Can I make this 5w driver circuit dimmable?

I've found a circuit here, and it looks perfect for powering a 5w prolight, because I'm using 4AA's NiMh.

Now, is there any way I can modify this circuit so I can vary the brightness of the led, as I do not want it to run at full power all of the time.

Here's the circuit:
http://www.zetex.com/3.0/appnotes/design/dn71.pdf

Mike,

Howdy and welcome. The simple answer is (like so much in life) "probably".

I assume you also meant to ask how? I'd suggest you try putting a few dozen Ohms in series with the feedback from R2 to the IC. Then add some current to the IC end (or remove it if you're one of those 'electron flow' freaks....) with a high value resistor to the battery. If you use say 50 Ohms for the series, try about 2000 Ohms to the supply. This should 'fool' the IC into thinking extra current is flowing in the LED causing it to throttle back.

Or not. But it's probably worth a try if you've a mind to fiddle.

Doug Owen

Ahh, I gotcha. With the example above, R2 is 350 Ohms, so if I up that with a POT, then the current should decrease, and the LED should dip. I've only got the prolight star so far, so I need to order the zetex chip and the rest of the bits so I test it all out.

Also, I've tried to get my head around these damn equations in the ZXSC4000 datasheet:

http://www.zetex.com/3.0/pdf/zxsc400.pdf

Basically, I need to increase to the output voltage slightly, as the dn71 example avobe gives out around 6.7v, when the spec sheet for my prolight star says it has a forward voltage of 7.1v. I'm guessing I need to maybe change the value of a resistor or the inductor slightly?