HI;
Any one can let me know the constant current LED dimmer using Transistor to control the brightness using a variable resistor.
Early reply is welcome.
Regards.
Reply me at <snip>

 

It is better to use Pulse Width Modulation to control the brightness of this LED.

• No power loss in the dimmer.
• The LED is always operating at the optimum current.

See the simplified diagram attached.

 

Not so simple. The Cree XML is a high power led. Say for instance that you had more than 6V available for input, and wanted to drive it at 1A, then look at the Zetex ZXLD1362 led driver. This constant current driver, and others like it, show how the adj pin can be fed an analogue voltage (i.e. pot) or pwm to vary the brightness. More involved solutions can be designed/simulated in National's WEBENCH led designer.

 

look at the Zetex ZXLD1362 led driver

Nice. Uses PWM internally, of course.

Actually, I'm looking for a chip just like this but that can be controlled directly from a micro-controller - probably with a serial data input - instead of analog. I will check out Zetex.

 

You could try a BuckPuck for the current regulation and a 555 PWM setup to control the current.

BuckPuck

And don't forget the massive heat sink!

The LED you mentioned, if white, can handle up to a max. of 3000mA but Cree currently recommends a maximum drive current of 2000 mA for XLamp XM-L white in designs seeking the eNeRGY STAR* 35,000 hour lifetime rating. (4Yrs) continuous - 24Yrs if used for 4hrs/day. At 1000mA and 4hrs/day it will probably outlive all of us!

 

Nice. Uses PWM internally, of course.

Actually, I'm looking for a chip just like this but that can be controlled directly from a micro-controller - probably with a serial data input - instead of analog. I will check out Zetex.

You can use a MC with the Zetek to control the LED.

 

Search for a led driver becomes somewhat easier when you define the project in terms of:
1) Low/med/high power leds
2) Constant current or voltage
3) Control configuration: pwm, I2C, SPI, DMX, etc.
4) Layout configuration: individual, matrix, rgb, serial string, etc.

Any of the large component houses will offer candidates to investigate. Looky here, a rare through hole driver (buck/boost) NCP3066-PG coughed up by a Digikey search.

 

Nice. Uses PWM internally, of course.

It's more than simply PWM. It monitors the actual current flowing through the LEDs via the shunt resistor, and continually adjusts the PWM duty cycle to make the current as constant as possible.