Old Dell 15" - 1 second lights up, then goes black

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

The original CCFL is about 310 mm long. I checked a spec sheet, and the rated brightness is 46,000 Cd/m^2

PlainBill

I was going to use 1206 SMD LEDs rated at 260mcd each. The tube is about 2mm dia and 310mm long = that means 2 * pi * r * h = 0.004m^2 = 223 cd for the tube. That means I'll need 1,000 LEDs+, which seems like way too much given that I've seen Edge LED screens with far less.

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Well the good news is, the LED strip I bought for £5 is really bright, and it looks like I can meet the original brightness of the CCFL. I burnt out some LEDs rather stupidly on one strip but it doesn't seem to have significantly affected it.

I'll need to drop 18V down to 12V at about 250mA, this means according to V=IR, I need a 24 ohm resistor, so I'll order a 27 ohm 3W resistor (the resistor will dissipate 1.7W, so I will attach it to the metal casing to help dissipate some of the heat.) I'll probably wire in a MOSFET to control the brightness too.

I need to figure out how to get the monitor to turn on without the inverter; it might be waiting for an "Inverter is okay" signal.

Now I know why to be annoyed when manufacturers want an extra £100 for an LED vs CCFL TV .

It's also interesting that this LED array only consumes around 4-5W but has almost the same brightness as 10-20W CCFLs. More efficient technology I guess.

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

Well the good news is, the LED strip I bought for £5 is really bright, and it looks like I can meet the original brightness of the CCFL. I burnt out some LEDs rather stupidly on one strip but it doesn't seem to have significantly affected it.

I'll need to drop 18V down to 12V at about 250mA, this means according to V=IR, I need a 24 ohm resistor, so I'll order a 27 ohm 3W resistor (the resistor will dissipate 1.7W, so I will attach it to the metal casing to help dissipate some of the heat.) I'll probably wire in a MOSFET to control the brightness too.

I need to figure out how to get the monitor to turn on without the inverter; it might be waiting for an "Inverter is okay" signal.

Now I know why to be annoyed when manufacturers want an extra £100 for an LED vs CCFL TV .

It's also interesting that this LED array only consumes around 4-5W but has almost the same brightness as 10-20W CCFLs. More efficient technology I guess.

Good job so far, now for a couple of points.

The signal card is not looking for an 'Inverter OK' signal.

The LEDs will always be on unless you use the 'inverter on' signal to control them.

There is a 4 pin version of the 78XX series of voltage regulators. One example is the Fairchild KA278RA05CTU. The additional pin is an enable / disable control. You can use that to drop the voltage and turn the LEDs on and off. A REALLY clever guy could even figure out how to use the brightness control to adjust the brightness (don't look at me).

PlainBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Good job so far, now for a couple of points.

The signal card is not looking for an 'Inverter OK' signal.

The LEDs will always be on unless you use the 'inverter on' signal to control them.

There is a 4 pin version of the 78XX series of voltage regulators. One example is the Fairchild KA278RA05CTU. The additional pin is an enable / disable control. You can use that to drop the voltage and turn the LEDs on and off. A REALLY clever guy could even figure out how to use the brightness control to adjust the brightness (don't look at me).

PlainBill

I wouldn't use a 78 series regulator because it's far cheaper to use a power resistor. Also, a 78XX wouldn't drop current, so thermal runaway is a real possibility - not pretty! If I were hyped up about efficiency, I'd use one of my buck converter LED drivers I designed, but I don't have the time at present.

For on/off, I figure I will just wire in the gate of the FET to the 18V_ON on the control panel. This comes out of the switch only when the monitor is on and could probably drive the FET into conduction.

Problem is, at the moment, the display is still white - no video signal is being sent to it. I cannot get the signal board to power up. I tried shorting various connections on the inverter board to turn it on with no luck.

How would the signal board know to shut down with a bad inverter/CCFLs, as was the case before?

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Good news is it is now working

Turns out it was a loose LVDS cable; it now works perfectly.

Interesting failure mode when powering the LEDs externally and the LCD is powered down. Very weird.

See video.
http://www.youtube.com/watch?v=ETL8q5nq1ro

I'm ordering the resistors and MOSFETs later, will see how it works when finished.

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Okay! The conversion is complete!

Assembly is all that remains... How many screws will be missing!? I tried to keep them in one place...

The circuit for the MOSFET on/off is really simple, and is attached.

I used an IRF510 because I had it on hand, if I were properly designing this I would order the right MOSFET. But the current is low enough that the MOSFET does not heat much.

I used a MOSFET because I'm not entirely sure that switch would be happy passing 280mA as at present, it just controls a MOSFET on the main board.

See the video on YouTube: http://www.youtube.com/watch?v=vevU_Fm6ap0

[sofTest]

Re: Old Dell 15" - 1 second lights up, then goes black

What LED strip did you use? I'm looking into doing a LED conversion on a 19", but is a bit unsure on which strips gives enough light. I've found 2-3 candidates, but it's nice to know what's been successful.

Thanks.

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

What LED strip did you use? I'm looking into doing a LED conversion on a 19", but is a bit unsure on which strips gives enough light. I've found 2-3 candidates, but it's nice to know what's been successful.

Thanks.

I used these:
http://www.giantcod.co.uk/high-densi...-p-404319.html

Perhaps if I were to do it again I would spend a bit more and get these as the display would look at bit brighter. It was originally fairly dim anyway.

http://www.giantcod.co.uk/ultra-brig...-p-406179.html

They are for model aircraft and I used the left over ones from the metre strip (about 30 cm) for my flying wing.