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help with small LED problem

Discussion in 'Electronic Basics' started by gustavo ramos, Oct 14, 2003.

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  1. Hello

    I'm replacing my scooter turn signals with leds.
    i want to use 4 leds on each turn signal. no interrupt relay needed, it has
    an electronic blinker integrated in the source wiring circuit. the scooter
    runs on 12V and 14amp/hour battery. i need to know what resistor (?) should
    i use to avoid burning my super bright leds.

    thank you!

  2. Four "regular size" LEDs that probably have specifications at the
    "usual" current of 20 ma and probably have a maximum continuous current of
    30 mA:

    Such yellow/amber LEDs usually have a voltage drop of 2 to 2.3 volts
    at 20 mA, and I would guess 2.05 to 2.4 volts at 30 mA. It is easy to
    overheat such LEDs in a cluster lamp at 30 mA and that can be done at 20
    mA, but for now let's assume that there will not be significant thermal
    abuse at 30 mA in a turn signal lamp.

    (If this is for a red rear lamp, the voltage drop at 30 mA would be 1.9
    to 2.35 volts.)

    A string of 4 LEDs in series at 30 mA would drop 8.2 to 9.6 volts for
    yellow, or 7.6 to 9.4 volts for red. Subtract from the power supply
    voltage to get the voltage dropped by the dropping resistor.
    The power supply on a "12 volt vehicle" that is running is normally
    close enough to 14 volts. 14 minus 8.2 is 5.8 volts, and divide 5.8
    volts (across the dropping resistor) by .03 amp (the current flowing
    through the dropping resistor) and this yields 176 ohms. The nearest
    common value is 180 ohms.
    Another issue: Power rating of the resistor. For good reliability, you
    want the rating considerably in excess of normal actual, preferably as a
    minumum at east 1.5 times actual to almost double actual. (Ever notice
    how hot a resistor gets at full rated power dissipation?) 5.8 dropped
    volts times .03 amp is .174 watt, and this seems more than my favored
    50-66% of a popular 1/4 watt rating - I would advise use of a 1/2 watt


    If you want the brightness to not droop when the vehicle is idling or
    otherwise not having the alternator keep up with the electrical loads,
    then consider a current regulator.
    One scheme is to use the LM317T "adjustable voltage regulator", which
    can be used as a current regulator. This IC has 3 leads and resembles a
    TO-220 power transistor. The "Input" lead goes to positive power supply
    of 12-14 volts or whatever. The "Low" (adjust/whatever) lead
    goes to the positive end of the current-regulated load, and the negative
    end of the load goes to "ground" / negative power supply. The "output"
    lead of the LM317T feeds nothing but a resistor connected from that lead
    to the low/adjust lead. For a current regulator, this resistor value is
    1.2 volts divided by the desired current, or 40 ohms for .03 amp (30
    milliamps). The nearest common value is 39 ohms.
    Be sure to include any/all capacitors recommended on the package of the
    Radio Shack pacaging of this item, catalog number 276-1778.

    Other notes: I believe there are regulations concerning minimum and
    maximum candela ("beam candlepower") radiated into all sorts of different
    specified directions, for every light that a road motor vehicle is
    required to have. Although legal risks may be and probably usually
    are minor, there is greater-than-zero risk of any or some combinations of
    these, and risks vary from one state to another and one municipality to

    * Vehicle fails inspection
    * You get cited for driving a vehicle that cannot pass inspection
    (a bad case is vehicle must be towed from the location of citation)
    * You get cited for some sort of inspection fraud (typically applies
    if you replace-with-alternatives or reconfigure the vehicle after
    passing inspection)

    This would seem to apply more to vehicles with noncompliant versions of
    retrofit HID headlights, headlights with the usually-noncompliant faking
    filter means, vehicles with blue-tinted headlight bulbs tinted enough to
    come in packages that say "for off-road use only" or "check state/local
    laws", vehicles with blue front turn signals (obviously illegal, and the
    packages that the bulbs come in say "check state and local laws"), etc.

    As for morality vs. legality, if you believe you can get away with a
    noncompliant turn signal, I do advise that it be adequately visible to all
    concerned. This includes adequate visibility to those on both sides of
    the cross street of the intersection where you intend to make a turn.
    And adequate visibility includes the turn signal being visible when the
    sun is illuminating the front of your vehicle full-blast from any
    direction that any concerned other vehicle operators ever need to see your
    vehicle from. And rear turn signals need to be adequately visible to
    bicyclists who would pass you on your right but would defer if they see
    you indicating a right turn signal. (And have no less turn signal
    visibility on your left so that left-turning cyclists will pass you on
    your right as opposed to your left if you get stuck waiting to make a left
    turn at an intersection that lacks a left turn lane.)

    - Don Klipstein (,
  3. happyhobit

    happyhobit Guest

  4. I'm not related to electronics by any mean, so i don't understand very much
    of what i've read...

    what is the sugestion of leds to buy, to make 4 turn signals each with 4
    bright white leds? and what other eectronic gear should be added to the
    the whole bike works on 12volt and 16a/h battery

    thanks for the help...and patience!!!!!!

  5. N. Thornton

    N. Thornton Guest

    LEDs with 5.8v out of 14v dropped by an R are going to be less
    affected by deltaV than filament bulbs, so I wouldnt get worried about
    this myself. If 1v were being dropped it would be different.

    Regards, NT
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