LED Dimmer

[tradinup]

I'm trying to vary the voltage (6v - 10v) on several leds (about 13 watts). I've used an LM338T with 150 ohm R1 and a 5K linear pot R2. The power supply is 12vdc 5 amp.

Under no load the regulator does exactly as I want (0v - to 10.5v), however when I connect the load the max voltage drops to about 5.5v. No heat generated, chip is cool and resistors are cool.

How did I mess this up?

 

[bountyhunter]

You can't drive LEDs from a voltage source, you have to drive them from a current source. They are clamping the output of your voltage regulator.

 

[tradinup]

So could I use the 338 as a current regulator to do the job?

 

[audioguru]

So could I use the 338 as a current regulator to do the job?

Its is on page 12 of the datasheet. They call it a Precision Current Limiter but it is actually an adjustable current regulator.

 

[Reloadron]

Page 13 of this data sheet. Just remember that R1 in the drawing is going to be a low value high wattage resistor, you can't use a small .5 watt pot if you plan to draw much current.

Ron

 

[tradinup]

I'm going to try 15W 100 ohm reostat

 

[meowth08]

I'm trying to vary the voltage (6v - 10v) on several leds (about 13 watts). I've used an LM338T with 150 ohm R1 and a 5K linear pot R2. The power supply is 12vdc 5 amp.

Under no load the regulator does exactly as I want (0v - to 10.5v), however when I connect the load the max voltage drops to about 5.5v. No heat generated, chip is cool and resistors are cool.

How did I mess this up?

Sir, kindly post the schematic of your design so I can also learn the application of LM338T. I'm just a newbie who wants to bridge the gap between theories they teach in school and application. Thank you.

 

[tradinup]

https://www.electro-tech-online.com/custompdfs/2011/12/lm338.pdf

As noted in prior posts, this data sheet gives good examples.