Calculate high power led resistor

[Yesing]

Hi!
How do I calculate the resistor to be used with the following high power leds?

-3W: 3.2 - 3.4V // 500 - 700mA
-5W: 6 - 7V // 700mA
Is it good practice to use resistors for these leds or should I used an led driver? I wanna post this issue in FAQ topic but it locked.

thanks!

[GibsonGM]

Hi and welcome!

The formula to find an LED dropping resistor's value is:  Supply voltage (Vs) - LED forward voltage (Vf) /  LED current in mA  = resistance

so  Vs-Vf / Iled = R

As long as you are not trying to dissipate more power than the resistor can handle, whether you use a driver or not depends on the application (basically, it's up to you to choose).  You can calculate the power used by the circuit and choose a resistor that is higher rated than the work you are doing with it...

If you describe or show the project you're using these in, others may have more advanced suggestions for you! 

[antonis]

Hi & Welcome..

Obviously, we don't deal with +9V (or even +18V) powered effect..
(unless we wish to illuminate the whole gig stage..)

I don't get your Power/Voltage drop/Current correlation (supposing 3W & 5W are refered on LEDs power..)

As Sir Mike  sais above, the power to be dissipated depends on Power supply voltage, LED forward voltage drop and current through it and is non-negotiable..
e.g. supposing you have to dissipate X Watts.. They can dissipated as a wholeness (at one item/device) or partially (at a combination of series items/devices).
(those items can be resistors, transistors, relays or any combination of them..)

[duck_arse]

no welcome for spammers, boys.

[antonis]

no welcome for spammers, boys.

Checked and Verified..??

[duck_arse]

homepage spamm link removed. post and thread would have followed, except someone's posted to.

[GibsonGM]

That first post always gets me, and such good questions, too!   

[antonis]

Supply voltage (Vs) - LED forward voltage (Vf) /  LED current in mA  = resistance

In kilo-ohms, I'd dare to add, your Highness..

[GibsonGM]

Good eye, Adonis...everything is in amperes and ohms unless specified, and I did specify mA...

[antonis]

To be honest, kudos should go to Stephen.. 

[GibsonGM]

I'm not a mathlete (I suck at math and forget where I convert or not, more in writing than in practice).  When I see the number on the calculator, I mentally convert it then and there if needed.  Don't ask me how it works, but it does (for me).   I play guitar far better than I do math  

[MrStab]

No, no, no. It's a biological equation. You must find the median between where most users experience retinal damage, and where they complain about dimness.

[Jarno]

Better way would be to use a constant current driver, right? Not so much for those few mA of a red led on an effects pedal, but for maximum efficiency in retina burning high power LEDs

This came to mind (retina burning)

[MrStab]

Perhaps, if efficiency were the key parameter, and PCB space and cost were lesser considerations.