Using Standard LED Power Adapter To Replace And Fix Old CFL TV Backlight Problem
About 2 years ago I was checking an old Samsung T200 monitor that had a strange backlight problem. It was brought to me by an old friend.
This 20 inch monitor still had old cfl tubes and the screen went dark right after about a second every time it was switched on. The white power led indicated that the monitor was still switched on but the screen showed no active backlight after that.
Samsung manufactured this monitor in 2008 and it was put aside for a long time because I couldn’t find what was wrong with the 20 inch T200 monitor. Because I already had checked both the 5pin STV407D mosfets that drive the HV backlight transformers controlled by the 20 pin FAN7311 IC. And also both HV transformers where fine.
And probably the Mckinley 20.1” power board was fine too and so there was no need to buy another power board that in used condition was still available on Amazon for $45.
Until I yesterday rechecked the HV output with a couple of good cfl tubes I managed to disassemble out of another scrapped for parts monitor. And I noticed that this time again these good cfl test backlight tubes went dark after one second but several seconds later they fully lit up. Indicating that indeed the backlight controller with HV transformers and both mosfets worked splendidly but the original T200 cfl tubes must have gone bad somehow.
So today I also opened the old Samsung LCD panel to be able to remove both cfl tubes for further inspection. Removing them from the LCD was not hard. Opening the monitor housing was the hardest part which I already did about 2 years ago. Both old cfl tubes were just like clipped on onto the top and bottom sides of the screen. And by carefully removing the metal frame with back cover from the LCD panel plates and also unscrewing the 3 screws with which the LCD pcb is still attached to the metal cover, without breaking the still attached flex prints, we create enough space to carefully remove both cfl tubes with their reflectors from top and bottom side. Which also needs some freeing up on the cfl tube side wiring that is also holding the backplate that we need to remove before we can completely remove both cfl tubes.
Further tests with the original now removed cfl tubes indeed showed that the tubes poorly lit up and compared to my good cfl testtubes hardly were bright enough, now only showing a weak pink light. So this time new backlight leds are needed to fix the T200 backlight problem.
Above led strips show what type of leds that I am going to use to replace the now bad cfl tubes. They came from large strips I bought on Aliexpress in the past and were cut from the too large strips because I only need two strips of about 45 cm in length for the top and bottom side of the 20 inch T200 Samsung. These backlight led strips count about 17 leds per 10 cm. I originally planned to use these long strips in another monitor. But the bought step-down converter that was supposed to power up these backlight leds went up in smoke just after a few minutes of stepping down the about 25V input voltage to the lower led voltage needed. Although the module was said to operate up to about 30V of input voltage, the module went so hot immediately without any decent cooling that it was just a crap product. And it neither had any cooling block or fan. Therefore this time I wanted to try a different power adapter to feed the led strips which I will show here, that not only works excellent but also doesn’t cost much and can easily be a DIY circuit too. First I will show the now new working led strips I placed in series onto the led power adapter that I will show after that too.
First photo in the top right shows the blue-ish white led light that escapes from the top left side.
Above photo shows the bottom left side with also second new working led strip.
Above photo shows how after replacement both led strips are wired and come out of the left backside of the monitor. The 4 cfl HV output connectors are no longer needed and hopefully can be de-activated without any issues. I placed both strips in series, red1 +input, white1 to red2, white2 gnd.
And now I’ll show what simple and excellent led backlight power adapter I found working very well.
It uses a capacitor instead of a transformer and makes it a cheap and simple to make working led backlight power adapter. The only thing I will like to add for safety reasons is a cheap but trustworthy fuse to present a good circuit breaker in case there might arise some overcurrent problem. This way preventing any fire risks in the backlight circuit. Since the used leds are white 3V leds like blue leds are, this circuit is excellent suited for feeding our backlight circuit.
Next I’ll also show what voltage and current I measured over both led strips in series.
So that looks quite reasonable in measured voltage and current wise. These strips are made out of in groups of 3 placed leds in series. If we take the last 3 leds of this particular strip and place about 8V over them, they will also lit up all other groups of each 3 leds in series at also about 0.01 A of current.
When we also need to cut these strips into the necessary monitor size in inches, we have to cut them in the correct 3 led group length. A last group cut into a group of only 2 leds won’t lit up on the strip end anymore. In other words… only all complete 3 led groups will lit up afterwards.
I also measured the C1 capacitor and the e-cap used in the backlight power adapter circuit.
The e-cap was marked as 3.3uF (400V !) so it was in good condition. The 560nF capacitor (~250V) would be even better if it was a safety X2 type which it probably isn’t. My transistor tester probably is bit out of tune because it keeps nagging that I need to calibrate the device again. Anyway a safety fuse and a X2 capacitor make this simple led power supply circuit safer when needed.
Above the now with new led strips and new led power adapter circuit fixed monitor. Back in action.
Above my DIY vga tester with 12Mhz over oscillating cheap PIC, but good working monitor tester. This vga monitor tester can supply different test patterns with the 2 pushbutton up/down control.
It was made on a cheap pcb using a wiring pen to make all connections. Last but not least follows the solder side of the here used cheap but excellent working led backlight power adapter.
Both smaller resistors on this board are 470 kOhm (to discharge the caps after this circuit is de-activated). The larger one is 470 ohm, and the diodes are all 1N4007 which can handle up to 1000V!
And this small backlight power circuit can easily be mounted inside the monitor. At best glued and or with tie-wraps safely attached to the monitor case.
I think that after this fix this monitor will work again for many hours when leds and the used led power supply circuit keep working.
It at least will work much longer than the useless step-down led backlight power module with these led strips ever did! Because that crap step-down led backlight module never was true to its promised specs!
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous article on Investigating In-& Outdoor DVB-T2 Antennas