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

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