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Just checking if my thinking is right on an automotive LED lighting design

Discussion in 'Electronic Basics' started by JazzMan, Mar 13, 2005.

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  1. JazzMan

    JazzMan Guest

    I want to make a set of amber DRLs for my car using LEDs in
    arrays. The LED is an AND185HAP, specs are If 50 mA MAX,
    iF 20mA typ @ 2.0-2.4V. Measured output on my alternator
    is 14.5V typ, so I want to run four parallel strings of
    six LEDs each, no resistors.

    Does this sound workable?

    JazzMan
    --
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  2. I believe you would be disappointed with the
    result and could lose (destroy) the LED's often
    enough to wish you had done it differently.

    The problem is that the LED current will vary
    quite a bit with battery voltage, which ranges
    from 11-12 Volts during engine off conditions
    to 13-16 Volts when the engine is on and the
    battery is being charged after a start. By the
    time you keep the upper end of that range
    from hurting your LED's (by using a limiting
    resistor), they will be too dim near the lower
    end of that range. Without the limiting resistor,
    over-voltage transients (exceeding the range
    mentioned) may well destroy or degrade your
    LED strings.

    The LED has the same exponential current
    versus voltage function as other semiconductor
    junctions, with a relatively small series resistance.
    If you can get a plot of the E/I characteristic, you
    will see the problem with voltage variation.

    If efficiency is your concern, rather than simplicity,
    a switching converter with controlled current
    output might be attractive. If you are not so
    concerned with efficiency, 1 or 2 fewer LED's
    per string and limiting resistor is a good route.
     
  3. JazzMan

    JazzMan Guest

    Efficiency is irrelevant to me, what I'm after is maximum
    brighness. The cost of the LEDs is trivial. The existing front
    marker light is a dual filament bulb that uses the dim filament
    for a parking light and the bright filament for intermittent use
    when signaling. A friend of mine bought a module that
    inverted that function and ran both filaments all the time
    with lights off, cycled the bright filament when signalling,
    and reverted to dim-on and bright-signal when the headlights
    are on. The problem he had was that his light housing melted
    because of the extra heat from running the bright filament
    full time as a DRL. My thought was to replace the bulb with
    as many LEDs as I can physically fit into the housing, which
    looks like it will take a 4" x 1.5" circuit board. Now that
    I think of it, I can probably run twenty strings of 6 LEDs
    if I pack them in edge to edge like the new traffic signal
    LED bulbs are built. 120 high-output LEDs should be bright
    enough. :)

    My experience with electronics beyond basic DC and some TTL
    is very, very limited from a design standpoint. I'm really
    good at laying out boards and fabricating complex circuits,
    if I have a schematic and parts list to work from, but I'm
    fairly deficient when starting from scratch.

    Anyone feel like referring me to a relevant schematic? :)

    JazzMan
    --
    **********************************************************
    Please reply to jsavage"at"airmail.net.
    Curse those darned bulk e-mailers!
    **********************************************************
    "Rats and roaches live by competition under the laws of
    supply and demand. It is the privilege of human beings to
    live under the laws of justice and mercy." - Wendell Berry
    **********************************************************
     

  4. Given your requirement, I would just cut the string
    length to 4 LED's and use a 1/2W 180 to 240 Ohm
    limiting resistor for each string. This will typically drive
    your LED's at 24 to 33 mA depending on the resistor.

    If you want better efficiency and are willing to build a
    power converter, take a look at this SMPS IC to get an
    idea of what would be necessary:
    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1705
    The datasheet shows a few more typical hookups, at:
    http://pdfserv.maxim-ic.com/en/ds/MAX629.pdf
    The part has a 250 mA current strapping option, and
    if you were to divide your LED collection into N strings,
    where N = 0.25 / (IperLED), and use a small resistor
    in each string to equalize the current sharing among the
    strings, your circuit would not be much more than what
    can be seen at the above links. (You choose IperLED
    according to how hard you want to drive the parts.)

    If you decide to go with a convertor, come back for
    a few tips on layout, or a review of your planned
    circuit prior to committing layout effort or buying parts.
     
  5. A closer study of the MAX629 datasheet shows that
    it is not rated for the input voltage you need for an
    automotive application.

    If you are eager to build a power converter to run
    your LED's, the MAX668 or MAX669, depicted at:
    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1901
    and whose datasheet is at:
    http://pdfserv.maxim-ic.com/en/ds/MAX668-MAX669.pdf
    can be made to do it.

    Its current limit is not well-defined in the datasheet,
    so, if you are interested in this alternative, say so and
    I (or somebody faster on the draw) will outline or
    explain how to set the current sense resistor to get
    approximately the current you want. This current
    should be predictable and stable enough for your
    purposes, although it is not strictly regulated.

    Replace these two links with above recommendation:
     
  6. PeteS

    PeteS Guest

    Well, if you are putting these in a vehicle, be warned that you are
    plugging into the power supply from hell (to quote an app note from
    Linear Tech).

    As a designer of equipment that ends up in vehicles, I can assure you
    that the 'nominal voltage' you get is there *most* of the time, but can
    be as high as 60V or so under load dump conditions (when loads get
    suddenly disconnected the reverse emf on the alternator windings
    generate a significant voltage), to say nothing of having large spikes
    at most times anyway.

    Because of that, I'd put a current limiting resistor in the sequence,
    otherwise as noted by others, you will regularly (and spectacularly if
    there's a really good load dump - as happens quite regularly at cold
    crank, engine pre-stall, accelerator kick). Most cars will dump the A-C
    if it's on for a maximum acceleration request, known to most of us as
    lead-footedness :)

    We'd usually generate our own power from the vehicle with a nice
    protected supply and use that, although you can use a string of LEDs
    with current limit.

    Cheers

    PeteS
     
  7. JazzMan

    JazzMan Guest

    Ok, I bought 100 LEDs yesterday to do this project. Specs
    on the LEDs are as follows:

    Vf @ 20mA 2.0V, max 2.4V
    output 11,150mcd

    How do I get one of these chips?

    JazzMan
    --
    **********************************************************
    Please reply to jsavage"at"airmail.net.
    Curse those darned bulk e-mailers!
    **********************************************************
    "Rats and roaches live by competition under the laws of
    supply and demand. It is the privilege of human beings to
    live under the laws of justice and mercy." - Wendell Berry
    **********************************************************
     
  8. If you want to drive them with 2.0V @ 20mA, then you can have 5 in a
    string, giving 10V total. That will require 20mA for each string, which
    means 400mA total current. Each string will require current limiting.
    You can do that with a resistor, but as others have pointed out, spikes
    will damage the LEDs over time.

    Another way to do it would be to use transistors to limit the current. A
    trivial 'current mirror' scheme might work here.

    A TL431A, with the reference connected to the cathode, serves as a nice
    2.5V voltage reference. Using that, you can build a little current
    source by setting the base of an NPN transistor to 2.5V, and using a
    resistor to limit current. This circuit is independent of the voltage
    input, and will draw a dependable current.

    By using that current reference, you can then use a set of PNP
    transistors to limit current across those strings. The base of all of
    these PNP transistors are tied together, and referenced using the
    current source above.


    12V -o---------o-------------o---------o--------------------.
    | | | | |
    | .-. .-. .-. .-.
    | | |10 | |10 | |10 | |10
    | | | | | | | | |
    | '-' '-' '-' '-'
    | | | | |
    | | | |
    | | | | |
    .-. >| |< |< |<
    | | 10k |----o----| .--| .. .---|
    | | /| | |\ | |\ | |\
    '-' | | | | | | |
    | | o------)----o----)--------------' |
    | | | | | |
    | | |< | | |
    | o----| Q1 | | |
    | | |\ LEDS LEDS LEDS
    | | |
    | |/ |
    o-------| |
    TL431A | |> |
    .---o | |
    | | K | |
    | R - .-. |
    '-- ^ | | 82 |
    | A | | |
    | '-' |
    | | |
    ----o---------o------o-------------------------------------------o
    GND
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    If there are spikes of 60V, the transistors and 10 ohm resistors will
    absorb them. I simulated a spike of 60V, and the LEDs went from 20 to
    22mA. That is well within their tolerances.

    You can adjust the brightness by adjusting that 82 ohm resistor, which
    sets the current limit for all of the strings. The transistors will be
    slightly different, in which case the current may be a bit different in
    the strings. However, the 10 ohm resistors will tend to even things out
    a bit. You could even go up to 22 ohm resistors without affecting things
    too badly, which would give you a larger margin for error, and also a
    bit more protection against spikes.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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