LM350 adjustable regulator system reduce Power LEDs voltage !!

Thread Starter


Joined May 3, 2015
Hello my friends,

I am an academician in Department of Agriculture and I aim to make growth chambers for plants which have light, airconditioning (with computer fan) etc.. One of the important thing is light intensity of chambers. So I use 9 pcs, 3W power led for one chamber and adjust the light intensity with LM350 - potentiometer system (circle below). Leds run with 3-5V and max current is 700 mA.

I use 12 V 16A power supply. As LM350 system, Vout is approximately 11.34 V. It is ok for me and I can also adjust light intensity with potentiometer. Just problem that when I run power leds, I get 8.4 V instead of 11.34V. So this is reduce the max. light intensity. If I run power leds without LM350 system, I get 10 V and it is enough for me. You will see my circle at below. I need your helps.




Joined Mar 14, 2008
Your problem is that the typical dropout voltage of standard linear regulators is about 3V, as you discovered.
You might try a low-dropout type regulator such as the LT1528 (a Google search of "low dropout regulators" will turn up others).
That particular device will allow adjustment of the output voltage to within about 0.4V or less of the input voltage at 700mA, which I would think would be sufficient for your purposes.


Joined Mar 30, 2015
In lieu of switching to an LDO regulator, you could change the way the LEDs are being driven. You could drive each LED with it's own current limiting resistor, or run them in pairs and have the odd LED in a separate leg with an appropriate current limit resistor to match the current in the other legs.

LEDs should not be driving in parallel without series current limiting resistors; unless the LEDs have been screened for matching forward voltage.

If you drive unmatched LEDs in parallel without current limiting resistors, one leg could hog current. Best case, this will cause the other legs to be dimmer. Worst case, one or more LEDs in the leg hogging current will fail and cause the other legs to be driven harder resulting in a cascading failure.