Driver voltage/LED Vf Question

So, I've got most of the basics down. Draw more current to output more voltage, etc... but what I can't find is this:

Does the sum of the Vf of the LEDs (in my case 4 Cree's in series at ~3.7v) need to exactly match the output of the driver?! Or is current really the only thing to worry about? Is there some sort of tolerance? Does it depend on the LEDs?

Thanks!

I'm oldfashioned and believe you should consider leds as needing a set current to drive them rather than wory re voltage drops like Vf across the LED.
If you want to live dangerously you can increase the drive current but the critical factor is heat generation and the need for adequate heatsinking with excellent transfer from the LED to the heatsink. Safest to run LEDS at the "typical" forward current. (If).
EDIT
If you have 4 Crees in series you will have the same current through each cree (say 800MA and the voltage drop across the lot of them will be 4 times the Vf for 1 Cree.) If you use a decent current regulator 9or driver) it should supply 800MA an a voltage of 4 times the Vf of 1 of the Crees you are using.

botsdots said:

So, I've got most of the basics down. Draw more current to output more voltage, etc... but what I can't find is this:

Does the sum of the Vf of the LEDs (in my case 4 Cree's in series at ~3.7v) need to exactly match the output of the driver?! Or is current really the only thing to worry about? Is there some sort of tolerance? Does it depend on the LEDs?

Thanks!

Click to expand...

You should worry about the current and the heatsinking. I think 1000mA is safe. But as you go higer - you get less eficiency.

If you are powering your LEDs with a constant current driver (which is the preffered way), then the only voltage you need worry about is the max and min output voltages of the driver (and of course the input voltage TO the driver, but Im sure thats not your question).
IF you have an 800mA CC driver, eg with min 2.5V and max 24V, you can connect one cree and it will run at 800mA, and you can connect up to 6 crees in series, which will also run at 800mA.
The CC drivers will adjust their output voltage to achieve the set rated current, WHITIN their specified voltage range. Like this: You have one cree, power up the circuit, from 0V 0A, the driver ramps up the voltage, and at about 3.7-4V it hits 800mA, and it stabilizes there. Now if you connect two crees in series, there is no current flowing until about 6V, and you hit 800mA at 8V or so.
Thats why you can only run one or two max crees from a flashlight board (low max voltage), and why some AC drivers need a minimum of 5 or so crees (high min voltage).
Hope I was clear enough! I had to go through the same questions as you and Im still learning!

PS To complicate things further a bit, for most buck and boost constant current drivers, the output voltage range is depndent on the INPUT voltage to the driver. eg if you have a buck (lowers the output) driver, if you put in 18V you get max 16V out, and if you put in 12V you get max 10V out.

Thanks for all the info.

So I bought the Shark before realizing I didn't know what I was doing. I plan on running them at somewhere around 800mA (+/-50) and what I think I'm hearing is that the Shark will adjust it's output voltage so that the current is regulated at 800mA. Is that correct? In that case, I don't really have to care about the voltage unless I'm supplying enough that the Shark just passes it through, unregulated.

Ahh... so much to learn.

:thanks:

From a quick reading of the shark specs:

You can adjust the output current with the on board trim pot. "Simply jumper J1 for LED driver applications and the Shark will be a constant current regulator with open circuit protection of ~22V and the trim pot sets the LED current."

For Vin:
"A general rule of thumb to use the Shark.

1) Determine the number of LEDs you want to drive in series. Example: 4 Lux3s.

2) Determine the series voltage of the LEDs at the desired current. Example: 1A which means ~4V/LED and 4LEDs = 4 * 4V = 16V (approximately)

3) Determine the operating and min battery voltages using the total Vf output. Operating is > 1/2 output and min is approximately 1/3 Vout.
Operating should be greater than 1/2 of 16 Volts or a fresh battery pack should be 8+ volts."


SO you need to supply it upwards of 8Vs, and below 16V, since:

"The only caveot is to ensure you have a boost configuration under all conditions. Letting Vin rise higher than Vout will put the converter in DD mode and will most likely Fry the converter board, LED and possibly the battery."

Sooooooo for your case 12V input would be ideal.

PS what is the intended use for this setup?