Trying to make my own LED tail light for my motorcycle.

Discussion in 'The Projects Forum' started by yamyvmax, Nov 25, 2009.

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  1. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    I have been trying to figure out the best way to make my own LED tail lights for my motorcycle. I have already made the board and have the LED's wired in. I just can't get the running light and brake lights to be what I consider bright enough. I got one of those 100 packs of LED's off ebay but am only using 72 of them. They are 20k mcd and have a Vf of 3.2 to 3.8. They are rated at a peak forward current of 75 mA and a max continuous current of 30 mA. I am running these in parallel and have a single ground wire and a running light power wire and a brake light power wire. I have tried hooking up a 33 ohm resistor and its pretty bright through the lense but the resistor gets hot quick and I cant imagine that the LEDs would last very long with that much power going through them. I have noticed that the LED's are much brighter when the light isnt filtered through the lense. But I am hoping I can make them bright enough that it wont matter that the lense is there. I have seen the LED lights on cadilacs and I want mine to look like that. Maybe wishfull thinking but I guess we'll find out.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Post a schematic of your circuit.

    When you have something already built, a schematic is pretty much a basic requirement.
     
  3. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    Hey thanks for replying! I am at work now and dont have any programs that I can make somthin up quick. I will make a schematic when I get home. Till then just on theory do you have any ideas? Pretty much what I did was ran all the LED's in parallel. There are 6 rows of LED's with different numbers in each row. I had to stagger the numbers because of the way i mounted the circuit board in the lens. All positives are connected to one wire and the same with the negatives. I have the regular running light power and brake light power going to the common positive. I have the different value resistors soldered into the wire so that if I needed to change them I could fairly easily without having to dismantle the board. I have a 33 ohm resistor in the brake light power wire and a 270 ohm resistor in the running light power wire. The brake lights are pretty bright but still not bright enough. The resistors are 1/2 watt. Im thinking of just trying to find brighter LED's and scratch what I have,but I dont wanna do that untill Ive exhausted all my avenues with this set up. Thanks again!!
     
  4. SgtWookie

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    OK, so your LEDs were specified as having a Vf of 3.2v to 3.8v at 30mA current.
    So, your typical Vf will be about the average of 3.2 and 3.8, or 3.5v at 30mA.
    I would not run more than 3 LEDs in series, which would drop 10.5v. A current limiting resistor would drop the remaining voltage.

    Since you have 72 LEDs, this means you would need to have 24 strings of 3 LEDs in parallel. This would require 30mA x 24 = 720mA total current when the brake light (maximum brightness) is on, and about 1/3 that much current when the park/running lights are on.

    However, you seem to have wired it up differently. Until you can give an example of how a few of your strings are wired, it will be difficult to recommend a course of action.
     
  5. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    Sorry yet again I am at work and havnt had a chance to make up a schematic. I pretty much have 6 rows of LED's going cross ways from left to right. Starting from the bottom up they are numbered as first/second row:12 third/fourth row:10 Fifth/sixth row:14. All rows are in parallel and linked in parallel. I have two resistors in the wires that power the board. There are no other electrical components on the board. I have one common ground and two power wires,one red for running light and one green for brake light. I am trying to figure out what the best configuration would be to make them as bright as possible. Thanks
     
  6. SgtWookie

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    Jul 17, 2007
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    A written explanation of the circuit really is not enough. A schematic is worth a thousand words, and much faster to get the needed information from.
     
  7. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    I just uploaded a schematic of my LED lights. Both resistors are 1/2 watt. Thanks!
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Holy cow, that's bad! :eek:

    You wired them ALL in parallel! :(

    Now I see why your resistors are getting super-hot.

    Let's see, average Vf = 3.5v @ 30mA, and you have 72 in parallel.
    Right now, your driving resistor is 270 Ohms, and your brake resistor is 33 Ohms.
    Let's do the driving resistor 1st.
    I'll assume 14v system voltage when your driving.
    14-3.5=10.5v
    10.5v/270 Ohms = 38.8mA
    Since you have 72 LEDs in parallel, 38.8/72 = 0.54mA - no wonder they are dim.
    Check the power rating: 10.5v x 0.0388A = 0.4074W; double for reliability = .8148W, you need a 1W resistor to get barely glowing LEDs.

    Let's look at the brake light resistor.
    10.5v/33 Ohms = 318.2mA - but you have 72 lights, so that's 4.42mA per LED.
    Check the power: 10.5 x .3182 = 3.3411 Watts, double for reliability = 6.6822 Watts; you need a 10w resistor to get very dim LEDs.

    You will need 2.16 Amperes of current to get them to maximum brightness
    If your electrical system is putting out 14v, then you need:
    Rlimit >= (14v-3.5v)/2.16A
    Rlimit >= 10.5/2.16
    Rlimit >= 4.86111... Ohms
    Since power = voltage x current, 10.5*2.16 = 22.68 Watts; double for reliability and that's 45.36 Watts. You'll need a 50 Watt, 4.86 Ohm resistor.

    Do you realize that your original light bulb only required 27 Watts, and you'll be burning up nearly that much just heating a resistor? The LEDs will only be using 7.56W, the resistor gets 22.68W.

    If some of the LEDs fail, they will all rapidly fail. It'll be a "domino effect".

    You really need to re-design the board so that you have three LEDs in series, and maximum current limiter resistors in each string. Then, a 2nd single resistor can be added to lower the current for all of them during normal driving.

    If you don't do this, you will not only waste a lot of power, but risk having the entire light fail catastrophically.
     
  9. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    hahahaha wow thats alot to take in. Would you be willing to help me design a circuit that would work well for what I wanna do with it? Thanks very much for helping me with my first design. If you have any tips or ideas I would greatly appreciate it.
     
  10. SgtWookie

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    Jul 17, 2007
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    OK, you said you were running them in parallel from the start, and I really couldn't believe it until you showed me the schematic!

    Going back to designing a better way to do it - actually, not that hard at all.

    Assuming 14v for your system, and a typical Vf 3.5v per LED, you could run 3 in a series string with a current limiting resistor.
    Rlimit >= (14v - (3x3.5))/30mA
    Rlimit >= (14 - 10.5) / .03
    Rlimit >= 3.5 / .03
    Rlimit >= 116.666...
    Not a standard value. Closest standard value >= 116.666... is 120.
    Power requirement: 3.5v x 0.03a = 0.105W; double for reliability, 0.21W. You'll need 120 Ohm, 1/4 Watt resistors for each string of three LEDs to limit the maximum current for the brake light.

    Then, you need to decide how many strings of 3 LEDs you'll have to figure out the resistance and power requirements for the running lights.
     
  11. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    Hey its been a little while and I've been trying to think of a way that I can make this work. What do you think? If you have any suggestions please feal free to let me know. Thanks again!!
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    OK, that's much better. :)

    Let's talk about your LEDs for a few minutes.

    Are they white? If so, most of the light they produce will be filtered out by the taillight lens.

    I don't know where you reside, as you have not put that information in your profile (hint, hint) - just your Country is enough, but your region or state is quite helpful.

    U.S. Department of Transportation regulations specify that brake lights need to be in the red-orange color spectrum. This is in the wavelength range of 615nm to 625nm. There is also some provision for amber turn signal lamps, but we won't get into that.

    If you use white LEDs for your brake light and park/running lights, you will lose most of the light output due to the necessary filtering of the color spectrum to only emit the red-orange that is mandated by law.

    Further, your white LEDs have a much higher Vf than the proper color LEDs do. This means that you are wasting a good bit of power by generating light in many frequencies that you won't be able to use.

    You will be far better off if you simply get LEDs that emit the correct wavelength spectrum to begin with. They will likely have a much lower Vf (forward voltage) than the LEDs that you are attempting to use now.

    Another concern is the LED viewing angle. It is likely that your LEDs have a very narrow viewing angle; perhaps 25°. These are not really suitable for brake/tail lights. You need a much wider angle LED.

    [eta]
    Have a look at these:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=160-1511-ND
    http://mouser.com/ProductDetail/Lite-On/LTL911VHKSA/?qs=sGAEpiMZZMs4quMj8r4lmihme1pmGm6xPhhictpwjfU=
    These LEDs are the right color, wide angle (70°), have a much lower Vf (around 2.2v-2.6v), higher current (70mA) and are REALLY bright. You won't need to use as many of them as the little T3 or T5 LEDs that you have now. They're around $0.40/ea when you buy 10 or more.

    You should get extras to have as spares, in case some of them burn out. Usually, LEDs are really reliable. However, if you goof something up, or if there is a fault in your bike's electrical system, you could lose a number of them pretty quickly.
     
    Last edited: Dec 5, 2009
  13. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    Thats awsome. Ok, say for instance I wanted to get the LED's from digikey. I see they are square shaped and have 4 posts? How do they work with 4 posts? Are all of them used or just 2 for power and 2 for mounting? What resistors would I use with these LED's? Could I still do the same configuration with those? Thanks for all your help so far!!!
     
  14. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    Ok so I just did some quick math going off of your example from before and heres what I got.

    Rlimit >= (14v - (3x2.15))/70mA
    Rlimit >= (14 - 6.45 ) / .03
    Rlimit >= 7.55 / .07
    Rlimit >= 107.85
    Next closest would be 110 Ohm.
    And power rating would be 1/2 watt because 2.15x.07=.1505 double for reliability would be .301 which the next closest would be 1/2 watt.

    Double check and let me know what you think. Thanks again!
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    That'll work.

    But now that the Vf has dropped - you can put more in a series string. :eek: :D Don't go more than 5 in a series string, and plan on a maximum system voltage of 14.2v for calculating your resistors.

    Two pairs of legs are connected together.
    Here is the datasheet:
    http://optodatabook.liteon.com/DataBookFiles/7197/LTL91xVxKSA.pdf
    Look in the datasheet at the photo on the 1st page, upper right. Two LEDs are shown.
    You can see that one corner of the square LED is cut off diagonally. That pin, and the pin connected to it by the strap going vertically, are the cathodes.

    Notice that the other vertical strap (towards the right) has two small holes in it. That strap is connected to the anode, and two other mounting pins.

    I strongly recommend that you measure each LED for Vf @70mA so that you can match them up prior to assembling them into your circuit. This is not hard to do.
     
  16. yamyvmax

    Thread Starter New Member

    Nov 25, 2009
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    Do I have to use both anode and cathode pins on each LED or can I just use one per? If I did 4 in series and built it the same as my schematic how would that work? How do I check Vf on an LED?
     
  17. SgtWookie

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    Jul 17, 2007
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    You don't have to use both. However, you should solder them into the board anyway to help provide for heat dissipation. Since you're soldering them anyway, why not connect them with wires?

    It should work OK. Just re-calculate the current limiting resistors for Ohms and wattage.

    Use a constant current source that outputs 70mA. If you don't have one, that's OK.

    You already know that the LEDs are supposed to have a nominal 2.15Vf @70mA.

    So, you can use your PC's 12v supply with a resistor to figure it out.

    12v-2.15v = 9.85v
    9.85v / 70mA = 140.71 Ohms
    You can use a 220 Ohm and 390 Ohm resistor in parallel to get 140.656 Ohms.
    The 220 Ohm resistor should be rated for at least 1 Watt, and be 1% tolerance.
    The 390 Ohm resistor should be rated for at least 1/2 Watt. 5% is OK.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    I installed 36 of those 4-pins LEDs in the case of a compact-cassette at 53mA even though they are rated at 70mA. I drilled many holes in the case for cooling. But the circuit board gets extremely hot and the case got "crazed" and the clear plastic case on the LEDs have turned yellow. I added a white LED but it got cooked by the others and turned a dim blue. Now I replaced the white LED with a blue one and it continues to work fine (no phosphor to get cooked).

    I think you will also have a serious problem with heat.
     
    Last edited by a moderator: Jan 13, 2013
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