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

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JDC

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Join Date: May 2015

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Country: USA
#1

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

03-20-2018, 08:23 AM

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.
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Andrew F. Ali

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#2

03-20-2018, 08:54 AM

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.
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JDC

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#3

03-20-2018, 09:29 AM

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

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

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#4

03-20-2018, 09:41 AM

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

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

Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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Andrew F. Ali

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#5

03-20-2018, 09:56 AM

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

Originally posted by budm

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

+10
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JDC

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#6

03-20-2018, 11:23 AM

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

Originally posted by budm

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

Thanks!
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tom66

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#7

03-20-2018, 12:36 PM

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.

Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
For service manual, schematic, boardview (board view), datasheet, cad - use our search.
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budm

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#8

03-20-2018, 01:55 PM

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).

Last edited by budm; 03-20-2018, 01:56 PM.

Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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Dumah Brazorf

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Join Date: Jun 2013

Posts: 179

Country: Italy
#9

03-20-2018, 04:11 PM

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.
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budm

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#10

03-20-2018, 04:16 PM

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.

Last edited by budm; 03-20-2018, 04:17 PM.

Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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Dumah Brazorf

Senior Member

Join Date: Jun 2013

Posts: 179

Country: Italy
#11

03-20-2018, 04:26 PM

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.

Last edited by Dumah Brazorf; 03-20-2018, 04:28 PM.
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tom66

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#12

03-20-2018, 04:30 PM

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

Originally posted by budm

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.

Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
For service manual, schematic, boardview (board view), datasheet, cad - use our search.
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CapLeaker

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Join Date: Dec 2014

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Country: Canada
#13

03-20-2018, 06:17 PM

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.
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chrismurphy

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Join Date: May 2014

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Country: australia
#14

03-20-2018, 07:57 PM

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

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

Last edited by chrismurphy; 03-20-2018, 07:58 PM.
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budm

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Country: USA
#15

03-20-2018, 09:19 PM

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

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

5 diodes for LED.

Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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JDC

Badcaps Veteran

Join Date: May 2015

Posts: 273

Country: USA
#16

03-22-2018, 09:01 AM

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

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

Leaking Member

Join Date: Dec 2014

Posts: 8146

Country: Canada
#17

03-22-2018, 12:39 PM

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

digikey, ali, mouser, element14, allied etc...
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jetadm123

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Join Date: Feb 2010

Posts: 2169
#18

03-22-2018, 01:43 PM

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.
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CapLeaker

Leaking Member

Join Date: Dec 2014

Posts: 8146

Country: Canada
#19

03-22-2018, 07:51 PM

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

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