LED Dimmer

T

tradinup

New Member
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?
 
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 said:
So could I use the 338 as a current regulator to do the job?
Click to expand...
Its is on page 12 of the datasheet. They call it a Precision Current Limiter but it is actually an adjustable current regulator.
 
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
 

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.
 
Last edited:
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