phillips 32inch led back light voltage drops

[elpiko]

hi all i have 32 inch phillips tv that turns on show logo then blank screen. so i took it apart, it has a total of 12 leds on two strips of 6 on a strip.

ive tested both strips with 24v and all of them lights up but was flickering in dont know if that was due to my power supply i tested it with not making good connection but all led lights up.

now when you turn on the leds flashes bright briefly then off, testing the voltage it spike up to 40v then drop to 13v and stay there. i tried testing the board without the leds connected i still get the spike of 40v then drop to 13v .

ive check caps they all look ok. any idea?

[budm]

Re: phillips 32inch led back light voltage drops

1) The output will Voltage will spike up if it sees no load, then the protection circuit kicks in and shutdown the boost converter circuit so the Voltage you see is the feeding Voltage (13V) to the boost converter.
2) Did you test the whole LED string will require more than 24V with current limiter by connecting the power source to the LED connector (LED connector not connected to the board) that goes to the power supply board.
3) LED should not be flickering when tested.
4) Do you still have the pictures of the LED strips and how they are linked together?
5) How many wires are there on the LED connector?

[elpiko]

Re: phillips 32inch led back light voltage drops

i tested one led strip which has 6 led on it with a laptop charge of 24 volt. it was lighting quite dim and fliker dont know if it was due to lose connection .

the two strips are totally separate from each other. each have two wires then goes in one connector where its connected on power supply.

[dick_barton]

Re: phillips 32inch led back light voltage drops

Dim and flickering sounds like a faulty led(s) on the strip.
How many leds were you attempting to test with the 24V supply?
24V on 6 leds is likely to fry the leds if they are 3V leds.

[elpiko]

Re: phillips 32inch led back light voltage drops

i tried to test a strip with 19 volt at first it did not light up so i tired 24v and it light up but dim. if i connect the power supply and connect both strip and power on all the leds light up for few seconds and then turn off.since the voltage i boosting up to 40v and i have 12 led in total that makes each led 3.3v i guess so the 19v should have light up a strip fine but didn't

[dick_barton]

Re: phillips 32inch led back light voltage drops

Take a video of the leds as you power them up. Check it frame by frame and you will find that some leds are not working.

[Repair_dane]

Re: phillips 32inch led back light voltage drops

Adjust the brightness on the video when recording, so you can see every single LED clearly. LEDS are very bright for human eyes and can be tricky to spot when faulty

[budm]

Re: phillips 32inch led back light voltage drops

"the two strips are totally separate from each other. each have two wires then goes in one connector where its connected on power supply." Are you saying that the two LED strips are connected in parallel?
The LED power supply is rated at 40V as shown on the print on the circuit board.
If the LED strips are connected in series then the Vf at high current will be 12 LEDs x 3V = 36V.
If LED strips are connected in parallel then then the LED will be 6V LEDs.6 LEDs x 6V = 36V.

[elpiko]

Re: phillips 32inch led back light voltage drops

so you think the problem is with the leds rather than the power supply? i will try to the the tv apart again to record the leds.

[elpiko]

Re: phillips 32inch led back light voltage drops

hi so i took the tv apart again but just to note that even if the back light is not working if i use a flash light i can see image on the screen. so i use an led bulb circuit to make an led tester. one of the strips works fine but the other one does not light up.

so i took the small cover off an notice one of the led is burned. since i had no spares led from an old tv i used one of the led from the bulb ive took the circuit from as replacement.

when i test the strip now all led light up but the replacement is a bit dimer than the original but still light up. now putting back the tv together i thought all would be fine since all the led are lighting up with my tester. but i get same problem all led flashes show logo the led goes off when i power on the tv.

could it be the replacement led ive used lower voltage than the other so its detecting a fault or something?

what if i bypass the tv circuit that powers the led and i use a separate one like the test ive made to power the led then my tv will work again would there be a downside to this?

[elpiko]

Re: phillips 32inch led back light voltage drops

hi so i took the tv apart again but just to note that even if the back light is not working if i use a flash light i can see image on the screen. so i use an led bulb circuit to make an led tester. one of the strips works fine but the other one does not light up.

so i took the small cover off an notice one of the led is burned. since i had no spares led from an old tv i used one of the led from the bulb ive took the circuit from as replacement.

when i test the strip now all led light up but the replacement is a bit dimer than the original but still light up. now putting back the tv together i thought all would be fine since all the led are lighting up with my tester. but i get same problem all led flashes show logo the led goes off when i power on the tv.

could it be the replacement led ive used lower voltage than the other so its detecting a fault or something?

what if i bypass the tv circuit that powers the led and i use a separate one like the test ive made to power the led then my tv will work again would there be a downside to this?