# Question about my 3W LED

Hi I'm making a constant current driver shield for my 3W RGB LED, I had a question about max current. From the LED's datasheet:

Forward Current Red 400mA Forward Current Green/Blue 350mA Peak Forward Current (1/10th duty cycle, 0.1ms pulse width) 800mA

I'm confused about the 'Peak Forward Current' part Can someone help explain this? Is this at full brightness?

Yes, quite simple.

The forward currents otherwise specified are continuous currents.

The peak current corresponds to being pulsed at the condition specified - if you pulse it for no more than 100μs, no more often than every 1 ms, then 800 mA for that duration is OK.

AllenI: Is this at full brightness?

Hi,

It may be maximum current but I doubt that will be full brightness. 800mA at 1/10 duty cycle is only 80mA on average. I have read somewhere that pulsing at a higher peak current/brightness can make an led look brighter than than it would for a continuous current with the same average level. But I can't imagine doing that could make it more bright at 80mA average current than at 350/400mA continuous current.

I would stick to the 350/400mA max. Use a big heat sink!

Paul

I have read somewhere that pulsing at a higher peak current/brightness can make an led look brighter than than it would for a continuous current with the same average level.

You might have read this but it is a common misapprehension and is wrong. To an order of a magnitude, pulsed LEDs look as bright as the average current passing through the LED.

That is to say that there is some very very slight brightness advantage with pulsed LEDs over the average but the effect is quite small and very difficult to measure with people looking at them.

Sorry, I'm still a bit confused. The part I am confused on is when I have the LED set to white at full brightness, the total current draw is 1.1A. I do have a heat-sink for all three channels. I'm not sure what a 1/10th duty cycle, 0.1ms pulse width would look like as far as brightness, but it doesn't seem like full brightness and I don't own an oscilloscope. 400mA+350mA+350mA=1.1A. I should be ok drawing 1.1A, right?

I had another question about Forward Voltage. I understand LEDs are current drawing. I'm using a 14V power supply as input for the CC Drivers. The Voltage shouldn't matter as long as the current does not pass the current limit on the datasheet. Is this the right thinking?

Max Forward Voltage: Red/2.8V; Green/3.8V; Blue/3.8V

I'm not sure what a 1/10th duty cycle, 0.1ms pulse width would look like as far as brightness,

It depends on the current, but if it were the same as the continuous current there would be about one tenth the power. That is about one seventh as bright.

............ is this the right thinking?

yes.