driving high powered LEDS

hi i am interestead in using some high powere leds with a If of 700mA and Vf of around 3v(typical max is 3.8v) power supply would be 12v would it be ok to use resistors or would i need something better? i am thinking of using 5-6 leds in parralell i should also add i would like to alter the brightness by pwm

Hi,

If you are interested in efficiency (and usually with high power LEDs we are) you would want to use a switching power supply or use LEDs in series to get close to the supply voltage of the power supply so you can use a dropping resistor without loosing too much efficiency. PWM can come in various ways depending on how you decided to start out.

efficiency is probaly not high on my list to be honest, i will give some background... i am setting up yet another aquarium this time in my office, i have some LEDS rated at 3w 700mA and i wanted to have a play with the lighting for the aquarium i also have a couple of high powered RGB leds i was going to use to adjust the viewing colour, i was thinking maybe driving each from a LM voltage reg via a transistor and maybe pwm the transistor with a pic to adjust brightness as i would like to gradualy increase the light level when first turned on and then decrease it gradualy when it is switched off but i am unsure how to design it. i have plenty ATX PSU's here rated from 250W-750W so would probaly use one of them.
I am not concerned about part count etc, for the heat sink i am thinking of alluminium carpet strips lining the lid and sitting the leds on those with maybe a fan or two but havnt decided yet as i am open to ideas!

I did something similar to give the effect of a TV being viewed behind curtains when I was out and didn't want the TV on.
I used a ULN2803 darlington device to power the 1 watt LEDs driven direct from a 8051 based micro using the PWM counter registers. PM me if you need some source code for the:

https://www.electro-tech-online.com/custompdfs/2012/05/C8051F321_Short.pdf.

I used 5 different coloured LEDs and using a series resistor to each one calculated the on current as each one had a different forward voltage.
Each LED had a heat pad underneath. I used Artic Silver (for CPUs) heat sink compound to strap them over a piece of single sided copper clad board cut in a thin strip on a guilloteen to disipate most of the heat.
Works fine without a fan and the software alters the brightness from a lookup table to emulate a TV screen watching a movie.
A word of warning, don't look at the LEDs when running as they are incredibly bright.
I used a 15W Traco power supply to run the device.

the more i think of it the more i see no reason not to drive them from a transistor if i run them in parralell from a 5v line of a ATX supply then i dont see much of a problem i would only have between 1-2v to drop over the resistor and to get the current for each led i want i could just parralell some 1W or 3W carbon resistors instead of the chunky 10W metal ones i have it might be easier to get the resistor value that way as well. as i would only be looking at around 3R to get me close to 700mA
i love the fake TV screen idea!! might be a market for that lol

LEDs are NEVER connected in parallel unless you buy a few thousand then measure and sort them into groups that have exactly the same forward voltage.

Which is why individual series resistors are used to limit the current. Not sure what the point is of your post.

to be fair to audiguru when i reread my post i can see why he thought i would be doing it that way i didnt make it clear that i would add a series resistor to each LED but rest assured a resistor for each it is

ghostman11 said:

to be fair to audiguru when i reread my post i can see why he thought i would be doing it that way i didnt make it clear that i would add a series resistor to each led but rest assured a resistor for each it is :d

Click to expand...

goody!

In my last Projects i used an LM3404 LED Driver chip.
It's a Swithing Power Supply for LED's, offers PWM Dimming and works up to a maximum LED Current of 1A.

When the input Voltage is high enough, You can put some LED's in series at the Output of the Chip.

When you need more Current you can try to use an LM3406 Chip. That works up to 1.5A.
With the LM3406 i have no experience.

wkrug said:

In my last Projects i used an LM3404 LED Driver chip.
It's a Swithing Power Supply for LED's, offers PWM Dimming and works up to a maximum LED Current of 1A.

When the input Voltage is high enough, You can put some LED's in series at the Output of the Chip.

When you need more Current you can try to use an LM3406 Chip. That works up to 1.5A.
With the LM3406 i have no experience.

Click to expand...

i might have a look at that but would depend on price as i am driving a few of theese leds

At Digikey ( USA ) the LM3404 is 3.28$ and the LM3406 is 3.95$.
At mouser the LM3404 is 2.71€ and the LM3406 is 3.26€.

if you are comfortable with HV, ditch the ATX and use an HV9910 and put them all in serial. better yet there are 1.5A HV chips now.

with the HV9910 you use and ext drive PFET. for multiple strands you should use current sources to force sharing. the old standby lm317 is not the best but it works.

Dan