Using a resistor to fake a Inverter into thinking a CCFL is good

[Nama]

A man was telling me to try this.
How would I go about it?.

[PlainBill]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

A man was telling me to try this.
How would I go about it?.

Ohms law. At a rough guess a CCFL runs at 650 - 1000 Volts and draws 3-5 mA.

E / I = R or 800/.004 = 200Kohms

I X E = P or .004 X 800 = 3.2 watts

So a 200K ohm, 5 watt resistor hooked to an appropriate connector should do it. Probably be a good idea to use a resistor rated for high voltage.

No warranty at this price.

Now, I've been thinking it should be possible to do something similar, except use two resistors (180K, 20K) in series to form a voltage divider so you could actually measure the output voltage without frying a DMM.

PlainBill

[Nama]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Thanks Mr Bill
I have been meaning to tell you I have access to thousands of LCD's and parts for them at the moment.
If you need anything let me know. That goes for all of you. Jacob

[EGuevarae]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Thanks Mr Bill
I have been meaning to tell you I have access to thousands of LCD's and parts for them at the moment.
If you need anything let me know. That goes for all of you. Jacob

Hi Jacob. Hi, Bill.
I have been thinking about this for quite some time. Here's my scenario:
I have a 24" LCD Panel (not mine) that was removed from a Samsung brand monitor (or so I was said. No plastic frame available, and no pics at hand). Six lamps in there, only one is fried. If I use a spare lamp that I have so the inverter don't have to shut down, you can not tell the 6th lamp is not lit. So it would be possible to just "fool" the inverter and with the appropriate resistor, use it like that?

[Nama]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Yes it does work, I can verify this. I had a tray of resistors and since Bill thought what was suggested to me would work I tried it today on a 22" acer Took me a few tries but it is flawless now!. I will be using this trick many times over from here on out.
So whats the model number on your samsung. I will see if I have a bezel for you, Jacob

[EGuevarae]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Yes it does work, I can verify this. I had a tray of resistors and since Bill thought what was suggested to me would work I tried it today on a 22" acer Took me a few tries but it is flawless now!. I will be using this trick many times over from here on out.
So whats the model number on your samsung. I will see if I have a bezel for you, Jacob

OK. First of all, thanks for the confirmation.
As for the model, I don't have it. I checked it on a friend's house, and it did not have a bezel (anymore). He did kind of a custom Jukebox and made a front "bezel" with acrylic and put some vinyl decorations so it looks like an old record player (I dunno how to put this in English. You know, the old machine that you put a quarter or two in and it plays your selected song). I will try to get pictures of it as it is kind of cool. Well, he did it quite colorful and with a great sound system, but did not make ventilation holes/grills, and the caps in the samsung went south : ([LCD heat] + [PSU Heat] + [MoBo's heat] + [amp & sound crap heat]) - [Air Ventilation] = crap caps giving in ....
Well, when he tried to fix it, he damaged one of the CCFLs (How? I don't know). Well, after the caps were replaced and some nice grills & fans were in place, we ended up with a panel with a dead lamp.
I think he'll be happy when I go to his place and with a dummy load solve his problem.
And thanks for the bezel offer!

[Bill Lee]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Dear sir,

Very interesting with your method to light up the backlight, I'm very new
to repair LCD, would you kindly show me which resistor in my BenQ DV3250
to replace? Image is attached.

[retiredcaps]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

'm very new
to repair LCD, would you kindly show me which resistor in my BenQ DV3250
to replace?

I suggest start a NEW thread and telling us what exactly is wrong with this monitor (is the power LED on, can you see the display flash, can you see it with a flashlight shined on it). We will also need a clear focused picture of both sides of the board.

[EGuevarae]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

I suggest start a NEW thread and telling us what exactly is wrong with this monitor (is the power LED on, can you see the display flash, can you see it with a flashlight shined on it). We will also need a clear focused picture of both sides of the board.

I agree. This thread has been dead for a year ...

[Bill Lee]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Glad for your quick responses. Please inform me how to start a new thread. ( Forgive me for so dummy...).

[PlainBill]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Glad for your quick responses. Please inform me how to start a new thread. ( Forgive me for so dummy...).

Click on Troubleshooting Computer Displays at the top of this thread. At the next screen you will see a blue button 'New Thread' on the left about half way down the page. Click on it.

Or you can just click on This Link.

PlainBill

[Bill Lee]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Thanks Mr.PlainBill, I'd just post the new thread. Please kindly be visit :
BenQ DV3250 backlight problem

[lord]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Hi,
About the resistor instead of CCFL, i tried 100k, 56k and some similar but they were all overheating (they were all under 1W). But i noticed that the current is always 0,2A (don't know why though).
Then i put in the 150R 1/2W and everything was fine (no overheating).

The reason i did this because i have a Qosmio G20 with a broken screen, and the replacement screen is hard to get because it has 2 lamps. So the laptop has a double inverter, and i need to disable one lamp, which i did. Put in the standard LP171WP4, everything fits and the job is done.


I noticed that some of you have a "dead lamp" problem. I fixed some lamps with resoldering the cables on the lamp leads (cold joints), but you must be very carefull since the tend to brake very easily.

Sory on my poor english!

Cheers!

Edit: Shit, the holes for the screws dont match
I have to make new ones...

[budm]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Typical CCFL used in monitor is about 8~10Watts
It depends on the running voltage that the inverter board will put out, usually about 800~1000VAC at about 8mA (Start up voltage is about 1500~2000VAC), so that is about 8Watts, so about 125K Ohms 8 Watts will be needed (@1000VAC). Your meter does not high enough bandwidth to measure AC voltage or AC current at 70~90KHz.
You can use CFL T5 4W lamp for testing or old CFL. At 150 Ohms, you will more likely burn up the high voltage winding of the transformer.
See pictures here:
http://s807.photobucket.com/albums/y...g%20old%20CFL/

Bad lamps/lamp assemblies:
http://s807.photobucket.com/albums/y...g%20old%20CFL/

[lord]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

OK. But the problem is that i tried out a couple of resistors, several resistances and wattage, but they were all overheating and through all of them ran the same current. When i put the 150 ohm resistor there was no overheating a no excessive current. All between 15-20mA. I don't know why would that burn the transformer. But then again, I'm not that kind of a expert.

[budm]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

It is basic voltage, resistance, current, and power math. Unless the transformer no longer puts out 800V and also the safety shutdown no longer function. You can calculate the current flowing through 150 Ohms resistor with an applied voltage of 800V.

[cfrantzol]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Hi everyone!

Here is the trouble I had and how I solved it:
A friend had a 17' Neovo F-417 that would flash the screen on for a couple of seconds then it would go black with the power light on and would latch on this condition until unplugged and restarted. He bought another one and since he knows I love dismantling things he gave it to me to play with.

I am relatively experienced with electronics (especially low frequency, linear and logic) but had never worked on an LCD or HT circuit before. So I had no other components of the sort (CCFL, inverter and the like) and could not test or swap anything.

Initially I changed the caps even though they seemed and measured ok. It worked fine for around 10 mins and then started over the same problem. So I swapped some caps, changed the rectifier bridge, resoldered all HT points, etc. What intrigued me was that after almost every change, it worked for a while and then reverted to the same problem. A couple of times it even worked fine for more than 30 minutes. This fooled me to think that there is a bad contact somewhere but I couldn't find it.

So I decided to test the lamps one by one (this monitor has 4 of them). I thought that since there was no pink tint or bad spot on the screen whenever it worked, turning each one on alone would reveal more.

I had no single lamp inverter to connect but I had a very old scanner stored somewhere since 2000. I took off its glass and removed its ccfl then used its inverter (complete with the scanner) to power each of the monitor's ccfls one by one. To do this I had to place the LCD on top of the partally dismantled scanner, in brief it was fun.

And it worked! Apparently those things are tolerant enough or I was lucky but it worked. With the help of the scanner's borrowed inverter I singled out one of the four ccfls which was completely used up. It lights very pinkish and turns off seconds later. All other three lamps appear ok.

So I googled and found this very informative thread and went to my bench to find a suitable resistor but I had none so I put 4 x 1.5MΩ/1W in parallel to form a 375 KΩ resistor to start with as low current as possible.

It worked again and the missing lamp is making very little difference on the display, it is bright enough and fairly uniformly lit. One could not easily tell if he did not know the mod.

So I wrote this post to reconfirm that it is possible to fire three lamps on a 4 lamp inverter with a simple resistor as the missing lamp. Furthermore in my case it worked with higher resistance and thus much less current than anticipated. Fyi this inverter uses an OZ960 controller chip.

As a footnote I have some doubts and maybe someone in this forum can answer them:
First will the amperage imbalance created by the decreased current in half of the output stage of the inverter create a problem in the long run?
And is it mandatory that the other 3 lamps will follow to their doom shortly or the failed ccfl could have been a weak one from the beginning?

Hope my confirmation will help other fellows to resolve their issues.

Thanks for reading!

[budm]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

The fault detector circuit will shutdown if:
1) The lamp is fired up by the start-up voltage (around 1500~2000V) but does not conduct the current, which will be the same as no lamp connected, it will shutdown.
2) The lamp draws too much current.

It will only take one bad lamp to shutdown the whole circuits. So in the first 3 seconds after the inverter is commanded to run, if it detects any of the fault in first 3 seconds, the unit will shutdown.
Instead of using pure resistor as the load, you can take the advantage of the Capacitor's Xc (Capacitance Reactance) as the current limiter and lower the resistor value so you will not dissipate so much heat on the resistor, the main key is that you will need to know the frequency the inverter circuit is running at so you can pick the correct cap with the the required Xc.
You will not damage anything with your setup right, just need to take care of the heat from the resistor.
You can use old CFL for testing the inverter circuits.
http://s807.photobucket.com/albums/y...g%20old%20CFL/

Here is the manufacture uses for testing their inverter:
http://s807.photobucket.com/albums/y...t=DSCF0614.jpg

Bad lamps:
http://s807.photobucket.com/albums/y...klight%20lamp/

[Chungalin]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

I was thinking about a different approach, instead of the resistor solution.

I have one bad lamp (out of six). Since every lamp output has two feedback lines (HV & LV) going back to the inverter controller, I've thought that I could cut the feedback from the bad lamp (now unplugged) and connect them to the same lines from a working lamp. This way, a working lamp would provide feedback for two lamps. Is it safe to do this experiment or better not even try? Am I missing anything?

[Chungalin]

Re: Using a resistor to fake a Inverter into thinking a CCFL is good

Sorry, can't edit my previous message to update.

I've tried my idea and it seems to work. I've dessoldered a blue, high voltage capacitor from the output of the high voltage transformer. Then made two bridges between two SMD diodes and two SMD capacitors. These were the first feedback checking components connected to the lamp output connectors.

Once powered on I notice a yellowish corner on the screen. I guess the bad lamp got hot for a long time before failing completely. Anyway, the screen is usable.