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luxeon emitters

Discussion in 'Electronic Basics' started by andy, Sep 18, 2005.

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  1. andy

    andy Guest

    can you get away with running 3 luxeon emitters in series from a 12V
    supply?

    I know that 12V means 13-14V in practice, so in theory it should be
    possible to set them up like:

    12V --->|--->|--->|---[===]--- gnd

    with the (variable) resistor set to whatever resistance was needed to get
    the right current from whatever voltage drop was left over. But is this
    likely to work OK in practice, or would it be better to use 2 in series
    and a more reliable 4-5V voltage drop through the resistor?

    Also, is using a variable resistor a good-enough way of dimming the
    lights, or should I build a more complex circuit?

    This is for lighting on a friend's canal boat.
     
  2. Bob Monsen

    Bob Monsen Guest

    The three should work fine. However, using a simple pot means that all the
    current will have to go through it. At 350mA, if your battery is 13.8V,
    and the Vf is 3.42 (see http://tinyurl.com/8a9ut for datasheet) then the
    pot will have to dissipate about 1.2W. Thus, one of those tiny radio
    shack pots will just burn up. If you do this, make sure the one you buy
    can take at least 2W.

    One simple way around buying a pot like this would be to use a transistor
    to set the current. One approach is to use a single NPN power
    transistor (like one in a TO220 case) and use the pot to set the base
    voltage. This has the disadvantage that the current (and thus the
    brightness) will vary widely depending on the temperature. However, it may
    be good enough. Note that the NPN will dissipate that 1.2W as above, so it
    may need a heatsink, depending on location.

    If more precision is required, a 1 ohm resistor between the NPN's emitter
    and ground can be used to measure the current. This can be used as
    negative feedback to an opamp, which then is used to control the base of
    the NPN with its output. By setting the voltage on the positive input, you
    can control the current with good precision, and the current won't change
    with temperature. A pot can be used to set the non-inverting input of the
    opamp. However, even more precision can be obtained by using a temperature
    compensated voltage reference. This way, your current will no longer be
    proportional to the input voltage.

    However, it just occurred to me that you could use a low voltage TLV431,
    which uses a 1.2V ref, and a 3.5 ohm sense resistor, like this:

    -----------------o-------------- 12V
    | |
    | |
    [led string] [330]
    | |
    c\| |
    |-b---------o
    e/| | K
    | ---
    o---------R-/ \ TLV431
    | ---
    [3R5 1W] | A
    | |
    ----o------------o--------------- GND

    When the current gets above 350mA, the TLV431 will turn on, pulling the
    base of the NPN lower, thus decreasing the current. This gives you an
    accurate, temp compensated current source which doesn't depend on the
    input voltage. It only requires a drop of about 1.5V at 350mA, so it can
    handle a voltage down to a bit lower than 12V with 3 luxeon emitters in
    series.
     
  3. andy

    andy Guest

    thanks - I'll try the transistor current source first, I think, and then
    try your circuit if that doesn't work.
     
  4. Jasen Betts

    Jasen Betts Guest

    you didn't mention which luxeons you were planning to use

    the nax forward volltagge is 4v so unless you intend using a switching
    regulator two in series is the highest I'd go - otherwise use a boos
    regulator and wire them all in series....
    from reading the specs it seems some of them may be effected
    by humidity (they mention humidity next to the longevity chart)

    Bye.
    Jasen
     
  5. Jasen Betts

    Jasen Betts Guest

    you didn't mention which luxeons you were planning to us

    the nax forward volltagge is 4v so unless you intend using a switchin
    regulator two in series is the highest I'd go - otherwise use a boo
    regulator and wire them all in series...
    from reading the specs it seems some of them may be effecte
    by humidity (they mention humidity next to the longevity chart

    Bye
    Jase
     
  6. andy

    andy Guest

    luxeon star III, white, lambertian radiation pattern.
    I've checked this since, and 4v is the max forward voltage, not the
    typical forward voltage, which is more like 3.6v, so I think I can get
    away with using 3 as long as the battery isn't run too low.

    What I'm thinking of doing now is to make a dimmable current source with a
    voltage reference, op amp, and transistor, and see if that works.
     
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