Newbie -> How to drive ~40 LEDs in car?

Discussion in 'The Projects Forum' started by Migee, Mar 30, 2013.

  1. Migee

    Thread Starter New Member

    Mar 30, 2013
    I've been killing myself trying to understand constant current drivers lately, I feel that it is simple on a high level but trying to understand what I need and reading datasheets throws me back to step 1. I tried (re)educating me on a lot of the simple electronics I knew in the past but I am still at a loss.

    At its core, I am trying to wire a few boards with 40 LED's that can be powered from my car. I still have no idea what kind of CC driver I need, or if I even need one in general... Here's what I know:

    The 8mm LED's are 20ma, listed with forward voltage of 1.9v (max 2.3v). I believe I understand that these will work much better with a CC driver than simply using resistors right up to the power source and nothing else (something I've done in the past). The voltage from the car fluctuates around 13-16v depending on its state as far as I know.

    First thing I don't quite get. Do I NEED a driver that is rated for 20ma output? Are there drivers with adjustable current? I assume that running a driver that puts out a higher current will toast the LEDs, if so, is there a way I can just run LED's in parallel to negate that? (Been so long since I've toyed with electronics I have to relearn this stuff!). I am also looking to have PWM added to allow me to adjust brightness as I need.

    Literally spent my entire last night trying to wrap my head fully around transistors and understanding basic current limiting devices. I do generally get the idea however there are certain aspects of them I feel are not explained enough (i.e. depletion zones I understand but then a electric field is tossed in the mix and I feel lost again. perhaps a question for a different thread).

    I am basically looking to see how this would be accomplished by the experts here. My idea is I can see the end result and work back from that, rather than trying to explore the right way out of 500 ways of doing this.
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    40 leds in series, at 2v each is 80v, so a constant current source of 20ma would work as long as the supply will get above 80v.
  3. #12


    Nov 30, 2010
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    Tip: You can possibly use groups of 4 2.3v LED's in series with some of the current limiting circuits shown in #12's link with 12v auto battery supply.
  5. #12


    Nov 30, 2010
    Ps, there is no law that says you have to run the LEDs at the maximum survivable current. Calculate for the highest voltage to produce 20 ma and resolve yourself to the fact that the LEDs will use slightly less current when the voltage is slightly lower.

    Either that or go with a constant current supply for each string. A simple 317 chip and one resistor makes a constant current driver.

    Then there is the fact that car systems have horrible voltage spikes. You must protect against spikes to about 60 volts.

    And then there is the fact that car modifications are not allowed on this site and a moderator might shut you down at any time. You are probably safe if you are just plugging in to the cigar lighter.
  6. Rbeckett


    Sep 3, 2010
    This thread is very close to violating the moratorium on car stuff. I suggest that you consider carefully what you are seeking....
    shortbus likes this.
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Newbie -> How to drive ~40 LEDs in car? Put them all in a box, throw the box on the passenger seat and go! :)
    #12 and absf like this.
  8. Migee

    Thread Starter New Member

    Mar 30, 2013
    Thanks for the help (and laughs!) guys. I'll close this topic I didn't realize automotive projects were not allowed :(

    Unfortunate, I can see the reasoning but it does have two sides.

    Edit -- looks like I can't close my own. Mods, have at it