high output LED powering options

Projects General Guidance
1

Hi there,

I am looking to power several strings of high output LED's (forward voltage = 3.56v @ 0.7A). I'm using an arduino for automation and internet connectivity for remote settings. I had planned on powering the LED's using meanwell's LDD-700H constant current power units, and feeding that circuit from a 48 volt DC power supply.

My questions are:

  1. I have read that it's best to allow 20% of the maximum voltage to allow for factory imperfections from 1 LED to another. The LDD-700H drops the input voltage by 3 volts, so if I feed it 48 volts, I have 45 volts maximum available. If I allow the LED's 80% of that 45 volts I get 36 volts which divided by 3.56 volts gives me a maximum of 10 LED's in series per circuit. Does this sound right or can I squeeze any more LED's per circuit?

  2. I read that it's possible to increase the brightness of LED's by using a pulsed power circuit. From:

http://www.piclist.com/techref/io/led/pulse.htm

"LED's pulsed for say 200% of their 'normal' current at a 50% duty cycle appear brighter than 100% current for 100% duty cycle even though the maths says that it's all the same average current."

Is there any truth to this with modern high output LED's (ie. cree 3v led's on star PCB's from ebay).

If so, can it be done with an arduino and would I have to build a custom constant current power supply, what components might such a circuit require? (ie. would a 555 timer chip work?). I'm questioning if it is possible, is it reasonable to put the effort in for a 'perceived increase in brightness', but I am curious regardless.

Sorry if these questions have been asked to death, I did search as best I could, here and elsewhere.

Thanks! :slight_smile:

2

Do you have a datasheet on the LEDs? Hard to answer the questions without seeing that to start.

3
  1. I have read that it's best to allow 20% of the maximum voltage to allow for factory imperfections from 1 LED to another. The LDD-700H drops the input voltage by 3 volts, so if I feed it 48 volts, I have 45 volts maximum available. If I allow the LED's 80% of that 45 volts I get 36 volts which divided by 3.56 volts gives me a maximum of 10 LED's in series per circuit. Does this sound right or can I squeeze any more LED's per circuit?

That sounds reasonable to leave a little "wiggle room", but the real issue isn't the applied voltage (the Mean Well power supply can probably supply the specified 45V). The issue is the actual voltage drop across the LEDs at the rated current.

  1. I read that it's possible to increase the brightness of LED's by using a pulsed power circuit. From:

Pulsing LEDs for effeciency, brighter appearance and multiplexed display

"LED's pulsed for say 200% of their 'normal' current at a 50% duty cycle appear brighter than 100% current for 100% duty cycle even though the maths says that it's all the same average current."

You'd have to check the CREE datasheet but my guess is you'll fry the led by doing that!

You can probably get away with 200% current and a 10% duty cycle, or something like that, but I don't know if you can find a compromise that makes it brighter and don't think you can make it significantly brighter unless you operate it as a "strobe", say on for 1/10th of a second and then off for 1 second... Long enough for the eye to perceive the brightness, but with short enough on-time and a low enough duty cycle that t doesn't overheat.

If so, can it be done with an arduino and would I have to build a custom constant current power supply, what components might such a circuit require? (ie. would a 555 timer chip work?).

There are dimmable constant-current LED power supplies. They use a 0-10V control voltage. Usually, that control voltage can be DC or PWM and it can usually be used to "flash" the LED. So, a 555 or an Arduino with a 10V "boost circuit" could work.

....I suspect if it made sense to do this, the feature would be built into commercial constant-current LED power supplies.