Turn Signal LEDs Resistive or Active Current Limiting?

Discussion in 'The Projects Forum' started by yoshirocks702, Sep 21, 2010.

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  1. yoshirocks702

    Thread Starter New Member

    Sep 21, 2010
    Hello everyone, I've been doing a ton of research for retrofitting LEDs into my scooters turn signals, I'm currently working on a prototype version of it.

    So far I bought 12 Amber Lumileds Superflux Model: HPWT-ML00-0000, I've decided to do a 4x3 array, consisting of 4 strings with 3 LEDs per string.

    From what I understand in the datasheet the LEDs should be runned at 50mA-70mA with a forward voltage between 2.19v-3.15v (2.6v nominal) @70mA.

    From some the service manual of the bike, max voltage is 18v, with 14v charging, and about 12.9v with a fully charged battery. The turn signal only has two wires, +12v power and ground.

    I've seen many different ways to power the LEDs, I'm wondering what everyone thinks is the best way to go. So far I've seen:
    1) The simple current limiting resistor in each string.
    2) The LM317 regulator with a resistor between the adj and out to make it a constant current.
    3) The circuit that uses an NPN transistor, a N-channel FET, and 2 resistors.
    4) The sense pass current regulator.
    5) The switching current regulator.

    I also want to ask about incorporating something called EMC transient protection which looks like a silicon diode in series with the LEDs, and a zener diode wired in parallel, is this a good thing to have or is it unnessesary?

    Here's some circuit diagrams of each, the LED array I'd like to use is 4 parallel connected strings with 3 leds per string.

    1) simple resistor method with that EMC protection

    2) LM317 method

    3) NPN transistor method

    4) and 5) Sense pass and switching method
  2. tom66

    Senior Member

    May 9, 2009
    We might want to get moderator approval before going further with this, as modifying the lights on your scooter probably comes under the rule about not messing with vehicle lights.

    But, anyway, neither of those is really a solution.

    There are several problems:

    • Wide range of battery voltages (12.9V to 18V) - this means you can't use resistors
    • Load dump transients (60V+) - these will kill a LM317
    • Fooling the turn signal controller - it will probably think the lights are blown with such low current; some controllers flash the lights faster if they are blown.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    That falls into an area we don't give advice on. It is best to use an approved LED bulb.
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