out how many will fit across at 20mA each . Again, I would run top and bottom of their own 12v power supply. I understand they are not as efficient But I have been trying to get this CCFL fixed for a month now and I am tired of getting things that do not work. It would be ALOT cheaper to replace with LED's and just be done with it.

Below is what I am talking about.

**broken link removed**

 

[duffy]

Do you have a series resistor in there? Do you know how to calculate it? Subtract the LED voltage drops from the power supply, then use Ohm's Law to set the current to the 20ma. If you didn't put one in before you ran it, you may have damaged the LED's.

Don't run all the led's in parallel, you can get 3 in series at 12V, and this will reduce the current load on the power supply by 66%. You need one resistor for each run of 3 series leds, then you put these series chains in parallel to each other.

Led's are not cheaper.

Prepare for a mottled display.

 

[Nigel Goodwin]

Also prepare for a very dark display

CCFL's are amazingly bright, LED's aren't.

 

[garrettp]

I guess that is one of the things I am trying to find out. How dim it will be with LED's. Although, anything is better then having to turn off all the lights in pure dark to see it. I have tried 3 inverters and other then trying to find replacement CCFL's (of which I am looking into pricing right now anyway), this is my only option.

 

[duffy]

Replacing the CCFL isn't that hard, I've done it before, and it's almost certainly the problem. Digikey sells the CCFL's. Give this a shot before you try to convert it to LED's.

 

[garrettp]

I have the CCFL assy out of one of the displays already. So no, that is not too difficult (keeping the LCD assy clean is though).

I will have to give digikey a call. I know in my past dealings with them, they are very very helpful.

Other then that, I ordered 100 - 0805 SMD LED's for $22 shipped. So, I will play with those for my external display along with different power sources. This one wont matter as much and I could even backlite it if I wanted to. It is just something to mess around with

 

[garrettp]

Below is a pic of the CCFL's I removed. And I just thought of something, I replace the LCD on the G4 with one from a G5 which was working and lit up and all. So, I guess this probably means that I keep getting bad inverters. I don't know. It could be a bad logic board but I also tried hooking up the inverters using an external PSU with different pin-in configurations. See, this is why I want to go with LED's

**broken link removed**

 

[duffy]

...And when I get close to the voltage rating of the LED's...

What do you mean by this?

 

[garrettp]

I have a power supply from a computer that I hard wired so I can use it for 12v and 5v. I have a 1000k pot and as I raise the voltage to the LED's (which are rated at 3.3v), the psu starts to strain. I can physically hear it shrieking. This would be around 3v. by the time I hit 3.3v the power supply would be annoying. Mind you, the reason I hacked this PSU is because it would not start up a computer any more . Below is the info on the LED's I ordered. I do not know if it is going to work but I will try on one of the monitors anyway. For $20 if it doesn't work at least I won't be out that much

0805 Super White, 460-600mcd
120 degree , 2.8-3.4v,20mA
size : 2.0*1.25*0.68

On a side note, I did just order an aftermarket inverter. The OEM inverters are very hard to come by. The big name stores do not have them and they are twice the price of the places that do have the ones that just don't work lol.

Thanks,
g

p.s. just out of curiosity, you don't have a sister named Kelly do you? Man would that be creepy.

 

[duffy]

No relatives named Kelly.

Doesn't sound like you had that pot hooked up right. An LED should light off 1K and a 12V supply without turning the knob at all.

Sounds like you made a voltage divider out of it. Did you have the LED on the wiper or the 12V? Did you try to run a bunch of LED's in parallel? Don't do it that way -

Led's never have EXACTLY the same forward voltage drop characteristics. One will run a little lower than the other, and take the lion's share of the current. What you want to do for this circuit is run them in sets of 3 in series, giving you about a 9.6V drop. 12V - 9.6V = 2.4V, so then you add a 120Ω resistor (1/4 watt is fine, 1/10th watt 1206 SMT is fine) in series with that. Connect 33 of these series chains in parallel, and it will pull .33A from the 12V supply, for a total of about 4 watts. That's about as efficient as you are going to get at this level.

I look forward to hearing how your LED conversion works out!

 

[garrettp]

I dated a girl named Kelly whom had a brother they called Duffy. He lived in Madison.

I will have to contact you when I get the LED's in house. I understand mostly what you are talking about but will draw up a schematic and have it approved here .

Thanks so much for you help and I will keep you posted.

I do have an after market inverter coming in the morning so I will play with that on the mac and save the LED's for my spare LCD. Also check out my other thread as I found a blown inductor on the mac's logic board.

g