Changing LED from blue to yellow (car ambient lighting) - what is the purpose of the resistors?

Question

I've got what looks like a fairly simple circuit I've pulled from a car door card. It provides variable brightness ambient lighting in blue, which I want to change to yellow for aesthetic reasons.

I'm assuming the car uses a variable voltage to achieve this but cannot measure without disassembling the vehicle as this is a loose door card part that I'm testing.

I should be able to get a yellow LED that is in the same package to resolder in place of the blue, but without knowing any specs on the existing LED I have concerns about too much current through the yellow LED if I just switch it out without changing resistors.

I've attached a schematic I've drawn up based on the board with measured values.

^{simulate this circuit – Schematic created using CircuitLab}

I've also powered the circuit with a very basic power supply and taken some readings as follows:

• Voltage across LED at 12.05 V:

  • 2.82 V

• Voltage across resistors:

  • R1 2.81 V
  • R2 9.25 V
  • R3 12.04 V (Edit: actually a ceramic capacitor)

• Total current through circuit:

  • 4 mA at 12 V

• Also:

  • 0.1 mA at 3 V
  • 0.6 mA at 4.5 V
  • 1.2 mA at 6 V
  • 1.9 mA at 7.5 V
  • 2.6 mA at 9 V

The circuit simulator app I've drawn the schematic on has the majority of current going through the 2.1k resistor (simulated,) which makes sense, with about 200uA through R1 and 1 mA through R3.

What is the point of all the resistors?

R1 and R2 appear to be a potential divider, but R3 in parallel with the LED confuses me. Edit: R3 turns out to be a ceramic capacitor - edited in schematic.

I've fairly basic knowledge of electronics in spite of tinkering for years and would expect a sufficient, single, series resistor would be fine to power an LED.

This same circuit is likely used for white and red LEDs in separate model variants of the car. I'm trying to get my hands on one with a red LED to analyse and that may be more suitable to a straight swap of the LED to yellow, also, given likely similar forward voltages.

Any thoughts would be appreciated.

Here's a picture of the actual board:

Answer 1

R1 could be for diagnostic purposes. A smart controller would be able to differentiate between a blown LED and loose connection.

The brightness is probably controlled by a PWM circuit.

Yellow and Blue LEDs have almost the same voltage drop, so the same series resistor should be fine. Almost any LED should be able to handle 5 mA. The biggest challenge will be find an LED with the same physical size.

Answer 2

What you call R3 with 12k is a ceramic capacitor. For DC it basically has almost infinite resistance (it measures as open circuit). The resistance of 12k you see on the multimeter is the combination of R1 and R2 in series. But I'm still not sure what R1 is doing...