Tuesday 14 May 2019

fixing interior 41mm festoon leds from china

As in the previous post, here I am modifying china made festoon LEDs, 41mm in this case. They start to blink after a few months of using them in the interior.  So I ordered new ones (1st were from banggood.com, 2nd set was from aliexpress.com) all of them failed. I have the last pieces which I am going to try with a lower current. From the factory these LEDs are driven at ~160mA with 4 parallel set of 3 LEDs, this means each LED is driven at 40mA.

Pictures of module


Removed heatsink.

Removed heat transfer compound.

useless CAN-BUS resistor

1st I removed stupid "can-bus" resistor, it is there just to generate heat. It's marked as 271, which mean 270Ohm, so the current through this resistor is 40mA while 12V is applied (12/270). As original bulbs are 5W, they run at 416mA on 12V, so this resistor is really just heat source.


PT4211 chip is used to drive these LEDs. Current is defined by Rs resistor value, installed one is 0.82Ohm, based on datasheet calculated current is calculated as 0.2/Rs, so in our scenario, it is ~250mA (0.2/0.83 = 0.2439A). This means current through each LED is 60mA. I try 2R2 and 4R7 resistor which corresponds with  90mA or 42mA respectively, this is combined current for all for branches, when we divide this by four, we get current for each led (22.5 or 10.5mA).

Light output

I use my Samsung to measure the output of these modules, without and with changed resistors.

Rsens = 0.82Ohm (original) 9500lx at 220mA (170 with removed 271 resistor)

Rsens = 2.2Ohm 3800lx at 66mA

Rsens = 4.7Ohm 1400lx at 27mA

Pictures of instaled modules

original modules with 0R82 sens resistor (i can't remember if this is 1 or 2 modules in the car)
1 module with 2R2 resistor
1 module with 4R7 resistor
There are similar LEDs on the Chinese sites to be brought, with different spacing between LED chips (check pictures). These use the resistive dropper consist of three parallel resistors(3x110 Ohm). Combine current over (4x3) LEDs matrix is 65mA with all three resistors in place, with just two resistor combined current is 50mA and with just one it is low as 25mA. Current over each branch of LED matrix is ~16,12 or 6mA.

Friday 10 May 2019

fixing license plate LED lights

Recently I found and buy these LED modules from china for my audi a4 b5 from 1999




I called this article "fixing" but in reality I only run these modules for 2months in winter so I can not say anything without datasheet for leds or real life experience. I change current via leds to 1/5 this means I will never be able say if they will fail with factory setup or not.

I have to created schematics from boards and investigate which resistor(s) need to be changed. Leds are driven in constant current mode using 2 NPN transistors constant current source.


Back of module

Front of module

Front of PCB

Highlight of traces on PCB

Original sense resistors has value of 2R4 and 1R6 in series so 4Ohm , transistor base voltage is 0.6V so this means LEDs are driven at 150mA. led matrix is 6sets of 3series leds ,   which makes each led driven at 25mA(150mA/6). I chaged sense resistor to 2 x 10Ohm (20Ohm total) which run them all at 30mA ((0.6V/20Ohm)/6 = 5mA each Led colomn).

with R2 and R3 resistors changed to 2x10Ohm the current is 30mA (6x5mA).

here are pictures of measuring current on bench, 2 unmodified modules

unmodified module

unmodified module

and here are 2 modified modules 1/5th of current:

1st modified module
2nd modified module

here is light output, still mostly lightning up ground :/ 

unmodified modules, current 150mA
modified modules, current 30mA 


some more measurements:

original module with 4Ohm "resistor" run a 150mA and give about 8000 to 9000Lux

after modification with two 10Ohm resisots module run at 32mA and output is about 1000Lux