Help with a circuit for custom car interior lighting with LEDs

[Javafox]

I'm working on adding some custom interior lights to my car. I'm trying to achieve certain behavior and since I'm an aerospace engineer, I don't have a lot of circuit knowledge. The first circuit I whipped up ended up burning out my potentiometer. Oops.

Let me describe what I'm trying to accomplish. My LEDs run on 12V. I have two circuits capable of outputting 12V: the parking light circuit and the dome light circuit. I would like the LEDs to be lit if either of these is on.

Here's the kicker: when only the parking lights are on, I want the LEDs to controlled by a potentiometer so I can vary their brightness. This is because the strips I have are too bright for ordinary night driving and need to be dimmed.

Then, when the doors are opened and the dome light circuit is on, I want the LEDs to see 12V. This will have the effect of increasing the brightness when the doors are opened, which would be awesome.

I've done the either-or case without the potentiometer with a pair of diodes. That works fine. But when I try adding in the potentiometer, it smokes.

I should also add that the voltage in either of these circuits is not a known constant. While the voltage in the parking light circuit is pretty close to 12V when they're on, the dome lights fade out and the voltage drops smoothly from around 12V to around 2V.

Can someone with more electrical engineering knowledge help me out a bit?

I attached a wiring diagram of my faulty, pot-smoking circuit for reference.

 

[Javafox]

I guess at a high level, what I'm really after is a circuit that emulates the MAX function with two inputs. I haven't been successful in finding examples of such a circuit. Do they exist?

 

[ronsimpson]

I think you need two diodes. Other wise you will have power from the parking light going to power the dome light. With two diodes your LED will have the larger of the two voltages.

I don't know why your pot is burning out. 10K or 10 ohms?

 

[Javafox]

The pot is 10,000 Ohms.

 

[audioguru]

A pot has a maximum allowed power rating that is very small, usually less than 0.5W. The pot can control an electronic circuit that applies adjusted power to your LEDs.
But we do not know how many LEDs and we do not know the maximum current taken by the LEDs.

If you have 30 strings of 3 LEDs in series and each string takes 20mA then the total current is 30 x 20mA= 600mA and the power is 12V x 600mA= 7.2W.

The electronic circuit can be a hot darlington power transistor emitter-follower or it can be cool pulse-width-modulation circuit.

 

[Javafox]

Here's a link to the exact pot I'd like to use: https://www.radioshack.com/product/index.jsp?productId=2062298.

Also, I measured the current pulled by the strings. Each "triple" draws 19.3 mA. I'll have around 28 "triples" initially, meaning my current draw will be 540 mA. That was a really good guess audioguru! I would like the ability to add more LEDs later as well, if possible.

 

[audioguru]

I assume the LED strips have built-in current-limiting resistors.
Here is a circuit using a darlington power transistor. The pot has very low current so it will not get warm.
The darlington transistor will get fairly warm and might need a little heatsink if many LEDs are driven.

 

[Javafox]

This only works for a single input, though, right? I'm trying to make it work with two inputs and take the max of the two.

And, yes, the LED strips do have resistors.

 

[Javafox]

Will something like this work for the dimming?

**broken link removed**

 

[audioguru]

This only works for a single input, though, right? I'm trying to make it work with two inputs and take the max of the two.

Make an OR gate with two diodes that feed the pot. Then either signal controls the LEDs.

Will something like this work for the dimming?

The PWM motor speed controller can be used to dim the LEDs.

 

[Javafox]

Great. I think I'm going to use that. Much easier for me to deal with than doing a home-brew circuit with transistors. Thanks for the help.

 

[Javafox]

Last question: This dimmer below has a message: "Will not work properly in vehicle circuits where you are grounding to the frame." I assume they wouldn't be making this up. Why would this be the case and will similar ones have this problem?