Electronic Circuits and Projects Forum

29th July 2009 05:59 PM

Hi everyone!

I'm pretty new to electronics and I'm trying to find a high power LED controller which can drive 5 Seoul Semiconductor P7 LEDs. The idea is to replace a standard halogen car headlamp with these very high-brightness diodes. The problem keeps falling back on the controller since I've had no luck at all in finding controllers which can put out enough power to drive them.

Ideally what I'd need is a controller which has following characteristics:

12vDC - 14vDC input
Current-mode controller
Capable of driving 5 LEDs, each drawing up to 2.8A current at up to 4.2V

So as you can imagine I haven't had such great luck finding somebody who produces such a controller. I'd really love to build one, though!

The datasheed for the diodes I'd be powering can be found here. I'd really appreciate any input that you guys could provide!

Cheers,

Corey

29th July 2009 06:29 PM

How much do these puppies cost?

Do you care if the lumens drop off if the alternator is not charging. If so, you could run three in series at 14.2 to 14.7V, and then brute-force linear regulate the current with only a few watts of dissipation in the regulator transistor.

29th July 2009 06:43 PM

Hi Mike, thanks for the reply!

Normally I wouldn't mind it if the brightness were to drop off a little, but that operating range is quite a bit too high. The P7s will run up to 4.2V. I should have stated earlier that I'd typically want them running at around 3.6 - 3.7vDC so unfortunately that's a no go.

As for the cost of the LEDs, it's $13.47 per diode shipped in 10 unit quantities. Individually they cost $15.37 shipped. I'd post the link for you but I don't know if it's allowed in the forums.

Cheers,

Corey

29th July 2009 06:52 PM

Three in series at 3.7V, running at ~2A is still only 6.8W of dissipation in a transistor used as a current regulator starting from 14.5V. Sounds quite doable. Think about how much heat an incandescent wastes...

29th July 2009 07:14 PM

Yeah, that is true. Either way each LED will be dissipating about 10W of power each anyways so it's not so much the power dissipation in the regulator that I'm concerned about...

It's just that if I were to go the route of using a linear regulator, then what would happen if there were large transients for whatever reason? I'm assuming that the voltage across the LEDs would spike, right?

Additionally, I'm pretty sure that I really need 5 diodes per lamp to get a satisfactory light output. Typically the P7s will output around 700lm, which, multiplied by 5 diodes is 3500lm of raw light. So after all the optical losses I can expect to be at maybe 2000lm which is right on target.

Most of all I'm just afraid for my diodes, though. I know that LEDs are very sensitive to transients and I don't want them dying on me prematurely

Cheers,

Corey

29th July 2009 08:29 PM

Here is a start at designing a controller. First thing, I need a model for the I/V behavior of the W724C0. I created a crude one. Compare mine to the data sheet plot.

29th July 2009 08:40 PM

Here are three in series with just a resistor as a current limiter.

29th July 2009 08:46 PM

Hello again Mike!

The I/V characteristics of the diodes are as follows:

It's actually pretty steep near the upper-end which is why I was mentioning that these diodes are really sensitive to transients.

29th July 2009 11:56 PM

Compare your plot to mine. I tried to make my model fit your plot at 2.5V/0A and 3.6V/2.8A.

I will play around with a constant-current regulator later when I get some time...

30th July 2009 01:55 AM

How will you cool the LEDs?
Liquid nitrogen?
That is a lot of heat in a small space. The LEDs will fry in a couple of seconds.

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