Hello,

I am in the process of trying to build a power supply to control some CREE LED's on my marine aquarium.

I have been working in MultiSIM to work out the specifics with the LM338 and LM317 regulator ICs.

So far I have it all working correctly, there is 24V 10A in, the 338 regulates the power to the required ~15V then the 317's regulate the current to 1000ma and 700ma for the wo types of LED's I will be running.

This in theory will work, I just need to check how much heat will be generated.

What I am wanting to do is to have the ability to dim the lights independently through a microcontroller.

There is a couple of options, I could just smooth the PWM output from the microcontroler and then run it through an opamp, or I could find a digital pot to use.

Any suggestions on the best way to do this? Is it even possible?

Another concern would be the additional heat generated when the lights were dimmed.

I am pretty sure I need to dim the LEDs on the current side (lm317's) and not the voltage side (LM338) but I may be wrong.

Any help would be greatly appreciates!

Cheers,
Dean

 

Hello,

You could also use special driver chips like the LM3404 or LM3409.
The LM3404 can be dimmed with a PWM signal.
The LM3409 can be dimmed with a PWM signal or a potmeter.

Greetings,
Bertus

 

Thanks for those bertus, I will have to have a play with them. I would still need to regulate the voltage to them first wouldn't I? With something like a LM338?

What sort of heat would be produced running the LM3404 at the full 1A? I am looking to run 4 or 6 1000ma LEDs @ 3.5v each (so 14v or 21v @ 1A).

Cheers,
Dean

 

Yes, but you can get a wide variety of power supplies. I'd go for a regulated wall wart, something in the 12V range at 5A. You can vary this quite a bit though, but there is no sense in reinventing the wheel.

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

If you would like I can draw something up using PWM that will generate a lot less heat. How experienced with electronics are you?

 

Hello,

When the 24 Volts powersupply ia a regulated one, there is no LM338 needed.
The LM3404 will run at 24 Volts and regulate the current.
You can hook up 4 leds to the LM3404 without a problem.
In the datasheet there are examples how to calculate the resistors and other components.
The heat produced will be minimal as the regulator is switching.

Greetings,
Bertus

 

Yes, but you can get a wide variety of power supplies. I'd go for a regulated wall wart, something in the 12V range at 5A. You can vary this quite a bit though, but there is no sense in reinventing the wheel.

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

If you would like I can draw something up using PWM that will generate a lot less heat. How experienced with electronics are you?

Hi Bill,

I just had a read over the link you supplied, there is alot of good information there!

How well do the concepts apply to the high power cree LEDs?

If you could draw something up to get me started that would be great, I had laid one out in MultiSIM (see attached) that worked but I am pretty sure now it is very inefficient!

Cheers,
Dean

 

Hello,

When the 24 Volts powersupply ia a regulated one, there is no LM338 needed.
The LM3404 will run at 24 Volts and regulate the current.
You can hook up 4 leds to the LM3404 without a problem.
In the datasheet there are examples how to calculate the resistors and other components.
The heat produced will be minimal as the regulator is switching.

Greetings,
Bertus

My 24v source is regulated (its a switchmode supply) and supplies 10A.

I think I am understanding it more now, as I am supplying a fixed (read regulated) voltage the IC will be able to adjust the current to compensate for this. Is this correct?

That will mean that if I supply the 24v I will be able to connect 1 or ~6 LEDs with no troubles and no altering of the circuit? If I put more LEDs with a Vf of greater than 24v - (the voltage the IC uses) then I will start to see the lights dim.

With the 10A supply I would be able to control 10 x 1000ma strings of 6, is that correct?

In relation to the dimming of the LEDs, the LM3404 has pwm control but I would need to smooth the output from a microcontroller wouldn't I?

I have seen some quad DAC's that run over SPI, would these be suitable to dim the LEDs? Any suggestions for a common one that would work?

Cheers,
Dean

 

A lot of it applies. There are other way to control current besides the LM317, though it is a good little chippie. Transistors are also good current limiters if wired correctly. In every case good heatsinking is key, especially with the LEDs themselves. They get quite hot too, and this affects their lifespan and performance.

Putting LEDs in series maximizes efficiency nicely. It is usually an all or nothing, it you have too many where the total Vf exceeds the power supply, they won't just dim a little, they will be pretty dark (maybe with a little glow).

Here is something I came up with for a guy who wanted grow lights for plants. It is completely analog, so there will be heat.

http://forum.allaboutcircuits.com/showthread.php?t=32066&page=3

PWM can improve the performance and reduce heat. I'll be glad to show you some ideas on that front a bit later.