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Connecting strings of LEDs in parallel

Discussion in 'Hobby Electronics' started by Jason Burton-Woods, Aug 4, 2004.

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  1. Hi all,

    Theres a project going on the forum about how to
    convert an incandesant tail light, into a bright LED light... but we could
    really do with some help from the experts?

    The guys are standardising on the following:

    Using a LM7812 to regulate the variable voltage from the bike down to a
    constand 12v, make 8 strings of 3 LED's and connect those strings in
    parallel... if you cant follow me then have a look at this schematic..

    There are two resistors used, on 150 ohm connected to the Brake light +,
    the other 150 ohm to the running light, these are in parallel so when the
    brake switch is not in operation then the total resistance is 1/2 that
    compared to when the brake light switch is operated.

    So, heres the question...

    From a number of difference web sites we've been looking at, we've found
    people saying its important to wire an individual resistor into each
    string.... and of course as the web would have it, we came up with a few
    that said its not necessary... Think you could clear this up for us??

    By the way, the guy whos running the project on this international forum is
    an aussie too... I'm sure you could give us some useful input into this!

    Thanks all in advance.

    P.s. Link to the thread is here:;action=display;num=1090891230;start=15#27
  2. Peter Jetson

    Peter Jetson Guest

    You need to have an input/output differential of almost three volts
    across an LM78xx voltage regulator, or it will not regulate.

    Doesn't a bike run a nominally 12 volt electrical system? If that's
    true, then you can't use an LM7812 regulator, because you'd need at
    least 15 volts on the input to get a regulated 12 volts on the output.

    Also, if a bike electrical system is anything like a car's, then it will
    be full of spikes and other nasties. An LM78xx voltage regulator is not
    the "regulator of choice" in a car - you need something with much better
    spike tolerance and resistance.
    Wouldn't I be just one more voice for one side or the other? How would
    that clear things up?

    The purpose of an individual resistor is to allow each string of leds to
    "share" the available current. If you can ensure that the forward
    voltage drop of each string of leds is not only identical "at rest" but
    also over the entire temperature range that the leds will be subject to,
    you will not need an individual resistor in each string. Since that is
    not something that you can easily ensure, using an individual series
    resistor in each string is proper design practice.

    If you don't use an individual resistor in each string, then each string
    will have a different forward voltage drop, and the string with the
    lowest forward voltage drop will "hog" all the current, and will be
    brighter than the rest of the strings.

    Since the apparent object is for each led to be equally bright, then you
    need the resistors. And, anyway, it's not like the extra resistors are
    going to cost you much, or take up much extra room. Why would you leave
    them out?

    Silicon Chip magazine ran an article called "LED Lighting For Your Car"
    recently (in the March 2004 issue - ). You may be
    able to find it in your local library. Or you could spend a few bucks
    on buying the back issue from Silicon Chip themselves.

    Also, I think I remember seeing commercially made led replacement lamps
    in recent Jaycar adverts, although a quick check only turned up this and that said that
    product was marked as "available soon".

    Finally, I have a vague recollection that I read somewhere that swapping
    an incandescent globe for an LED replacement may affect a vehicle's
    compliance with Australian Design Rules, and could therefore render a
    vehicle unroadworthy.

  3. gcd

    gcd Guest

    here are my thoughts, no doubt others will have their own.

    Are you looking to using white LEDs or colour LEDs?
    Are these in a tailight with reflector?

    Using LEDs in parallel does require a resistor in series with each parallel
    chain. Without the resistor, the circuit will balance itself out to achieve
    the same voltage across each parallel path, this may result (and most
    probably will) lead to different currents through each path, leading to
    differing intensity for each parallel path.

    Without knowing answers to the above questions, i'd opt for white LEDs in
    place of the existing bulb and maintain the reflector.

    If you want to use parallel chains for redundancy then I'd look at some
    dedicated ICs for the job. The Fairchild FAN5606, 5609,5610 series is a
    starting point. Or a host of other ICs made for driving LEDs as a current
    source. ie Maxim, LTC, etc,etc

    Or the simple cheap way - you could use for each chain 4 x white LEDs in
    series without any resistors. The forward voltage of a white LED will be
    between 3 - 4 V depending on make and optical power rating and accordingly
    the forward current required will vary from 20mA to 50mA+ typically.

    For a tail light you may only need a total of maybe 6 or 8 LEDs, again
    depending on the optical power and if the reflector is still in place.

    Without a resistor or better still a pulse width controller , the intensity
    and colour will change with voltage changes.

    just some starting thoughts

  4. Unbeliever

    Unbeliever Guest

    With a dropout voltage of 2V, the 7812 will not guarantee a constant 12V
    unless the battery terminal voltage is above 14V. Perhaps you might
    consider a 7809 and a lower value resistor.

    LED brightness depends on current, but how well the strings share current
    depends on the consistency of the voltage drop of each string. This is
    pretty much a manufacturing/testing parameter and will depend on the actual
    devices used - you could "suck it and see". If they're from the same batch
    you might get away with it, depending on the manufacturer, particularly if
    they've been "binned" or selected carefully. Personally, I'd use individual
    resistors on each string, particularly in an application where vibration
    might cause failure and necessitate replacement down the track. They won't
    need to be 5W though. a 1/4 watt 1k2 resistor will give about the same
    current through 1 string.
    BTW, from experience with my 1975 Ducati 860GT (b4 electric start) you'll
    have more trouble with those crappy Italian connectors than with the LEDs.
    I ended up replacing the entire loom with a Suzuki loom. Reduced my
    problems manifold.

  5. GM

    GM Guest

    Why not use a constant current source - eg LM317 - look at the
    datasheets for example applications. They are pretty cheap.
  6. Bill Bailley

    Bill Bailley Guest

    Perhaps even a resistor/zener to limit the applied voltage to the strings
    when the alternator generating. Zeners actually gobble up spikes!! I like
    simple better.

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