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Using resistor to slow down flashing turn signal on my bike

Discussion in 'Electronic Basics' started by [email protected], May 4, 2007.

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  1. Guest

    I'm replacing the front turn signals on my motorbike with LEDs. The
    LED light is rated at 0.4w at 12 volts, and the globe it is replacing
    is 21w.

    When I connect the new LED light to the original wiring, the turn
    signals (on the dash, front LED and back globe) all flash too fast. I
    am wanting to slow these down and was hoping someone could provide
    some advice on what type of resistor I need to apply to the circuit to
    increase the resistance to the same as the original 21w globe.

    I'm not sure if I've provided enough information for this calculation,
    so if more is needed, please let me know.

  2. Bob

    Bob Guest


    Since power = volts * volts / resistance then resistance = volts * volts /
    power, so

    R = 12V * 12V / 20.6W (note that 20.6W = 21W - 0.4W)
    R = 7 ohms

    You can probably get away with using a resistor rated at only 10W since the
    blink rate will make the effective power closer to only 10 watts (or so).
    Typically, you don't want to run a resistor at its max power rating, but in
    this case it's not a critical application, and if you select a wirewound
    resistor it will (most likely) never fail even at its rated power. Also,
    you'll probably never exceed its rated max temperature either.

    Having said all that, it seems like a real waste of power just to be able to
    use LED lamps. I've resisted putting these types of lamps on my bikes'
    blinkers because it seems to defeat the advantage of using LEDs.

    Someone must have come up with a flasher replacement that works properly
    with the lighter load drawn by LEDs. Have you looked for this type of device
    (I haven't). Let us know if you find anything.

  3. Lord Garth

    Lord Garth Guest

    Assuming you mean 2 globes at 21 watts each, one forward facing,
    the second rearward. Lets round that down to 40 watts.

    Power in watts is volts x current

    40W/12V= 3.33A

    The LED load is insignificant at 33mA so I am ignoring it.

    Voltage is resistance x current

    12V/3.33A= 3.6 ohms but it will have to be quite a resistor at 40 watts
    plus a safety
    margin! Since they are operating intermittently, you can get by with a some
    lower rating however I think you'll do better to find the minimum load at
    which the
    flasher operates or replace the flasher with an electronic version.
  4. Guest

    Bob/Lord Garth,

    Thanks for your replies!

    I've spoken to the local auto electrician and he has advised me that a
    resistor will not work in this situation. Since the flasher cans
    require power usage to create/break the circuit (creating the flash),
    using a resistor will only reduce the power usage even more and
    therefore speed up the flash even more. I hadn't thought of it this
    way previously but it seems to make sense. He has advised me to
    replace the flasher can with an electronic one, and this should
    apparently solve any problems.

    I'll let you know how it goes!

  5. What the electrician is saying is true if you were to put the resistor in
    series with the LED pack.

    What I think you need to do is to put a resitor in parallel with the LED

    The resistor will look like a bulb as far as the flasher is concerned so the
    flasher will work normally.

    The LED pack in parallel will simple light whenever the resistor is

    Resistance of bulb is given by : POWER = Volt x Volt / Resistance

    So is 0.15 ohms.

    So if you were to put a 0.15W resistor (rated at 21W) in parallel with the
    LED they I beleive the LEDs will flash normally.

    Note trying to keep simple - I know I have considered the parallel
    resistance but in reality the power in resistor (21W) is so big compared
    with the LED (0.4W) that it makes little difference to the calculation.

    Obviously, this is a waste of power (21W rather than 0.4W) but without
    changing the flasher circuit I can't see how you can avoid this. Besides
    the indicators are not on for long.

    Also, 21W is the power when the flasher is on. Because it actually, flashes
    on and off you may be able to get away with a lower rated resistor.

    Resistor could be something like this

    This is a 0.1 ohm but should work okay.


  6. Allot of people suggested using a resistor to increase the amount of current
    drawn by the circuit. You can do that, but it might be easier and cheaper
    to just use an 1156 bulb (or something similar) instead of the resistor.
    Just wire a bulb in "parallel" with the LEDs somewhere along the wire
    feeding each turn signal. You can wrap them in foil and hide them inside
    the headlight bucket or under the seat, wherever you can find a spot.

    OTOH, you could fabricate a new flasher module that doesn't depend upon the
    load to determine the flash rate. You may be able to find something already
    being manufactured that plugs in place of your stock module. Have you
  7. John G

    John G Guest

    Of course fabricating a flasher that does not depend on the load would
    be a contravention of the rules in most jurisdictions that require the
    flash rate to change to indicate the failure of a bulb.
  8. I never knew that to be a requirement anywhere in the US. A couple of years
    back I did a little research on DOT regulations regarding turn signal flash
    rates, but I don't recall seeing anything indicating that a failed bulb
    should cause the remaining bulbs to flash faster. Only that bulbs should
    flash between 60 - 120 flashes per minute. Could you cite a reference?
  9. default

    default Guest

    I have a bike and put leds in the front turn signals and tail light.
    You can just put power resistor in parallel with the LEDs to simulate
    the full load to the flasher. Cheap and dirty.

    My goal was to save power the alternator only puts out 10 amps on my
    bike and 5 goes to the headlight, the rest supports the ignition and
    running lights. Using LEDs cut about 2.5 amps load to 600 milliamps
    total - turn and tail light.

    The auto parts place wanted $17+ for the electronic flasher. I opened
    the box and through the smoked plastic cover there was a large
    electrolytic, small TO92 device, a few two wire parts like resistors
    or diodes, and a large relay with very fine wire.

    I built my own for ~$2. One bipolar transistor, one zener, one
    mosfet, small electrolytic a few resistors - no relays.

    Works like a champ and can switch between ~200 milliamps to 20 amps,
    between 11 to 15 volts supply, with very little variation in rate. I
    potted it in epoxy in a small plastic case.

    By now you've probably solved your problem. But if you want to roll
    your own I can post the schematic on the binary group or email it.

    My design is for a two wire flasher - should work in any application.
    My bike was wired for a two or three wire flasher. Two wire design
    was fun because the flasher is essentially shorting out its own power
    supply when the lamps are on.
  10. Martin

    Martin Guest

    I would Like to see your flasher Circuit.

    could you mail it to
  11. default

    default Guest

  12. A 21 W resistor!

    (That's 6.8 ohms at 25 Watts, wired in parallel).

    The lamp is cheaper.
  13. Ross Herbert

    Ross Herbert Guest

    There are various standards applying to vehicle turn flashers and
    lamps. The CEC catalog refers to the appropriate standards applying to
    each of their flasher products. SAE J590 appears to be the original
    flasher standard.

    A list of standards applying to lighting on passenger vehicles is here so you can Google for
    those which you think might apply.
  14. default

    default Guest

    I notice all the common automotive flashers they are selling meet the
    SAE 590 standard whether heavy duty, or led or whatever, including the
    one that is supposed to be able to flash one to twenty lamps.

    I'm guessing if you have 20 lamps on the same flasher and one burns
    out, there will be no change in the flash rate. Although, it could
    mean that if all the lamps burn out you'd be alerted, presumably the
    small indicator lamp in the dash won't be enough to keep it toggling -
    and some of those light during the off period of the turn signal.

    The common thread on the product list is the 60 -120 flashes per
    minute (no mention of on versus off period - but that may be in the

    Scrolling down from the multi-led lamp flashers they show some fixed
    load flashers that are supposed to alter the flash rate when a lamp
    burns out - they also meet the 590 standard
    Found it for $59; decided I didn't want it enough.

    Both of these meet the same SAE J590 standard - one is designed to
    alert the operator of a lamp outage the other isn't. Lamp outage must
    Not be a requirement for turn signal flashers.

    EF43 20 Lamp 3 Terminal Electronic Flasher
    * FMVSS 108 SAE J945, J590, and 1690 Class A
    * Heavy Duty Design Rated for 100 hours continuous operation.
    * 60-120 Flashes per minute
    * Fits all Standard Round Sockets
    * 11-15 volt, 45 Amp, 1 to 20 Lamp Capacity

    EF34 Fixed Load Electronic Flasher Turn Signal/Hazard Warning
    * FMVSS 108 SAE J945, J590, and 1690 Class A
    * 3 Terminal Double Flash Rate Design for Lamp Outage
    * Flash Rate Electronically Timed
    * Fits all Standard Sockets found in newer import cars & trucks
    * 11-15 volt, Fixed Load 2 lamps for turn signal / 4 lamps for hazard
  15. Thanks, after digging around some, I was able to find that FMVSS do
    aparently require some kind of turn signal "lamp outage" indication on at
    least some vehicles. Seems silly considering that there is apparently no
    warning required for failed brake lamps! Of course outage sensors aren't
    required on vehicles over 80" wide either, allot of common sense in that one
    too, huh? ;-)

    I had primarily been looking for flash-rate and duty-cycle information
    before and didn't even think about that aspect of it. No biggy though, I
    can deal with it if necessary. Automotive high side drivers are chock full
    of handy features like that. :)
  16. Ross Herbert

    Ross Herbert Guest

    On Sun, 6 May 2007 11:43:34 -0500, "Anthony Fremont"

    The flasher specs would most likely be in SAE J823 Flasher test.
  17. Martin

    Martin Guest

    Could you send it once more, hotmail deleted all my messages because I
    had not logged in recently enough.
  18. default

    default Guest

    Sent it again, this time as an html file with schematic and
    description on the same page
  19. Martin

    Martin Guest

    The description came through, but not the schematic.

  20. Lord Garth

    Lord Garth Guest

    Use a binaries group...alt.binaries.schematics.electronic would be fine.
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