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.

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