Electronic – How to add second, lower power source to car fog light

circuit-design

I'm trying to make my fog lights work with the parking lights, so I have one continuous strip of light. While I'm handy enough to craft it, designing the circuit so that I retain proper factory functionality is beyond my skills. I'll try my best to explain what I need, please go easy on me if I mess up some terms.

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  1. The rear parking lights consist of one 21/5W (leftmost, also brake light) and one 5W bulb (center). The fog light uses а 21W bulb (rightmost, in the trunk partition of the taillight).
  2. Since the fog light is driver-side only, I'll add another 21W bulb to the passenger side.
  3. I'll run a wire from the driver-side 5W parking light, into the trunk, through the components that you kindly suggest I use, and finally to both fog light bulbs.
  4. There needs to be a resistor somewhere along the line, and the bulbs need to be wired in paralell, due to the bulb holder design.

The result of this mess – when I switch the parking lights on, the fog lights also light up – using 5W of power each (hence the resistor), so they are exactly as bright as the other two bulbs.

Here come the tricky parts:

  • I have no clue how to calculate the proper resistor needed to power two 21W bulbs, wired in parallel, at 5W
  • Maintaining the fog light functionality – the driver side bulb should be able to draw full power from the factory wire when needed
  • I would like current to only go one way – no current should go from the fog light to the parking light, or from the parking light to the headlight switch module
  • The original fog light circuit needs to be the default – the CANBUS system checks the bulb at startup and I refuse to live with a "bulb out" warning

I would like to use simple components – relays, diodes and so on.

A million thanks!

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

A basic circuit which ought to work would be as above. The diodes should be rated at least 2A - the 1N5400 is 3A. The tricky bit is going to be the resistors. If you put 5W through a 21W lamp, it will glow feebly. You would need some trial-and-error to find what actually works. The resistors need to be high power wire-wound ones.

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