how to test lcd led backlight?

[Peter Chiu]

Re: how to test lcd led backlight?

Let's go back to {how to test lcd led backlight?}.

I try to use CCFL Tester ....to light up the LED section (This is the super thin LED screen , 2 stripes from top and bottom reflect to all screen with thin pieces of prism plastic. don't know the name, yet.(from 60' thin LED TV)).

one LED section has 60 LEDs (top and buttom total 4 sections , one section 3.3v x 60= 181.8v DC votage needs be to light up the LEDs . I use my 50v dc lab power supply cant do anything!)

But, strangely , just by connect the black ground wire to the LED strip , I accidentally touch the on button..... so the CCFL tester is turn on.......the LED strip light up a little bit .... show in (thumbnail 2) .....I just connect the ground wire .....and push the on button.... the LEDs light up faintly........ the it's a miracle happen right in my eyes....








......Working...... from my theoretically..... use HV and low current power source ( the low wattage is the key)... .....to drive unknown LEDs section

controversially... I use Lab power supply... kill a lots of 10w leds by over power them!

[budm]

Re: how to test lcd led backlight?

For Mains Isolation--Again Cheap Cheerful, --with possibility of voltage customisation (within limits)

Two std low-voltage transformers--Say a pair of lighting Toroids or whatever you've got around--providing they the same O/P voltage, and the same/similar VA rating/size.....

Connect the first Secondary to the Secondary of the Second.

Connect the First Primary to the mains...

You'll have an isolated 240V (Assuming mains is 240V) at the primary of the second.....

I use two 650VA toroids like this--as a Mains Isolation Transformer to run sets from--Works pretty well!

That is what I am experimenting right now (two 120V:6V), the output (120VAC) is fed into voltage doubler circuit, I am using VARIAC right now but will build the drive circuit to drive the 6V winding with feedback to maintain current.
I use this method for other project to get high voltage with low current.

[Alastair E]

Re: how to test lcd led backlight?

For Mains Isolation--Again Cheap Cheerful, --with possibility of voltage customisation (within limits)

Two std low-voltage transformers--Say a pair of lighting Toroids or whatever you've got around--providing they the same O/P voltage, and the same/similar VA rating/size.....

Connect the first Secondary to the Secondary of the Second.

Connect the First Primary to the mains...

You'll have an isolated 240V (Assuming mains is 240V) at the primary of the second.....

I use two 650VA toroids like this--as a Mains Isolation Transformer to run sets from--Works pretty well!

[tom66]

Re: how to test lcd led backlight?

One thing is not mention using Cap for Vdrop is the Voltage on the load at the first half of the cycle when power is first applied, the cap will be like a dead short (think of the power supply cap when you first apply the voltage to it, you will see current surge when power is first applied) for that first half of the cycle so you will need the clamp at the output.

That is a good point, at zero crossing, the circuit has no inrush but at all other conditions it does. To avoid this you can attach an output rectifier, although this may be increasing the complexity too far; C2 is a 10uF/400V electrolytic.

[keeney123]

Re: how to test lcd led backlight?

To me it would be simple to standardize these LED strings so they're are not so many different types that achieve the same thing, to light up a screen and be able to make a few adjustments for ambient light. Back in the 80's Apple thought they would be smart by changing the RS232 pins. They lost that battle because IBM standardized those pins though the Electronic & Electrical Engineering Handbooks. When Apple started not selling computers because one had to buy their products, then they changed their design to match the rest of the world. Standardizing things saves time and money and makes technicians and customers happy.Most companies do this type of protectionism because they want people only to buy from them. Besides the obvious problems this causes it also stifles innovation. Instead of relying on their companies ability to come up with innovative ideas they are relying on there protectionism in order to be the leader of the industry.

[Peter Chiu]

Re: how to test lcd led backlight?

I thought about that too.

Digital way:
Use PIC 18F8720 microcontroller write some program. go up 1 mA and step up, until Cds cell detect the LED light up , with IGBT now available in a range of voltage ratings from 300 to over 1200 volts should be enough. With lcd display can give voltage and current mapping. Basically, it is micro-controller (with IGBT) power supply.

Analog way:
Cheap and dirty, use CCFL lamp tester, which is an oscillator(like IC 555). Efficient way to make any HV voltage but low current. If I set up the current limited to 100ma even I inject 1500v, the voltage will drop a point call wattage balance, so few resistor will do the job. Other option is use feedback current limitation circuit,now most switching power supplys all have this future.
LED driver circuit from TV will be a good way to do modification to universal LED tester.
All we need are LEDs light up for 5 seconds to do the inspection. When I made it ,I will let you know.

......LED string, or the string may be connected in series/parallel setup.....

I think will come in the service manual or we can find in Ur best collation.
great site for LCD panel info:
http://www.gblcd.com/data_center_lcd_panels.htm

We waiting for Ur......... great site for LED panel info: ............

Links help:

http://electronicshelponline.blogspo...-led-back.html



http://www.dhgate.com/product/high-p...0-1|2731706408

http://www.linear.com/product/LT3956

[budm]

Re: how to test lcd led backlight?

One thing is not mention using Cap for Vdrop is the Voltage on the load at the first half of the cycle when power is first applied, the cap will be like a dead short (think of the power supply cap when you first apply the voltage to it, you will see current surge when power is first applied) for that first half of the cycle so you will need the clamp at the output.

[budm]

Re: how to test lcd led backlight?

"To: budm
A HV adjustable voltage with adjustable current source (at 240v x 1.414 = 340volts DC) for Lab, is not cheap.
U Just like a high rank kung fu fighter.... one touch ......he won. An HAM (Radio Amateur) can use very simple parts, like a piece of cooper wire ,a crystal and make a radio.

And your secret weapons are the current limited lamp and non polarized full bridge !!!"
That is not my design to use direct power line to drive the LED, it ALASTAIR ideas I suggested to add the bridge so the polarity of LED string will not matter, by the way, there is no filter cap so the there will no 340VDC, it will be on average DC since the LED will only light up half of the line frequency only. One thing I will not do is to use direct line operate to test the circuit without isolation or safety equipment in place.
I am still in the progress of making high voltage constant current source.
"It's kinda of hard to Calculate and to find the right capacitors for different LED screen applications. " that is the key since you do not know how many LEDs are in the string so the current source has to have high enough Voltage to provide enough total Vf of the string, but if you have to open the panel to get to the string then you might as well just check each LED.
So you have LED backlights that you are not getting any lights so the purpose of trying to testing the LED string is to find out if the string is bad due to bad LED or bad connector or the LED driver is bad. Some board setup can have 8 wires going to the LED assembly so that is hard to find out which pair of wires go to the LED string, or the string may be connected in series/parallel setup, so using LED lamp tester is only small part of troubleshooting, figuring out how the LEDs are connected and driven are harder part.
I use DCA to find out if the string has open circuit or not after I am sure the power supply is OK.

[tom66]

Re: how to test lcd led backlight?

For up to 80V across the LEDs the current will remain roughly constant, within about 20%. This is sufficient for a simple test of a backlight array.

Of course, I'm not a massive fan of using this circuit without an isolation transfomer, as it puts the LEDs at mains potential. Should someone attach, for example, a grounded scope probe to the set (it's easy enough to forget) they could end up with a live-to-earth arc inside the panel. This could cause an electric shock, or damage the diffusers, LEDs, LCD panel. So, it will be best to use this circuit only with an isolation transformer.

[Peter Chiu]

Re: how to test lcd led backlight?

Thanks ....Good to know...... another way to achieve the goal.

When capacitor at
DC.... current =0
AC..... a small amount of current flow through it (proportional to its capacitance).*

Capacitance (Farads)=1/ (2 x 3.14 x (50hz or 60hz) x Impedance of the capacitor Xc)

It's kinda of hard to Calculate and to find the right capacitors for different LED screen applications.

http://www.vintage-radio.com/repair-...per-calcs.html

Next project:
How to use LED screen to grow {plants} hydroponically, and they won't find out by high electricity bill.

[tom66]

Re: how to test lcd led backlight?

Another alternative; Use a capacitive dropper. 1uF in series with AC mains is about 100mApk at 230V and 50mApk at 115V. Common to find 1~2.2uF X-caps on power supplies. D1 rectifies the output via charge pump action (can be 1N4007 or so.)

This is the same circuit as used in most cheap made-in-china LED ballasts. It has the advantage of being lossless, in theory. So the capacitor only needs to be rated for a peak voltage, rather than power rating. R_disch will discharge X-cap after mains is disconnected.

[Peter Chiu]

Re: how to test lcd led backlight?

To: Alastair E
15w/240v(AC) =15w/340v(DC)= 0.044A=4.4mA maybe too low !!!!
According to my calculation .....331V(DC)*0.289A=95.659W ...better use 100w 240V(AC) Bulb ,But I will use 60w lamp for safety. .....wow!!!! if works, this tester almost free...!!!!

For the beginner: AC lamp or DC lamp ?.... Caution.... use Only incandescent lamp.......... Basically just a piece of high resistance wire, so it does not care DC or AC.

To: budm
A HV adjustable voltage with adjustable current source (at 240v x 1.414 = 340volts DC) for Lab, is not cheap.
U Just like a high rank kung fu fighter.... one touch ......he won. An HAM (Radio Amateur) can use very simple parts, like a piece of cooper wire ,a crystal and make a radio.

And your secret weapons are the current limited lamp and non polarized full bridge !!!!
And my secret weapon is light Dimmer. (a cheap current limiter..should say a wattage limiter).

...............Caution..........LIVE circuit......240V AC

Next round we will Compete again...Ha....Ha.....

Next project 1:
How to use laying-around- CCFL-lamp-tester (900v--1500v) to test LED screen......

Next project 2:
How to use LED screen...... to light up my life.......

[budm]

Re: how to test lcd led backlight?

Ah--Sod it. How we would do it red-neck style in deepest darkest Wales.....

15W 240V Bulb in series with a rectifier, connected to 240V mains supply.--wire DC output from bridge to the LED String (No res cap) and a Switch across the rec DC side...

Switch is wired in such a way as to be across the LED String when first power is applied--to allow the lamp to light and therefore, be at max 60mA

Switch 'Off' position--LED's should light and lamp will dim.
--Observe polarity of LED string and the supply from the rec.....

Purpose of Switch is to avoid the high current that could pass with cold lamp--that could over-current the LED's...

Cheap, Simple, No Fuss! I'll try it maybe tomorrow with some old edge-leds from panels Ive got around....

Use the bridge so you do not have to worry about the polarity of the LED string. Hopefully the Hot resistance will stay high when switch over occurs. The lamp can be replaced with active device to allow to set the current, the problem will be that the Vf of total string will have to be met.

[Peter Chiu]

Re: how to test lcd led backlight?

Wow! cheap like me. Ha! Ha! I like it.

because Vs and Is are critical

VSNS Pin9

Voltage sense input pin. The voltage signal on this pin feeds the over voltage detection circuit. A threshold of 2V is set internally for fault detection. When the voltage sense signal reaches the threshold of 2V, a clamping current source will be activated to pull down COMP signal to limit the sensed voltage. In the meanwhile, the fault monitoring circuit will start counting the accumulated fault duration and set up the fault
flag when the fault count reaches pre-determined counts.

ISNS Pin10

Current sense input pin. A full wave rectifier circuit is built in to allow AC input to this pin. The regulation point at this pin is 421mV which is amplified by 4.75 times and then fed to the error amplifier to compare with a reference signal of 2V to regulate the LED current. This signal is also used to detect open LED, over current and short circuit conditions. The threshold for open LED is 100mV, the threshold of over current is
600mV, and the threshold for short circuit is 930mV. A fault flag will be declared if ISNS signal stays below 100mV or above 600mV over defined counts, and the operation will be cut off immediately when ISNS signal goes above 930mV.

http://www.google.com/url?sa=t&rct=j...89381419,d.cGU

you gonna need adjustable dummy load! If not, let us know....A big saving !!!!
Maybe lx27901 (Comp Pin11) to ground, to limited the error signal ?

http://circuitsbook.com/adjustabledummyload.html

http://hackaday.com/2010/08/06/dummy...nd-heat-sinks/

Next project :
The poor man’s BGA rework station with Optical Alignment System from scrap BY dumpster diving.
Hint 1: Optical prisms could be fonnd in old DLP TV.
Hint 2 :Cheap usb webcam(with cable broken can be find in dumpster too.) will be A+
Hint 3 : Keep all the hair dryer U found in the dumpster, I personally use pop corn
machine but now is hard to find (collectible item?).
Hint 4: Old PC XT, we will use the case and power supply (Easy to find).
Hint 5: This one need to be cautiously , because the ex-owner maybe dead. ( inhaling machine u found in the dumpster ,destroy it, just get vacuum pump out. I use my son's machine.)
Untill now should be cast $0 .............To be continued...............

[Alastair E]

Re: how to test lcd led backlight?

Ah--Sod it. How we would do it red-neck style in deepest darkest Wales.....

15W 240V Bulb in series with a rectifier, connected to 240V mains supply.--wire DC output from bridge to the LED String (No res cap) and a Switch across the rec DC side...

Switch is wired in such a way as to be across the LED String when first power is applied--to allow the lamp to light and therefore, be at max 60mA

Switch 'Off' position--LED's should light and lamp will dim.
--Observe polarity of LED string and the supply from the rec.....

Purpose of Switch is to avoid the high current that could pass with cold lamp--that could over-current the LED's...

Cheap, Simple, No Fuss! I'll try it maybe tomorrow with some old edge-leds from panels Ive got around....

[Peter Chiu]

Re: how to test lcd led backlight?

Let's go back to {how to test lcd led backlight?}

Samsung 55' LED TV (BN44-00499A)(Panel No.DE500BGM-V1), they used 331v DC289mA to driver the two groups of LED s ( distinguish as D1 group and D2 group)
Marked D1+ D1- D2+ D2- , and D1- D2+ are connected together. controller is LX27901 .
Use dummy load to light up one grooup of LEDs ,then another group LEDs have problem.

The best links I could easy understand.

http://www.google.com/url?sa=t&rct=j...DxyMIY0VtDnWDw

[tom66]

Re: how to test lcd led backlight?

Not as simple as that. Introduces yield issues, LEDs are made from different types of semiconductor (GaAs is quite common), GaAs doesn't make good transistors.

One possible solution would be a zener diode across the LED, the zener would conduct, heat up then anti-fuse shorted if the LED went open.

[Peter Chiu]

Re: how to test lcd led backlight?

LED is a diode too, when manufacture do the masking, it wont cost them not too much to do it.
Basically just a P-N junction.

[tom66]

Re: how to test lcd led backlight?

? Auto-recovery LED screen ? Resettable Fuse ? Auto-bypass LED?

ST has LBP01 part can bypass any defected LEDs

Benefits
■ Keep LED strings on in case of LED open mode failure
■ Reduced maintenance cost
■ Increase lifetime of the lighting system

https://cdn.badcaps-static.com/pdfs/...34696944c0.pdf

But, new LED TV used 2000 LEDs to make more brighter LED screen. I predict ( Auto-bypass LEDs will be use to build Auto recovery LED screen) in the future !
(I mean, LED will build in bypass circuit in the chip which just two more diodes. )

Tips: So next time don't put a cooper wire cross the defect LED put a diode.This allows the right amount current to flow through the other LEDs.

Problem is, who's paying for this? Manufacturers won't care too much unless it lowers cost of production or reduces in-warranty failures.

[Peter Chiu]

Re: how to test lcd led backlight?

Constant Current Driver for LED Backlights Tester

https://cdn.badcaps-static.com/pdfs/...eeae3f24ee.pdf