Replaced individual LED with resistor....but how?

[JDC]

I'm sure this has been addressed many times before, but if I wanted to replace an LED with a resistor, how would I go about determining the right value?

I know how to replace them with other LEDs.

[Andrew F. Ali]

Re: Replaced individual LED with resistor....but how?

Is this LED the backlight for a TV? If yes I would suggest a diode instead of a resistor. The resistor depending on the Current and Voltage would have to be a physically large one that would dissipate heat and eventually destroy the PSU and the polariser sheets etc.

[JDC]

Re: Replaced individual LED with resistor....but how?

Sounds good! So...a 3.3v diode?

[budm]

Re: Replaced individual LED with resistor....but how?

At least 2W 3.3V Zener connected in reverse bias.

[Andrew F. Ali]

Re: Replaced individual LED with resistor....but how?

at least 2w 3.3v zener connected in reverse bias.

+10

[JDC]

Re: Replaced individual LED with resistor....but how?

At least 2W 3.3V Zener connected in reverse bias.

Thanks!

[tom66]

Re: Replaced individual LED with resistor....but how?

Resistor and zener would do the same in this application, a resistor would handle the power better.

But to know the value of the resistor you need to know the current and that's not always easy to measure.

[budm]

Re: Replaced individual LED with resistor....but how?

The current is set by the Backlights level setting so Vdrops on the resistor will keep changing so the overall Vf will changed more compared to if the load is all LEDs which the Vf will vary not very much (have to look at the Vf curve VS current).

[Dumah Brazorf]

Re: Replaced individual LED with resistor....but how?

Are leds all connected in series or are there some groups in parallel?
In the first case, assuming the led driver is current driven, you can simply short the bad led out.

[budm]

Re: Replaced individual LED with resistor....but how?

The LED are fed with constant current using constant current control element (PWM Controlled MOSFET which is connected to the Cahode return of each LED string) in the low side (sink) so if you bypass the bad LED that means there will be more Vdrops on the MOSFET which can cause it to overheat.

[Dumah Brazorf]

Re: Replaced individual LED with resistor....but how?

It's only one led less...
More important is to teach the customer to set the brightess no more than 70-80% if they don't want to kill the others soon.

[tom66]

Re: Replaced individual LED with resistor....but how?

The current is set by the Backlights level setting so Vdrops on the resistor will keep changing so the overall Vf will changed more compared to if the load is all LEDs which the Vf will vary not very much (have to look at the Vf curve VS current).

That's true but the current will be constant. The backlight current is usually PWM rather than direct modulation. In any case, if it is modulating the actual current, then it won't make much difference as long as you design it to work well at the full current setting. Any power dissipation increase in the driving transistor, for a couple of LEDs, will be absolutely marginal. Whereas a resistor will handle the power dissipation better.

[CapLeaker]

Re: Replaced individual LED with resistor....but how?

I only can say that IF I replaced a bad LED with something else in backlights, it always was a Zener diode in reverse bias as budm suggested. The resistor is too much headache for my liking.

[chrismurphy]

Re: Replaced individual LED with resistor....but how?

5 x IN4004 diodes in series or similar work well, but don't use a resistor

[budm]

Re: Replaced individual LED with resistor....but how?

https://www.badcaps.net/forum/showthread.php?t=46403

5 diodes for LED.

[JDC]

Re: Replaced individual LED with resistor....but how?

Where's a good place to get a few 2W 3.3V zener diodes?

[CapLeaker]

Re: Replaced individual LED with resistor....but how?

digikey, ali, mouser, element14, allied etc...

[jetadm123]

Re: Replaced individual LED with resistor....but how?

Try a 1N5333 zener. Rated 3.3V @ 5W. Readily available and a little more mainstream than a 2W zener.

[CapLeaker]

Re: Replaced individual LED with resistor....but how?

1N5913 3.3V 3W zener at digikey part #: 1N5913BRLGOSCT-ND
4523 in stock