Led back light

[Rickig]

Hi I have a back lit led tv that I suspect a led has gone down as the backlight no longer illuminates
My question is what s a safe voltage to put across each led to test it and when I find it what size resistor would be best to solder in its place to bypass that led.
Thanks

[ReeceyBurger123]

Re: Led back light

2.2k ohm resistor or 3w zener diode in reverse bias to bridge over the Led. To check each led you can use your Dmm put the probes on the respective polarity of the Led and if its good it should slightly illuminate.

[Rickig]

Re: Led back light

And if it doesn't?

[steviewonder]

Re: Led back light

then the LED is duff..a 3v supply to each individual diode should provide illumination.
Often,the led itself is visually burnt.
Good luck!

[flocko]

Re: Led back light

LEDs will be open, short or working, if its short your meter will show this, if it is open it will show that also, and if it glows its working. You fail to tell use the make and model number of the set you are trying to fix? not all LEDs are the same voltage ?

[ReeceyBurger123]

Re: Led back light

And if it doesn't?

Well if it doesn't light what is it ? Dead !

[retiredcaps]

Re: Led back light

To check each led you can use your Dmm put the probes on the respective polarity of the Led and if its good it should slightly illuminate.

Be aware that multimeters vary greatly in the output voltage when in diode mode. For example, the Fluke 87V will output more than 7V in diode mode. I have a Brymen meter that only outputs 1.5V in diode mode.

[Rickig]

Re: Led back light

That's what I was referring to... What if the meter doesn't output enough voltage to illuminate it. So 3v should be ok?
Bush bmk40

[budm]

Re: Led back light

You should put the current limiter resistor in series with the power source when you try to test the LED.
http://led.linear1.org/why-do-i-need...r-with-an-led/
https://www.sparkfun.com/tutorials/219

[ReeceyBurger123]

Re: Led back light

The meter will have enough juice to dimly illuminate the leds.

[stj]

Re: Led back light

i test led's with an old coincell from a motherboard - i have lot's of used ones.

[Rickig]

Re: Led back light

Ok I will check them all and when I find the bad one I just solder the traitor in its place and it should resume normal function? Minus some brightness of course

[ReeceyBurger123]

Re: Led back light

It wont be too noticeable the brightness difference and what do you mean by traitor ?

[Rickig]

Re: Led back light

The meter didn't power them had to use a battery. Have 6 LEDs out. 8 strips of 5 led and 3 on 2 strips gone. I've removed the bad ones... Any idea where a can get replacements?

Spell check -- traitor = resistor

[Rickig]

Re: Led back light

http://www.maplin.co.uk/p/3w-high-power-white-led-n79jq

Will these do?

[budm]

Re: Led back light

http://www.maplin.co.uk/p/3w-high-power-white-led-n79jq

Will these do?

That is really big because it is 3W, you need the dimension of the old LED and the solder pads, 1W should be OK to use.
What does the one in the panel look like?

[Rickig]

Re: Led back light

http://www.maplin.co.uk/p/white-led-n67jq

These?
My board has 5 LEDs and says vf max 18v so I assume each led is 3.8v Ish?
Says if= 320ma the above is if 350ma. That ok?

[budm]

Re: Led back light

http://www.maplin.co.uk/p/white-led-n67jq

These?
My board has 5 LEDs and says vf max 18v so I assume each led is 3.8v Ish?
Says if= 320ma the above is if 350ma. That ok?

So the LED strip has all that info printed on it, 18V MAX, If = 320mA? 18V/5 LEDS = 3.6V
So you will be running at lower current which will be fine, I also doubt that they are running them at MAX If.

[Rickig]

Re: Led back light

Yes says that on it.
Original is if320
The maplins are 350 that OK?

I noticed on maplins page it says heat sink required..... Will I really need that's?

[budm]

Re: Led back light

"The maplins are 350 that OK?' yes, you are going to be running at the lower current, but I do not know if it will have color shift at lower current drive or not. The existing strip is probably using the copper pads for heatsink, so if this LED should be able to handle it, we still do not know what your LED strip look like, the original probably does not have good thermal management either so they burn out.