Heat dissipating circuit for hi-brightness LEDs - Page 1
Forum section: electronics / projects

author: 69morpho
author_rank: Newbie
timestamp: June 01, 2014, 05:24:19 pm
content: 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

author: georges80
author_rank: Frequent Contributor
timestamp: June 01, 2014, 06:24:20 pm
content: For high power LEDs you really want to use a current regulated switching driver. Buck drivers are probably a better choice if you are running from nominal 12V supply. With an linear driver you'll be dissipating a lot of heat in the driver's pass transistor/FET if you have 12V in and only need ~6V out (assuming 2 series LEDs), at 1A drive you would have about 6W dissipation in such a driver. A switcher would be around 90% efficient for that situation and the losses in the driver would only be about 0.6W....To run DMX you need to turn that into a PWM signal that can then go into the driver's PWM input. So, smart to find a driver that has a PWM input since it will then properly handle various duty cycles correctly. Obviously the PWM frequency and duty cycle range need to be within the driver's PWM input capability.The other option is to design your own DMX decoder (I've done it in the past and it isn't too complicated) and then create a PWM control signal for the LED driver's PWM input.cheers,george.

author: mariush
author_rank: Super Contributor
timestamp: June 01, 2014, 06:41:02 pm
content: Those leds are 3.5-3.7v each at 700mA - you can pulse them for a few us at up to 1.8A but really the maximum recommended is 700mA.Like georges says, you're better off powering everything with higher voltage, say 12v or 24v and then using a buck driver to limit the current going into each led. Not only you can then use thinner wires and you'll have less voltage drop with the wire distance, but it will also be more efficient compared to boosting small voltage to something higher.Lots of buck drivers have a pwm input allowing you to adjust the actual current, here's just a few such led drivers that I found on a quick searchhttp://uk.farnell.com/zmdi/zled7000-zi1r/led-driver-buck-0-75a-sot89-5/dp/1898426http://uk.farnell.com/diodes-inc/al8805w5-7/buck-async-sot-25/dp/1904023http://uk.farnell.com/diodes-inc/ap8803wtg-7/ic-led-driver-buck-1a-5tsot23/dp/1825359These don't do DMX but you could easily use a cheap microcontroller, like a PIC16 to listen to the dmx communication and pwm a signal to the led driver. the pic would need 2.5v to 5v to work and considering it will only use a few mA, a simple linear regulator will be fine if your voltage will be higher than 5v.