utorok 14. mája 2019

fixing interior 41mm festoon leds from china

as in previous post, here I am reversion china made festoon leds, 41mm in this case. After few month of using as interior lights they start blink.  So I order new one (1st were from banggood, 2nd from aliexpress) all of them failed. I have last set which I am going to try with lower current. Original these leds are driven at ~160mA with 4 paralel set of 3 leds this mean each led is driven at 40mA each.

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 current which this resistor generate in 12V environment is 40mA (12/270). As original bulb are 5W they run at 416mA on 12V so this resistor is really just heat source ....


Using PT4211 current is defined by Rs resistor value, manufacturer value is 0.82Ohm, based on datasheet current is calculated as 0.2/Rs so ~250mA (0.2/0.83 = 0.2439A). This mean 60mA over each led. I try 2R2 and 4R7 resistor which mean 90mA or 42mA.

Light output

I use my samsung to measure output of these modules with changed resistors

Rsens = 0.82Ohm (original) 9500x 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 cant remember if this is 1 or 2 modules in car)
1 module with 2R2 sens resistor
1 module with 4R7 sens resistor

piatok 10. mája 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).

R1+R3 change to 2x10Ohm resistors, change current to 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