Low dropout adjustable voltage regulator.

[FranciscoPerez]

Hi there!

I'm building a voltage regulator for my girlfriend's led table lamp and would appreciate some advice.
Power supply secondary is 4v, 3Watt. I tried using a LM317 I had available at home, but this doesn't seem to do the trick. Without load (the led) I cannot read more than 3.2V output with my voltmeter.
Loaded, it gets even worse, and I get a voltage range of 1,2v - 2,6. I expected the lower limit (as written in the lm317 datasheet) but thought I'd get an upper voltage near  4v.



Do you know another adjustable voltage regulator with less voltage dropout that will suit this particular application??
Thank you!!
Best Regards,
Fran

[HotBluePlates]

I'm not knowledgeable of solid state voltage regulators, so I can't recommend a lower-voltage regulator.

However, I think you're misunderstanding something about these. Maybe it doesn't apply to the LM317, but I've seen some regulators of this type called "3-volt regulators". This does not mean the output of the regulator is 3v, but that the voltage across the regulator must be 3v. In other words, the regulator maintains 3v between its Adjust pin and Output pin.

So if you start with 4v, you probably won't have enough voltage to drop across the regulator to get 3v out. It would be much better for you to start with 6-7v (or even more), and regulate that down to 3v.

See this video for a tutorial on the LM317.

[FranciscoPerez]

Thanks for answering HBP!

Don't know if LM317 is a 3-volt regulator, read the datasheet and did not find any references to this.
Anyway, I think I didn't explain things very clear.

Power suply that feeds the Led delivers 4V/0.75A , so I would like to regulate these 4V in a range from 1,2v to 4V in order to obtain different intensities of led light depending on the rotation of the potentiometer. (I did not build this power supply, and must use this one).

However, using the LM317 I get a range from 1,2v (pot at max counter clockwise) to 2,6v (pot at max clockwise) and would like to get closer to the 4v to go from barely no illumination to full led illumination.
All voltage ranges are being measured from LM317 output to ground.

Hope it's clearer now, should've taken more english lessons!! 

Thank you!!

Kind regards,
Fran

[sluckey]

If your intent is simply a dimmer you don't need a regulator. Just use a series pass transistor...

[FranciscoPerez]

Hi sluckey,
Yes a dimmer is exactly what I'm after. That's great, thanks!!

I downloaded the 2N3055's datasheet but don't know where to look at for finding max load current values.
Power supply secondary is rated 0,75A so would be fine if I use a NPN that is able to handle 0,75A, right?
Room here is an issue, so would like to use a NPN as small as possible.

About the potentiometer, would a 5K pot be suitable for this? I got some of them around, and they're switched so would be perfect to use if possible.

Thanks for your help guys!!! 

[sluckey]

I mentioned 2N3055 because it's waaay overkill for that LED dimmer. It would run pretty cool. A TIP 29 comes in a TO220 package and may be more suitable. Just about any power NPN would work.

5K pot should be fine.

[FranciscoPerez]

Worked great!

I finally used a BC639 and added a 12K resistor in series with the pot to ground to adjust the voltage range. The transistor gets hot, but not so much that you can't touch it so I think I'll use it.
Thank you both!!

Kind regards,
Fran