LED Driver... go simple or complex?

[Gryd3]

I'm working with a microcontroller that will control a solid state relay that will be used to control automotive lighting.
I am confident I have enough knowledge to get this half of my circuit done, but I am struggling with how I should tackle the lighting side...

My goal is to make my controller compatible with any other lighting product, which means I will be switching 12-14.4V on and off. This will require me to make my own LED lighting modules that accept this an in input voltage.

I struggle with SMD components and wish to keep the component count low.
I will be using 1W high power LEDs that will require 350mA, and some square LEDs that require 70mA.
They will not be mixed, I am making a puck with 3-5 of the square LEDs, and a puck with 2 or 3 of the 1W LEDs.

-I can get away with using a resistor and calculating the values based on a 14.4V supply (and slightly underrate the LEDs to help them last longer) but am concerned with heat, and longevity of this method.

-I can construct a basic analog constant current supply that I had found online at http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/ ... but again, am concerned with heat generated. I greatly prefer the controlled power source though)

-I have started looking at switching power supplies... but have been presented with solutions requiring lots of parts that I don't completely understand. I like the sounds of this method as it will keep heat down, and allow for a controlled powersource.


I was hoping I could run this by someone with more experience to find if it is worth trying to over complicate this keep heat down, and keep the LEDs running hopefully longer. These LED modules will operate in 2 states, Constantly on, and flashing... so any method I choose will have power cut, and re-applied at no faster than 30Hz.
Will the flashing cause issues with a linear or switching power supply?

What are your thoughts on how I should power each LED puck?

 

[(*steve*)]

Have a look here first https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

The main issue with driving LEDs efficiently in an automotive environment is the variable voltage you will encounter. Added to this, for longevity of the LEDs you need to ensure that they neither get too hot, not have too much current flowing through them.

A linear constant current source will allow you to maintain a constant current, but may require a significant voltage overhead. This will cause losses, but other than heat, this is not a huge issue. A better solution is to derive a higher voltage supply and run your LEDs in series -- this is also the most complex method. Somewhere in the middle is to use a switchmode constant current source for the LEDs.

The LEDs are best kept cool by using an efficient driver (switchmode) or keeping the driver remote from the LEDs, AND by thermally connecting the LEDs to the metal of the car body. In most cases the car body will not get hotter that acceptable for the LEDs. The worst case are those silly daytime running lights. Other lights tend to be on intermittently, or only on when it's darker (and therefore likely cooler)

 

[Gryd3]

Thank you *Steve*, insightful read, and refreshes my memory on a few details,
Due to the desired construction method, the LEDs will not make viable thermal to the car body, as they will be enclosed in a small puck/housing.
To help mitigate wasted power from the resistor, or linear supply I had planned on connecting the led's in series to bring the total Vf closer to the lower end of the supplied voltage range of the vehicle (12V).
What I'm more curious about is if a switch mode or linear power supply would be a viable option considering power will be switched off/on at 30Hz for flashing mode. I am under the understanding that due to how the switch mode power supplies work, cutting power may not immediately turn the LED off due to the supporting components involved to smooth the output. These LED's will never be run continuously for any more than a few seconds at a time, as the primary use will be the flashing mode.
At the moment, I am viewing the Linear supply as my most viable option.

 

[(*steve*)]

You've probably got to consider the worst case of someone putting on their hazzard lights for an extended period on a hot day.

The puck should be thermally conductive so that the waste heat can be safely transferred away.

No problems with switchmode regulators turning off fast. Current drivers never have large value capacitors on their outputs anyway.

 

[Gryd3]

Thanks for the tips Steve.
I'll do some more digging and post a couple schematics that I feel will work for me. Hopefully someone can pick it apart and provide some feedback.

 

[Gryd3]

Well, I do not have a schematic... I don't have software to produce one other than paint XD.

I'll be using a pre-made switch-mode power supply... as I have fat fingers and want to keep size down
http://www.dx.com/p/7029-mr16-3-x-1w-led-drive-power-black-green-5-pcs-180543#.U63bUPldXON
with the following LEDs
http://www.dx.com/p/1w-6300-6700k-90lm-white-led-light-bulbs-20-piece-pack-133082#.U63Z6fldXON
http://www.dx.com/p/625nm-1w-40lm-red-led-light-bulbs-20-piece-pack-133096#.U63bSPldXON

The plan is:
Create a housing to hold the LEDs and power supply with a metal back to dissipate LED heat.
Wire 2 or 3 LEDs in series and connect to the output side of the pre-made power-supply (Polarity is important here...) A long pigtail will be connected to the input side of the power supply once the posts are removed (Polarity is not important, as there is a rectifier on the power supply circuit)
This completes the 12V LED module.
Once the modules are made, they will be connected to a controller which will control 4 outputs, and change the pattern based on an analog input provided from a center-tapped potentiometer (Attiny85, from the Adafruit Trinket**). The outputs will switch a Solid-state relay. (This relay will have diode protection for reverse polarity, and as a bypass in-case an inductive load is driven)
http://www.digikey.ca/product-detail/en/CPC1709J/CLA280-ND/1277135

The Solid state relay will introduce a delay in turning on the LED modules... but this also allows me to control more than just the LED modules I am constructing. (Other after-market lights can be switched)



**Trinket can be a pain-in-the-@$$ to program... as the bootloader resets every 10 seconds and windows is not fast enough to work without being creative... I have had better luck on Linux, but it is not officially supported! AVRDude is a lifesaver

 

[(*steve*)]

When you get those drivers, carefully check them over the expected voltage range. I've found that some of these cheap drivers are not all they say.

Those LEDs WILL require heatsinking.

 

[Gryd3]

When you get those drivers, carefully check them over the expected voltage range. I've found that some of these cheap drivers are not all they say.

Those LEDs WILL require heatsinking.

I've ordered a couple in the past and they seamed alright. Output was consistant. and they run pretty cool. One was dead on arrival mind you.
The LED's I linked do not have that fancy star plate, so yes, I do plan to thermally fix them to the metal back plate of the LED puck I plan to make. The LEDs will be sandwiched between a piece of clear acrylic and a metal back plate. I have access to a CNC machine, so I can be creative and mill a pocket in the clear acrylic for the LED to rest and mill some aluminium to come in proper contact with the back side of those little LEDs

All in all. It does not appear like any immediate concerns were brought up.
When I get some parts made, Ill post pictures. (I tend to delay my projects, but I have a friend that is very interested in the results so hopefully I can get this done )