Discussion in 'The Projects Forum' started by russ3270, Apr 19, 2009.

  1. russ3270

    Thread Starter New Member

    Apr 19, 2009
    Hi everyone,hope somebody can help.
    im trying to convert a touch screen gambling machine into arcade machine and would like to build an led light panel on a 300*150 pcb.
    i have blown one set of leds up already lol.
    p.s. is a 12v regulated inline,laptop type,5a max which im using to power up pc speakers(sound is sorted now)and i would like to run light panel off same supply as there seems to be plenty overhead.
    im after making a panel of 30 white leds,specs of which are forward v 3-0-3.4(max4.0) reverse current 20ma, set out as 10 strings of three,series parallel arrangement.
    im keen after frying last set of leds using series resistor technique,to use current regulation with resistor value of which to set 20ma output.
    i would be grateful if someone would check over my specs to try and prevent another load of friodes!!
    ie forward voltage ok? regulator dropout taken into consideration etc,am i ok having 10 lm317 one per string?
    regards russ
  2. Wendy


    Mar 24, 2008
    Schematic please?
  3. SgtWookie


    Jul 17, 2007
    Do you mean 300 LEDs x 150 LEDs? That's one heck of a lot of LEDs! :eek:

    Being able to keep one's sense of humor when dealing with electronics is a valuable character trait ;)

    Ok, laptop supplies may or may not be regulated. They generally give around the specified voltage when the specified load is applied, but under light loads the voltage may be considerably higher. This might be where you ran into trouble on the last go-around. However, posting your schematic will help us help you much more effectively.

    OK, I've slightly edited your specs, but they still don't quite make sense.
    LED specs from reputable manufacturers will have a minimum, maximum, and typical Vf at a certain current, along with maximum current and other specifications. When dealing with arrays, you normally want to go with the "typical" specifications. Since you haven't spelled out the "typical" specifications, you really need to give us that information.

    An LM317 has a minimum 1.7v drop across itself when used in voltage regulation mode, and a minimum 3v drop when used in current regulation mode. The calculation for either voltage or current regulation has to take into account the Vref for the particular LM317 being used. The Vref is nominally 1.25v, but can be anywhere from 1.2v to 1.3v, and still be within specifications.

    But for starters, why don't you post your schematic so we might discern where things went so awry for your 1st experiment.
  4. russ3270

    Thread Starter New Member

    Apr 19, 2009
    Hi and thanks for replys so far,
    youre right if we couldnt retain a sense of humour when things went belly up we'd all be crazy..
    300x150 was the size of the pcb i wish to mount 30 leds on.
    lm317l min ref voltage 1.2v,1.25 is typical and 1.3 max youre spot on!!
    ref led the info doesnt seem as good as i would of hoped but thats all there is im afraid,current at a given voltage on the curve would of been nice help please.:)
  5. russ3270

    Thread Starter New Member

    Apr 19, 2009
    sorry forgot to mention ps is regulated:)

    have put rough hand drawn sketch in my public album??
    hope you can access it there,dont laugh too much im no expert and my artistry leaves a lot to be desired,i cant even draw matchstick men very well
    Last edited: Apr 19, 2009
  6. SgtWookie


    Jul 17, 2007
    OK, here's the schematic you posted in your albums:

    It's easier to post it as an attachment to the thread using the "Go Advanced" button.

    I see the problem; you started connecting the LM317's as current regulators (each just having an R1), but you took the current from the OUT terminal instead of the ADJ terminal.

    This caused the LM317's to run "wide open", dumping as much current as they could through the OUT terminal trying to get Vref (1.2v to 1.3v, nominally 1.25v) established between the OUT and ADJ terminal. Since there was no path from the ADJ terminal to ground, no amount of current would be able to establish a valid Vref.

    When you're using an LM317 regulator in voltage regulation mode, you need two resistors, R1 and R2. A standard value for R1 is 120 Ohms; if Vref is from 1.2v to 1.3v, a 120 Ohm resistor will ensure the minimum current of 10mA to provide guaranteed regulation. Since I=E/R (Current = Voltage/Resistance), then 1.2V/120 Ohms = 10mA. See how that works?

    OK, now in voltage regulation mode, the minimum dropout voltage from the IN to OUT terminals is 1.7v. Just accept that as a fact of life for a moment. (for LM317's, this is true. There are newer regulators with much lower dropouts, but you're dealing with LM317's, so let's stick with the facts.)

    The regulator tries to keep Vref between 1.2v and 1.3v (nominally 1.25v) by sourcing current from the OUT terminal. So, you always need a current path from the OUT to ADJ terminals, and from the ADJ terminal to GND.

    There's also a small amount of current flow from the ADJ terminal (Iadj), usually somewhere between 50uA and 100uA (0.05mA to 0.1mA). If you're using fairly low values of resistors for R1 and R2, this current doesn't have a lot of effect on the output voltage.

    So basically after that, it's the resistance from the ADJ terminal to ground that determines the output voltage. The minimum output voltage is Vref (1.2v to 1.3v, nominally 1.25v.) Then it's the current flow through R1 times R2, plus R2 times Iadj, that makes up the remainder of the output voltage. It sounds more complicated than it actually is. It'll trip you up until you get your head wrapped around it.

    Current regulation mode is different; only one resistor is used (R1). R1 is connected from the OUT terminal to the ADJ terminal. Output current is taken from the ADJ terminal; hence nearly all current passes through R1, except for up to about 50uA to 100uA sourced from the ADJ terminal.
    The generic formula for R1 in current regulation mode is:
    R1 = Vref / DesiredCurrent, where 10mA <= DesiredCurrent <= 1.5A.
    Don't try to use the LM317T as a constant current source for less than 10mA or greater than 1.5A; it won't regulate properly.

    CurrentOutput = Vref / R1; 130 Ohms <= R1 <= 0.8 Ohms

    Determining Vref for your individual regulator:
    1) Start by connecting a resistor between OUT and ADJ, using 1.25 as a temporary Vref, that should produce the desired current.
    2) Ground the ADJ terminal.
    3) Connect a 3.5v to 5v source to the IN terminal.
    4) Measure voltage from the ADJ terminal to the OUT terminal; this is your regulator's Vref.
    5) Re-calculate R1 using the now-known value for Vref.

    Note that in current regulator mode, you have a total dropout of at least 1.7v + Vref; since Vref may be as high as 1.3v, the two together can be as high as 1.7v+1.3v = 3v.

    You must also calculate the wattage requirement for R1, particularly when low values of resistance are used for R1. If your current is under 100mA, you can get by with 1/4W resistors.

    WattRequirement = (E^2/R) x 2
  7. russ3270

    Thread Starter New Member

    Apr 19, 2009
    Hi and thanks for your help thus far.
    i will conduct experiment this weekend to test vref of lm317l's.
    just to clarify that ive gotten my head around it,
    assuming my ps is 12v 5a regulated,and that lm317's v ref is 1.3v giving a dropout total of 3v including working voltage,that should leave me an overhead of 9v?
    does my diagram (modified to put led strings on adj terminal:D) to light 10 strings of 3 leds look feesible?
    i would use near as i can to 62.5 ohm to try and achieve 20ma current flow,are you aware of any issues with having 10 strings of lm317 with 3 leds on each?
    regards+thanks russ
  8. SgtWookie


    Jul 17, 2007
    It depends upon your white LED's actual Vf. If it is indeed over 3v at the rated current, you'll run into problems, as the LM317 is going to take roughly 3v away from the supply voltage by itself.

    You'll probably be better off to use simple fixed resistors. However, first you'll need to determine the Vref of one of your LM317's to make a constant, current regulator, then create the regulator using the known Vref, and then use this regulator to determine the actual Vf of your LEDs at their rated current.

    Once you determine the actual Vf of your LEDs (which may vary up to 10% in a lot), then you can calculate the value of resistor you'll need in series with a string to limit the current.

    A 3v drop isn't excessive. I suggest you don't try to run more than three white LEDs in series with a 12v supply.
  9. russ3270

    Thread Starter New Member

    Apr 19, 2009
    Thank you very much for advice,i will make a small scale test circuit ie one string measure and test that and if all seems ok i can replicxate x10 strings.
    will post back to let you know how i got on.
    regards russ