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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.

schematic

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:

enter image description here

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  • \$\begingroup\$ I edited your schematic, it is easier to read now. \$\endgroup\$
    – jusaca
    Jul 10 at 10:05
  • \$\begingroup\$ Are you sure about the circuit? I really don't see any reason for R1 and R3 to be there. \$\endgroup\$
    – jusaca
    Jul 10 at 10:05
  • \$\begingroup\$ Thanks for the schematic edit - that helps. I've included a picture of the board now. Have checked over the components with a multimeter for continuity and it checks out against the diagram. May have to desolder the LED to get a true look at it, but I can't find any other connections. \$\endgroup\$
    – MrFlibble
    Jul 10 at 10:43

2 Answers 2

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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.

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  • \$\begingroup\$ The smart controller aspect is likely given that it's a modern vehicle (2018). I will try to slow motion video the installed lights to pick up some PWM flicker. RS Components appear to have a selection of LEDs in the same package size with forward voltage of 2 - 2.5 so I will try a straight swap without altering resistance and report back. \$\endgroup\$
    – MrFlibble
    Jul 10 at 17:14
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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...

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  • \$\begingroup\$ I've edited the schematic to reflect R3 being C1 of unknown value. When I simulate the circuit (I'm using PROTO app) it just says shorted C2, so I've just taken it out (treating it as infinite resistance). Majority of current is through the 2.1k R2 so I think I'll try swapping that out with appropriate resistor when I change the LED, test and monitor for heat. \$\endgroup\$
    – MrFlibble
    Jul 10 at 11:21
  • \$\begingroup\$ R1 is limiting current flowing in the LED. \$\endgroup\$
    – Miss Mulan
    Jul 10 at 13:16
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    \$\begingroup\$ @MissMulan R2 is the current limiting resistor \$\endgroup\$
    – jusaca
    Jul 10 at 13:21
  • \$\begingroup\$ They both limit current. \$\endgroup\$
    – Miss Mulan
    Jul 10 at 13:26
  • \$\begingroup\$ @MissMulan R1 adds diagnostic current, it doesn't limit anything. \$\endgroup\$ Jul 10 at 19:59

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