UN55D6000 Blinking Standby Light

[tom66]

Re: UN55D6000 Blinking Standby Light

approximately 20 white LEDs They are non linear so can only roughly be approximated with a resistor.

[mmartell]

Re: UN55D6000 Blinking Standby Light

If everything works out I intend to replace them. However my electronics math/theory is still in its infancy so I need my hand held figuring that resistor value.

Before you said the strings were in parallel I figured a 120ohm 8 watt resistor (!) but even if that WAS right for a series string wouldn't it be different in light of the fact they are not wired that way ?

[tom66]

Re: UN55D6000 Blinking Standby Light

You need to know the current through the LEDs to calculate the resistance which produces a desired voltage drop.

e.g. you need to drop 165V when 0.1 amps flows, R=1650 ohms (V=IR, I=V/R, R=V/I) and power dissipated would be VI or V^2/R or in this instance about 17 watts. So choose a 25 watt wirewound resistor.

However, you do not know how much current each LED string draws; you need to measure that before calculating R.

[mmartell]

Re: UN55D6000 Blinking Standby Light

The board says 330v/280mA for that circuit is that not what we need ?

[tom66]

Re: UN55D6000 Blinking Standby Light

Probably a design maximum.

[mmartell]

Re: UN55D6000 Blinking Standby Light

But this is chicken and egg I can't measure the current flowing through the string without the string working. Is it advisable to use the design maximum or like 80% of it to make the calculation ?

[tom66]

Re: UN55D6000 Blinking Standby Light

Can't you measure it through the working one?

[mmartell]

Re: UN55D6000 Blinking Standby Light

Power supply won't even fire up with the backlights the way they are. In fact I originally thought I needed a main board, then thought, no the power supply is fubar then realized the backlights were possibly the problem the whole time. I think confirmed by the shorted led's.

So in fact this is the first kink in the chain that needs ironing out before I can say any of the other boards are good or not.

[tom66]

Re: UN55D6000 Blinking Standby Light

So, how did you get the LEDs to light up? Did you test them individually?

[mmartell]

Re: UN55D6000 Blinking Standby Light

With my DMM as you suggested

[tom66]

Re: UN55D6000 Blinking Standby Light

Oh... my error then. I don't know what you can do, unfortunately .

[mmartell]

Re: UN55D6000 Blinking Standby Light

So I guess you don't suggest using a resistor based on say 80% of rated current capacity ?

Or what if I wanted to halve the PS output so I could use the one known good string, would the value be 165v/.28A and then V^2/R for the wattage or is that not advisable, too ?

[tom66]

Re: UN55D6000 Blinking Standby Light

How would you halve the PSU output?

[mmartell]

Re: UN55D6000 Blinking Standby Light

Can't you use resistor(s) to drop the voltage 165v or are we back to the unknown amperage thing ?

[tom66]

Re: UN55D6000 Blinking Standby Light

Yes, original problem returns with finding I.

[mmartell]

Re: UN55D6000 Blinking Standby Light

Ok tom thanks for your help and patience as always. I will try to find another strip, or one to harvest good led's from anyway.

[mmartell]

Re: UN55D6000 Blinking Standby Light

Well of course I got impatient and put a wire across the two open diodes...

SUCCESS !!

[mmartell]

Re: UN55D6000 Blinking Standby Light

Sort of...

As can be seen the led's on the bad strip are quite a bit less intense on the top half as the bottom half. Can that be explained by the fact the bottom has 19 of 50 led's out ?

[tom66]

Re: UN55D6000 Blinking Standby Light

If those bottom LEDs are short circuits, the LEDs should be just as bright (circuit will drive a constant current regardless); if they are in between, then the LEDs might be dimmer.

[mmartell]

Re: UN55D6000 Blinking Standby Light

All 20 tested short (continuity) though I didn't ohm them.

Strangely enough I'd doubt you could tell the difference in brightness from the bottom half left strip vs the right strip with regular programme material even though almost half are out. Can't leave it the way it is though as I DID notice a difference with the top left portion of the screen when the set was working.

Need a source for these led's if I can't get another strip...