hp f2105 conversion to smd led

[klein]

hi
i have had this f2105 for a while and when i got it i searched google and everyone said the ccfl's were bad. and at that times i didn't wont to put $60.00 into it, a while back bought a strip of water proof flexible smd 5050 leds. so last night i took the screen apart and the strip of leds fit right in the ccfl tray, this morning i put it back together for testing and hooked the two led strings to a computer psu, after testing i find the the psu on the monitor is putting out 18.98v and 3.3v and i need 12v @ .75 amp

do you think a voltage regulator will work? i was thinking of using a TO-220 L7812CV will it have to be heat sinked ?

psu for monitor is a 4h.l1s02.a20 but i can't find a pdf on it.

where can i pull the voltage for the regulator i was thinking right off the plug for the ccfl inverter being it is a spare part right now.

[jetadm123]

Re: hp f2105 conversion to smd led

Various versions of the LM7812 voltage regulator will put out 1A (or more) and take a input voltage up to 35V. If your requirements are 0.75A, then I would suggest you place it on a heatsink. You might be able to remove one of the inverter transformers and install a small heatsink in it's place. If the inverter board input voltage is fed through a single fuse, then you should be able sever the connection there to kill power to the inverter board and connect your voltage regulator as you suggested earlier.

I don't know if the LED string brightness varies with the input voltage you feed to it, but if it does, then you might want to purchase a variable output regulator.

[klein]

Re: hp f2105 conversion to smd led

hi jetadm123

i was going to take the inverter pcb right out it is a separate board and the monitor works with out it hooked up, and take the power from cn7 of the psu pcb.

am i correct that the back or mounting hole of TO-220 L7812CV is ground the same as the centre leg of the regulator ? if this is true couldn't i just screw it to the steel plate that is holding the pcb boards that would give it a 10" x 21" heat sink.

variable output regulator, i'm not sure how much voltage these will take they ate set up and resistored for 12v. but i do have a small strip of 3 smd's i might try so higher voltage on. i'v been looking for something to test my new variack on 1-130v 20 amp and a d5sb 20 bridge

[jetadm123]

Re: hp f2105 conversion to smd led

hi jetadm123


am i correct that the back or mounting hole of TO-220 L7812CV is ground the same as the centre leg of the regulator ? if this is true couldn't i just screw it to the steel plate that is holding the pcb boards that would give it a 10" x 21" heat sink.

The ground should be the same. To verify, set your meter to resistance and place one probe on the tab and the other on the center leg. You should read approx 0 ohms. How close is that metal plate to the LCD panel underneath? I don't know how hot the regulator will get, as heat could affect the display itself.

[klein]

Re: hp f2105 conversion to smd led

there is 3/8 of an inch there between the plate and the lcd back

[mariushm]

Re: hp f2105 conversion to smd led

The power supply takes 19v and outputs 600v on those connectors to the CCFL lamps. Be careful what you do there, I hope you don't plan to connect the leds to the old lamp connectors.

The controller on board takes 19v and generates AC to the lamps at about 200 Hz, then uses PWM to change the brightness based on an input voltage from the other board... with leds you would ignore that adjust wire.


Otherwise, leds are current driven devices, not voltage driven. The brightness is adjusted by limiting the current going through them or using pwm (turning leds on and off so fast your eyes don't notice it).

I don't know why are you guys talking about 12v linear regulator - if you plug too few of those leds to 12v, you're just gonna blow them up by giving them too much current.

Let's say each of your SMD leds tops at 50 mA and their forward voltage is 3.3v. You can put 5 of these leds in series for a total of 15.5v voltage drop.
Now you need to put a resistor in series with the leds to limit the current going through them, so you use Ohm's law:

I = V / R ... I = (19v - 5x3.3v) / R therefore R = (19-16.5) / 0.05 = 50 ohm
So you'd pick a standard 56 ohm resistor.

The power going through the resistor would be IxIxR = 0.05x0.05x56 = 0.14A or 140mA, therefore you'd need a 0.25w resistor.
This online calculator agrees with me: http://led.linear1.org/led.wiz

I'm not sure how you plan on adjusting brightness because this method won't allow you to adjust anything.



Could do some PWM with a attiny or a pic, by switching an npn transistor on an off hundreds of times a second.

[Khron]

Re: hp f2105 conversion to smd led

The controller on board takes 19v and generates AC to the lamps at about 200 Hz, then uses PWM to change the brightness based on an input voltage from the other board...

Actually, the AC supplying the CCFl's is closer to ~50kHz. The PWM-dimming frequency's around 200Hz

[klein]

Re: hp f2105 conversion to smd led

hi guys

as stated before i'm going to take the inverter pcb right out.

led are water proof flexible smd 5050 leds, they were already set up for 12v ready to go i just cut 3 leds off cause i need them shorter.

this was just quick and dirty experiment but i'm not taking that screen apart again lol

do you guys think i should put caps before and after the voltage regulator as the pdf shows for test conditions .

[jetadm123]

Re: hp f2105 conversion to smd led

I was assuming that klein was using a aftermarket automotive type 12V LED strip. Those strips have built-in current limiting resistors. Klein- was I wrong?

[klein]

Re: hp f2105 conversion to smd led

jetadm123


you are 100% correct bought from e*b*y.

[Th3_uN1Qu3]

Re: hp f2105 conversion to smd led

You could use the DIM signal of the monitor, that way you can connect the brightness of the LED strip normally, from the monitor's menu, just as you would normally do.

Basically you need a PWM- or voltage-controlled current source. Edit: Since you say the strips have the limiting resistors built-in, you can just drive the ADJ pin of a LM317 from the DIM signal and a suitably configured resistive divider, if you don't mind the heat dissipation.

[jetadm123]

Re: hp f2105 conversion to smd led

Just out of curiosity, since you put the monitor back together, can you please post a photo of what the monitor image looks like with the 2 LED strips lit? I'd like to see how bright and uniform the light is.

[klein]

Re: hp f2105 conversion to smd led

ok here they are :

lights out
lights on
lights on movie playing

you can see some of the leds at the top and the bottom but when i put the strips in i didn't stick them to the pans, if they were stuck to the pans you wouldn't see the led i believe.

i have a couple 100, 5050 smd's i'm thinking of making a picture frame with thoes and a 21 inch sq dell, i will try my hand at pwm on that, these pre maid strips the leds are 11.5mm apart

[klein]

Re: hp f2105 conversion to smd led

accentual monitor looks better then the pictures

[tom66]

Re: hp f2105 conversion to smd led

It looks really good.

I would just use a power resistor though instead of a regulator. More reliable, it has to dissipate the same heat but is more capable of it. For 18V to 12V at 750mA you need an 8 ohm resistor; 8.2 ohm will do. The resistor will dissipate 4.6W, so choose a 7W or 10W for reliability.

[jetadm123]

Re: hp f2105 conversion to smd led

One of the better repairs I've seen using a LED strip as a substitute. And much cheaper than new ccfl's!

I would still stick with the voltage regulator as the current 19V output is un-regulated and may vary widely under load. Even though the LED strip requirement is 12V, I didn't see what voltage range it will work under. Maybe 12-15V?

[budm]

Re: hp f2105 conversion to smd led

So are the LEDs are setup as 5-LED string, since LED is the current device, you can use LM317 as a constant current source, and set it about 30mA. You can change the brightness by adjusting the current control resistor.
Or you can use Voltage regulator to feed the LED string which already has limiting resistor built-in, by varying the supply voltage,you then basically vary the current.

[tom66]

Re: hp f2105 conversion to smd led

I would still stick with the voltage regulator as the current 19V output is un-regulated and may vary widely under load. Even though the LED strip requirement is 12V, I didn't see what voltage range it will work under. Maybe 12-15V?

It's unlikely to vary under heavy load, like with a big LED strip, and 1V is only an extra 125mA.

[klein]

Re: hp f2105 conversion to smd led

with my variack 1-130v 20 amp and a d5sb 20 bridge , at 15v they were getting hot, noticeably hot i don't think what ever they are in bedded into to make them water proof holds the heat in. now you can get the strips that are not water proof they might work better if your going to feed them more voltage.

this is the first large monitor i have taken apart and it has a 1/2 or better thick piece of plexie / lexon glass that the leds shine into that may help with the light distribution

[jetadm123]

Re: hp f2105 conversion to smd led

What's was the minimum voltage required for the string to light?