LM317 power supply

Discussion in 'General Electronics Chat' started by anvit2a, Jul 28, 2011.

  1. anvit2a

    Thread Starter New Member

    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 !