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Power for LED backlight

02/23/2022 9:39 PM

Hi Folks

I have a monitor that went dark and discovered the the PC board was not outputting enough for the LED strips for the back lighting. I am searching for what type of external driver I can use to power these.

This is the part number 06-32C2X6-618-M08W14

There are 2 strips 24" long. 6 LEDs per strip with power connector in the middle of the strip. This is for a 32" display.

I have another display that works; with 9 LEDs per strip, the voltage across each LED was 5.86V and the PC board driver connector was 53 volts going to each strip.

On this display I discovered the two were in series. The PC board output was 33V and one of the capacitors was bulged out so I figured the driver output was done in.

So I was thinking (6 x 5.8v = 34.8v) for the strips. I took them out of series and connected them in parallel to a 24V supply and thought it would at least have them half bright.

Well that was a bad decision because it burned out the last LED on each strip after about 2 minutes.

I have ordered more light strips and plan to have another go at powering them with an external driver but I am having difficulty finding any details on the current or voltage specs

Thanks

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#1

Re: power for LED backlight

02/23/2022 9:52 PM
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#2
In reply to #1

Re: power for LED backlight

02/23/2022 10:13 PM
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#3

Re: power for LED backlight

02/24/2022 3:45 AM

5.8 Volts is very high for the forward voltage of an LED.

On the board you're working on it looks like the forward voltage of each is 33/12 (although of course you can't really rely on that 33V reading from a faulty power supply).

Dried out (swelling) electrolytic capacitors is easily the most common cause of failure in power supplies. If you can use a soldering iron they're easy to replace, and cheap to buy from Digikey, Mouser or other local electronics outlet. You can read the value, Voltage and temperature rating off the side of the can. Try to replace with long life / higher temperature rating. The life expectancy doubles for every 10 degrees C difference between the ambient and rated temperature. I'd just replace them all.

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#4

Re: Power for LED backlight

02/24/2022 7:46 PM

LEDs have a threshold voltage, which depends on the color. It has to do with different color photons having different energy. When the voltage exceeds that then the current increases rapidly. White LEDs consist of a blue or UV LED which excites a yellow/white phosphor resulting in white. There may be a difference in color and voltage between manufacturers.

If you drive LEDs with a constant voltage supply, a small error calculating voltage can result in a large error in current. It's best to drive LEDs with a current regulated power supply.

https://www.ti.com/lit/wp/slva130/slva130.pdf

https://electronics.stackexchange.com/questions/453467/two-different-led-types-diode-test-on-two-different-multimeters-shows-just-1

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#5
In reply to #4

Re: Power for LED backlight

02/25/2022 12:48 AM

Yes, I thought 5.8 Volts was really high too. The chart Rixter had was more in line with what I remembered. So next week in the shop I'll check the amps on the larger display that still works and wait for the replacement LED strips to come in. I may even have a capacitor in my spare parts to throw on for a trial.

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#6

Re: Power for LED backlight

02/25/2022 7:12 AM

The voltage may make the LED conduct forward and turned on, but It is the proper amount current that makes the LED glow brighter..

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#7

Re: Power for LED backlight

02/25/2022 10:48 AM

That is usually a blown electrolytic capacitor on the main board that causes the back light to go dark. Pull the board and look for puffed up caps and just replace them.

I have also encountered broken solder joints on many boards using lead-free solder for through board solder joints. I don't thing the new alloys are as good at preventing tin pest as lead solder.

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#8

Re: Power for LED backlight

02/26/2022 2:01 AM

I sympathise with your predicament as I work on circuits often. I have found that low value electrolytics fail more often then the higher value. My method on multilayer boards is to remove the suspect electro by cutting off close to the board. The leads can then be unsoldered and the holes cleaned out with a desolderer, a solder sucker or desoldering braid.

I have found that the non lead solder tends to form dry joints and fail in what we term frogs eggs, where the solder forms into clumps of silvery grey speckles.

Surface mount capacitors are a whole different kettle of fish but they can be replaced if one has the right heating gear or very fine soldering iron tips. As for LEDs I have come across a few in strings where one may become open circuit or worse still short and take a few more out from excess current.

Happy repairing.

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#9

Re: Power for LED backlight

02/28/2022 10:01 AM

As Rixter has stated, LED's are current-driven devices. If you are going to drive them from a voltage source (vs a current source), you should always have a current limiting device in series, such as a series resistor. Otherwise, you risk exactly what happened, you burn out LED's because the max forward current is exceeded.

Your best bet would be to repair the existing current driver in the monitor, so that you will still have adjustable brightness settings from the front panel. If you can't fix the existing driver, look for a constant-current source supply that will output the correct current, in the voltage range needed for the LED strips.

Tom

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