LED Dimming

[kushy04]

I've purchased the following:

MR16 1*1W 320~350mA Constant Current Regulated LED Driver 12~16V Input / Three of these

and

3W LED Emitter on Star (Multicolored RGB) / One of these

-----------------------------------------------------

My question is this, to power on the LED and change colors of the light as I wish, I was planning to attach a dimmer switch to each lead like this:

[12V BATTERY]-----[LED DRIVER]-----[DIMMER]-----[RED LED]
OOOOOL------------[LED DRIVER]----[DIMMER]-----[GREEN LED]
OOOOOL------------[LED DRIVER]----[DIMMER]----[BLUE LED]
------------------------------------------------------

Would there be an easier way to accomplish this? I guess the real problem is that this ameteur isn't confident enough in his electronic skills to know for sure if the setup will work... So any assurance would be greatly appreciated.

 

[jankj]

I am thinking the dimming should be done by the driver. Such as a multi-mode driver, or a continuously variable driver.

The drivers job is to output a pre-determined amount of current (amperes or milliamperes). In your case it is supposed to deliver no less than 320 mA and no more than 350 mA, regardless of resistance. If the resistance (ohms) increases the driver will increase the voltage so the current stays the same (remember the old Ohm formula voltage = resistance * current, or U = R * I). So adding a variable resistance in series, as your scetch indicates, will not work.

However, your LED may be mounted in parallell with a variable resistance.

12V positive
...........|
LED driver 350 mA
...____|___
...| ............|
LED ......... Variable resistance (dimmer)
...| ...........|
12V negative (ground)
(edit: Lots of "."s inserted as superfluous spaces are removed, making ascii art nearly impossible...)

Three of those circuits, and you have accomplished what you want.
If you increase the resistance in the dimmer, more of those 350 mA will go through the LED and less through the dimmer. Any current going through the dimmer is wasted (heat), so it is a terrible inefficient design.


BUT there is one serious drawback to this design: The resistance of the LED is temperature sensitive, so any change in temperature requires that you re-adjust the variable resistance. And you have three of those to fiddle with. The LEDs will heat up when you run current through them, so it will be a terrible fiddly procedure to get the kind of lightning you want. EDIT: Also, as the current/voltage relationship in LEDs are highly non-linear a very small change in the variable resistor will give a large change in light output. Adjusting for your "perfect color" will be next to impossible with this setup. Might work for a toy or test project, but it will not be practical at all.


Someone with a better knowledge of electronics may provide a dimmer circuit which works better for your application, but to me it sounds like you should buy a new set of drivers with continuously adjustable output or with multi-level output.

 

[lolzertank]

By dimmer switch, do you mean like the dimmers that go into the wall to control lightbulbs? Those will not work on DC.

 

[kushy04]

I was planning on using a potentiometer to vary the resistance but was unsure with the properties of led's so I guess that won't work. Is there another way to make use of this RGB LED on my motorcycle?

 

[jankj]

What exactly do you mean by "change color", btw?

If you want to change from white (all three colors fully lit) to blue, red or green, and the combinations red+green, blue+green, blue+red (yellow, cyan and magenta) then you don't need potentiometers at all. All you need is three on/off switches:

12V positive
..____|____________________________
..|.............................|......................|
Driver 1..............Driver 2............Driver 3
..|.............................|......................|
Red LED...............Green LED.....Blue LED
..|.............................|......................|
Switch 1..............Switch 2........Switch 3
..|.............................|......................|
====12 V negative (ground) ======

That'll work, and it's simple, and it will give you 6 clean colors plus white. Things only start to get tricky if you want to produce more colors, using a partially dimmed LED. Then I strongly suggest you replace those drivers with something more sophisticated, capable of infinetly adjusting between 0 and 350 mA...


There ya go - stick with 6 colors + white, keep things simple and use the parts you have. Please post some results here when you're done :candle:

 

[lolzertank]

I was planning on using a potentiometer to vary the resistance but was unsure with the properties of led's so I guess that won't work. Is there another way to make use of this RGB LED on my motorcycle?

Power potentiometers are NOT cheap. IMO, the best way to dim a RGB array is PWM. I guess you could try to make a PWM generator with a 555 (or three), the potentiometers and some MOSFETs.

 

[kushy04]

Any suggestions on the best way to install the rgb LED under my motorcycle? I was thinking of drilling a hole underneath the bottom fairing and cutting the end of a flashlight off with the RGB LED installed into that. Just unsure how to fix the flashlight end onto the bottom fairing so it won't fall off... I'll post pictures of the entire process as I'm pulling this small idea/project off the paper and into existance... Any suggestions(better or worse) or ideas about any aspect of the project is appreciated, it will only make me think from a different prospective and help the end product become better.

 

[JimmyM]

The best thing to use for continuously variable color on 3 channels would be a microcontroller. It can do PWM out to an FET (with an RC filter on the gate so it operates in linear mode) and measure current as feedback. All you need to do is drop the 12-14V input to 5V (5V switchers are cheap and can supply all the current the LEDs would need). The micro controller runs on 5V and the output FETs will not have to dissipate as much power (heat).
Then you can program any number of color sequences you want and select them via a pushbutton.