LM317 power supply

Thread Starter


Joined Jul 28, 2011

I'm new on this forum and really hope you guys can help me a bit out here to understand a few things better.

I'm planning to make a few power supplys for a few diffrent things.
I need to make at least these;

* Power supply for 3x LED lights, with one "brightness controller"
* Power supply for 2x Canon PowerShot A495 (3V 1,5A)
* Power supply for 2x 4.3" LCD screens (12V, maybe 1A ?)
* Power supply 4,5 V and 1A

I'm not sure if i need more but i really want this project to learn me how to make and understand a bit more about these things. I wanna do it this way because everything can be power into the circuits by the same DC. I'm planning to use a computer power supply. most of the power supplys should be adjustable with a potentiometer.

The first and maybe the most difficult thing is the LED power supply/driver.
I will be using these leds; Cree XP-G R5 Cool White
It is a 3W led with Min. lumens 139 @ 350mA and a max drive current of 1500mA or a bit more like this:

Forward voltage (@ 350 mA)
V   3.0 MAX 3.75 Forward voltage (@ 700 mA) V   3.2   Forward voltage (@ 1000 mA) V   3.3

The max voltage is then 3,7V and the led might be driven at 1000mA.

For this i have been looking at the data from LM317 at page 22;

And also this "guide"; http://www.instructables.com/id/RGB-Color-Controllable-High-Power-LED-Room-+-Spot-/

I really need to get around this and figure out what to order so i won't order anything i don't need (this project has to be as cheap as possible :D )

If i look at the datasheet at page 22 i have a current limited 6V charger.
I have calculated that R1 (resistor 240 in datasheet) could be 240Ω. (calculated with http://www.reuk.co.uk/LM317-Voltage-Calculator.htm)
and R2 (resistor 1.1K in datasheet) should be 2064Ω which means a potentiometer with 2k Ω.
But how do i fix the current? in the datasheet it is the 100 resistor, 2N2222 and 1* ? I should adjust to 1000mA ? This part i cannot seem to find a way to calculate... (It is not possible to make this adjustable very easy and cheap?) or maybe i should not even adjust the current? This is where i am right now :)... i really want to be able to make this so i can adjust the brightness of the leds. I just want to understand the datasheet perfectly to be able to adjust to my own purpose.

and also the princip for Power supply for 2x Canon PowerShot A495 (3V 1,5A). I want to make this part adjustable too. atleast at the voltage. Maybe it is not needed to be able to adjust the current? then i maybe should just use the simple adjustable regulator from page 1 in the datasheet? if i can understand this part i should be able to do the rest myself i think.

another question... what is best be able to "slowly" adjust the voltage(/brightness?) with a potentiometer? I mean should i use like a R1 at 240Ω and R2 (potentiometer) at 2k Ω? Or is it better with higher values like R1 = 1200Ω and R2 = 10kΩ
Have i got any chance to foresee what the best solution is?

Really hoping there are some smart people here that will just try to help me out a bit :) ...

All the best !