High Power LED PWM

[simonB]

Well yet again I feel dumb as heck... how to light an LED for crying out loud !??

So let's start with the LED, a Luxeon Rebel Star.
Typical Forward voltage is 3.2v.
I intend to run it at 2 current levels, 350ma & 700ma.
Proposed power source will be 3*AAA rechageable cells = 3.6v.

Here's my calcs for the resistor when running at 350ma:

(3.6v-3.2v)=.4v
0.4v / .350 = 1.1 Ohm

and 0.6Ohm to produce 700mA

Can someone firstly tell me if that's right ?
tbh I feel silly putting such a small resistor.... but I'm inexperienced and easily paranoid !


Next problem....erm 'challenge'
I want to then control the brightness of this LED with PWM from an ATTiny.
Now that will involve a transistor.
I am completely lost as to how to choose the best transistor for this.

Mosfets, as I understand, would be overkill and possibly need a driver ??
So I'm sticking on the line of transistors.....

I found this one : TIP31C
From what I can tell, it will handle the current/voltages that I need, but I'd really be grateful of a second opinion !

http://www.hobbytronics.co.uk/tip31c


Thank you for any advice

 

[(*steve*)]

check the tutorials for a large tutorial on driving LEDs. The short answer is that a constant current driver is best

in your case the resistor might be ok because the battery.voltage will drop pretty quickly and the chance of thermal runaway happening is low. However you're unlikely to get the full current either

 

[BobK]

As Steve implied, you power supply is a bit weak for a 700mA 3.6V LED. Are you trying to make a flashlight? This site has lots of very inexpensive parts for doing so.

Deal Extreme

Bob