SgtWookie... Help me! My LED's are getting hot..

Discussion in 'The Projects Forum' started by Cr_rydah02, Apr 19, 2011.

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  1. Cr_rydah02

    Thread Starter New Member

    Apr 18, 2011
    First of all, im a second year Industrial Design student and this is a school project of mine down at Arizona State University.. I had absolutely no knowledge of how electricity works untill about 2 weeks ago when i decided to tackle this assignment.. and im aware that i still dont know a thing! haha

    So, I got the bright idea to wire up some LED lights on my Dirtbike.. A little backround on my dirtbike.. there is no battery.. a stator generates the power when the motor is running.. a regulator/rectifier converts the AC to a DC... I spliced into the power wire coming off of the regulator/rectifier and my multimeter showed a steady 14.5 volts..

    Knowing this info, i purchased (4) 5mm LED bulbs. These bulbs are rated at 18,000mcd, Continuous Forward Current of 30mA, Peak Forward Current of 70mA , Forward Voltage is 3.4V - 4.0v maximum...

    so i wired these 4 bulbs in series ( Power wire was soldered to Anode on first bulb, and the cathode on the 4th bulb soldered to a ground)...

    I figured that 14.5 volts would be soaked up my the 4 bulbs wired in series (3.625V x 4 bulbs = 14.5 Volts) so the bulbs are well within the voltage rating, Right Sgt? I didnt think i would need a resistor on this circuit... is this untrue?

    So here are my questions (which im sure will seem ignorant to an electrical genius such as yourself haha) but anyhow, are the bulbs themselves supposed to get warm rather quickly..(within 2 minutes)? How do i fix this? or do i even need to fix this? i purchased a multimeter, but i think i blew a fuse in the multimeter when i was playing with it at first.. so it wont let me check the mA being pulled through the circuit...

    Any help will be very much appreciated!!!

    Best Regards,

    PS: I saw in the other posts that you often asked for a visual diagram to help diagnose our failures more quickly! im including the electrical diagram from the shop manual for the dirtbike (ignore the drawings on there, those are from the previous owner of the manual, however i did splice into the same place for the 14.5v power supply if that helps at all) my circuit is different though, im including a photo of my circuit drawing.

    <snip> Moderator's note: Posts to the forums can have photos attached. Choose the "Go Advanced" option, then "manage Attachments".
    Last edited by a moderator: Apr 19, 2011
  2. JDT

    Well-Known Member

    Feb 12, 2009
    SgtWookie doesn't seem to be replying - perhaps he thinks you should not be let loose anywhere near electricity! And, how come you can do a whole year of Industrial Design without being taught anything about electricity?

    Never mind.

    LED's should never be connected directly to the supply. They must have resistors in series to control the current. Yours are getting hot because there is far too much current flowing which will quickly distroy the LED's.

    So, add a resistor. Consider the voltage drop of this resistor adding to the voltage drop of the LED's. Read up on Ohm's Law. Also consider the fact tha as the engine speed of your bike changes, the output voltage of the alternator will also change. Quite a lot, probably.
  3. Wendy


    Mar 24, 2008
  4. Jeff7

    New Member

    Apr 17, 2011
    V = IR
    Start low, and assume 3.4V/LED.
    3.4 * 4 = 13.6V
    14.5 - 13.6 = 0.9V
    So the resistor needs to drop 0.9V.
    Assume 20mA for now.
    R = V/I = 0.9V/0.020A
    R = 45 ohms

    P = I^2 * R = 0.020*0.020*45
    P = 0.018W
    A 1/4W resistor would be ok in this instance.
  5. Cr_rydah02

    Thread Starter New Member

    Apr 18, 2011
    hey JDT.. thanks for your condescending attitude, much appreciated by someone attempting to learn. Perhaps you should post a thread in an industrial design forum so you can better understand what they do. Were not electrical engineers (although i am TRYING to learn a little bit about this stuff) I am aware of ohms law and how to use it, ive done so in some testing i did with 9v batteries and component LED bulbs. When i hooked up the multimeter i was looking to get a range of volts when the motor was at different speeds, but it didnt matter if i was at idle, or 8,000 rpms.. it read steadily between 14.1 - 14.5 volts. Will the output of the current change when the motor is at different speed.. i believe this would be measured in mA.. correct? Like i said before, i had wanted to test this but i need to pick up a new multimeter today.

    Jeff7 - thank you for the help.. ill certainly pick up a pack of resistors today and see if that helps!
  6. SgtWookie


    Jul 17, 2007
    I am sorry, but I no longer recommend attempting to use LEDs to replace incandescent lighting on vehicles.

    It is also against the Forum policies to discuss such modifications to vehicle safety lighting, as per this thread:

    I can only suggest that you return your dirt bikes' lighting to how it was configured when it was delivered by the dealer. Suggestions other than that may be in violation of established Forum policies.

    My time on the Board is very limited nowadays, and will be far more limited in the very near future.
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    Woops - I missed the dirtbike part. Our policy is against homemade LED swaps in automotive applications.
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