Efficient Current Limiting Circuit for CREE Q5 LED With Minimal Heat Dissipation

Hi there,

For a little art project I have to individually control a couple of CREE Q5 high-brightness LEDs with DMX. You can run them between 700-1000mA and therefore the LEDs have to be heat-sinked. If I run them at 12VDC and use a current limiting resistor it has to be a pretty big resistor Wattage wise and heat-sinked as well.

New flashlights use the same LED with a step-up converter so you can run the flashlight even from a single 1.5V battery. I could just use the driver circuit from a flashlight and then DMX-control the brightness via an external DMX driver circuit. I tested this and it works flawlessly. I’ve been told, though that those step-up converters don’t really like external input from DMX drivers. As mentioned before it works but might not work for long.

In order to avoid those big heatsinks that might even need a cooling fan I was wondering if there is another current limiting circuit out there that would dissipate all the heat generated without a heatsink and can handle external input from a DMX driver? Another option would be a current limiting / heat dissipating AND DMX driver combo circuit that directly accepts DMX data input.

This is for a super-low budget project so price is key.

Any ideas? Thanks for your help in advance,

Erwin

To efficiently power a high brightness LED [HBLED] you need a switch-mode current regulator. LEDs are current-mode devices, and though they do have a characteristic forward voltage, this is not to be relied on for driving an LED. All LEDs are best run by a current regulator. For low power LEDs (such as those designed to run at 20ma nominal), a series resistor provides enough current regulation in most cases, and because of the low amount of power, a low wattage resistor doesn't dissipate much [if any] heat.

But, for the kind of LED that you're talking about, you'll need a switch-mode current regulator. I'm not familiar with DMX enough to know how it achieves brightness control [probably with Pulse Width Modulation [PWM]], so I can't advise you there.

BUT, if your DMX controller does provide a PWM signal and if the voltage levels are compatible, then check out the following on eBay:

* High power LED driver dimmer PCB, charge control board HV99 photocell UVLO AC DC [http://www.ebay.com/itm/High-power-LED-driver-dimmer-PCB-charge-control-board-HV99-photocell-UVLO-AC-DC-/231196811419?pt=LH_DefaultDomain_2&hash=item35d467609b]

You might try contacting them and explaining your needs. Perhaps they can help you

I also found the following:

* http://cache.freescale.com/files/microcontrollers/doc/app_note/AN3321.pdf

It's rather technical, but it gives a good explanation of the concepts and might provide ideas.

This is the kind of thing you probably need and it might work to feed the DMX driver output to the "PWM" input:

* http://ledlighting-eetimes.com/en/accurate-pw...-controllers.html?cmp_id=71&news_id=222908330

Can't vouch for this, at all, but might provide some ideas.

BTW: The example circuit shows it driving several LEDs in series, but it will also drive only one LED.

The email generated by this "Reply" will have an error:

"it might work to feed the DMX driver output to the “DIM” input"

should be:

"it might work to feed the DMX driver output to the “PWM” input"

I corrected it above.

Just a little update on the flashlight dimming project:
I got those 7W 300LM Mini CREE LED flashlights and I tested 2 different DMX controlled current limiting drivers from Shanghai EUChips Industrial - www.euchips.com
PX704 - has 4 channels and you can set the current either to 350 or 700mA
PX705 - has 3 channels and you can set the current to 300, 350, 500, 650 or 700mA
EUChips has been extremely helpful and you can order directly with PayPal. Contact Ivy Chang at ivy(_at_)euchips.com if you need further information.

Cheers,

Erwin

It is important to understand how an LED works specifically a hi-brightness LED that requires a lot of current. In order to be about to control the heat being dissipated, you can use simple heat sinks under that particular LED. If there’s space for a fan, you can use that too but it’s not recommended generally for just keeping an LED cool. A fan will take more power than the LED itself.