12 Volts to 3.3 Volts - Help with Chip Selection

Discussion in 'The Projects Forum' started by campus189, Mar 14, 2012.

  1. campus189

    Thread Starter New Member

    Mar 13, 2012
    This is my first post, so be gentle,lol
    I'm a noob at this and need help :(

    Want to power 24-led's for an emergency lighting project for when the power goes out.
    Will be running off a 12 Volt deep cell marine battery that is charged by solar panels.

    Led Specs- 3.2-3.4v 20mA each X 24 =480mA total

    I am currently using cigarette lighter adapters ( See Attachments) to go from 12 Volt to 5V.

    The reason i'm using them is because I have alot of these & can get more cheap.

    My output voltage on the adapter I have.....

    Pin 1 ----Ground
    Pin 2 ----2.1 Volts
    Pin 3 ----2.8 Volts
    Pin 4 ----5.2 Volts

    Heres my question to you..

    There are so many chips to choose from, i don't know where to start..

    From what I can gather, My best options would be these chips listed below.

    All these are TO-220 style

    LM2678S-3.3 Max Voltage In- 12V,---5A
    LM1117-3.3 Max Voltage In- 15V ---800mA
    LM1086-3.3 MAx Voltage In- 5V,----1.5A
    LM3940 MAx Voltage In- 5.5V---1.0A

    Anyone got an idea which one to go with ?
    If I can keep from using lots of resistors & other components, that would be great.

    It looks like if I used the LM2678S-3.3 I would have a heatsink with alot of wasted power.
    But, I wouldn't have to use the adapter above to go from 12v-3.3

    It looks like the LM1117 or LM3940 would be best for this.

    With the LM3940, it calls for a minimum output capacitance required to maintain stability. Which is 33 μF.
    Any recommendations on capacitors ? or should I go with the minimum?

    According to datasheet, value may be increased without limit.
    Larger values of output capacitance will give improved transient response.
    The ESR of the output capacitor will cause loop instability if it
    is too high or too low. The acceptable range of ESR plotted
    versus load current is shown in the graph on datasheet. It is essential
    that the output capacitor meet these requirements, or
    oscillations can result.

    Or would I be better off modifying the 12v adapters I have?
    I have been working on this for a while now, modifying the 12 volt adapter and KJ6EAD has been helping me here

    I'm trying to K.I.S.S. (Keep It Simple Stupid) , lol

    Any help would be appreciated..

    Thank You
  2. alan1975


    Jan 1, 2012
    I'm not an expert but in my opinion converting 12V to 3.3V is a little overkill because of waste of heat.
    I suggest connecting LED in series!
    For example if You connect 3 LED together, (3.3 * 3) and You will get 9.9V.
    Now you have to kill only 2.1Volts using resistor (from 12v) or You can also use adjustable voltage regulator, to get 10V. Still just 2.1v * current.
    Just calculate:
    12V-3.3 / 0.5A
    12V-9.9V / 0.5A

  3. #12


    Nov 30, 2010
  4. campus189

    Thread Starter New Member

    Mar 13, 2012
    Well thanks anyways :(
    Got the issue solved elsewhere :)
  5. Audioguru

    New Member

    Dec 20, 2007
    Using a 3.3V regulator to power many 3.3V LEDs in parallel with no current-limiting resistor?? Don't Doo Dat!