Results 1 to 6 of 6
  1. #1
    No talent hack
    Reputation:
    Join Date
    Nov 2010
    Posts
    950

    One more time.... clarify my mind.

    I mowed my yard last night using a bike headlight.... I will be doing this more often, so it is time to build a lawnmower headlight. I want to build a dual LED light and need my mind straightened out and clarified for the wiring between the 2 LEDs and more importantly WHY they are wired.

    I know there is parallel and series wired. This will be built with a pair of XMLs and a driver capable of putting through 3000ma (cheap one from FastTech given the purpose of the build).

    Option 1:
    Driver+ --> +LED- --> +LED- --> -Driver

    Option 2 (both LEDs wired to the driver)
    Driver+ --> +LED- --> -Driver
    Driver+ --> +LED- --> -Driver

    Option 3 (taken from my days in car audio and subs; driver to LED then to second LED):
    Driver+ --> +LED+ --> +LED
    Driver- --> -LED- --> -LED

    I think options 2 and 3 are actually the same in parallel and option 1 is series.

    I understand why I did this back in car audio. Parallel pulled more power out of the amp by reducing the effective ohm rating. Now in DIY lighting, why is one selected over the other and which should be the better option and WHY? (I am big on the WHY because it helps me commit to memory the info instead ofbeing lost in the future.)

    As always, thanks guys/gals.... this has been a great hobby.
    Fat guys need bikes too.

  2. #2
    mtbr member
    Reputation:
    Join Date
    May 2011
    Posts
    257
    I was concerned for your neighbours, bright lights & a lawnmower at night, but then I realised this is a bike forum, the lawnmower must be pedal powered

    Simple explanations work for me. The voltage forward for the LED rises with the current, as does the light output & the heat. LEDs in series, current is the same, voltage is cumulative, LEDs in parallel voltage is the same, current is shared (not necessarily equally). With a battery powered LED, you need to control the current, which is why most multi LEDs are wired in series.

    1 XM-L @ 3A = ~3.35Vf. So 2 x XM-L in parallel = ~3.35V & 6A. 2 x XM-L in series = ~6.7V & 3A.

    These figures show what you need in a driver. It seems to me that, as you say, option 2 & 3 are the same. Using your criteria of a cheap driver from Fasttech, here are 2 driver options

    Option 1 - 2 XM-L in series buck driver with 2.4A output, 6V~8.4V input. Not quite 3A, but in practice 2.4A is not bad - less heat, easier on the cells & the difference in output won't be high.

    Option 2 - 2 XM-L in parallel linear driver with 5A output, 2.7-4.5v input again 2.5A per LED is not bad.

    Poorman's circuit is a different way of wiring LEDs in series but using a cheap linear driver. Led1 is wired in series with the driver on the input side & LED2 is wired on the output side. as always a circuit diagram makes it clearer

    An XM-L has a lower Vf than the P7s in the diagram, but it still works for 2 XM-Ls.

    It means you get to save $2 and use this driver at 2.8A

  3. #3
    No talent hack
    Reputation:
    Join Date
    Nov 2010
    Posts
    950
    Disclaimer: I can mow (legally) up until 9pm... and with my work schedule, I have to mow late. It gets too dark to mow by ~8pm.... I am not mowing at midnight, but if I lived in the country, I would.... and just like riding at night, it is fun mowing at night.

    And thank you for the diagram. The driver you linked to at the bottom is the driver I have too.

    Is there any reason why the Poorman's circuit uses both sides of the driver (input and output) instead of just wiring in series on the output side?
    Fat guys need bikes too.

  4. #4
    mtbr member
    Reputation:
    Join Date
    May 2011
    Posts
    257
    Fair enough, I was just teasing you about the mowing.

    The reason the poorman's circuit uses both sides of the driver is that nothing good will come from putting 8.4V through it. The driver uses a Tiny 13A chip together with AMC7135 chips to regulate the current. While the driver page says "3.0V~4.5V" the chips are rated to 6V. What the LED wired to the input side does is drop the input voltage by between 2.6V & 3.35V, keeping the driver in spec.

  5. #5
    No talent hack
    Reputation:
    Join Date
    Nov 2010
    Posts
    950
    I used the Poorman's circuit for this light (yes, my lawnmower light). I just buttoned her up this afternoon and actually mowed my entire yard with it after dark tonight. Chinese made lights be damned, I really enjoy building my own.... even if my lights aren't nearly as cool looking as some of the lathe/mill made beauties around here, this is a ton of fun and I REALLY enjoy it. This has helped me get MUCH better with a solder station too. Some pics:



    Hammond box style light. Cut the box down to just fit what I needed in it. The blue heat sink is a north bridge cooler (or at least half of one) that I had from computer crap (it is actually a Zalman NBF-47). Since I am moving slower with the mower (4mph tops), I wanted more surface area to cool the light. The screws used to mount the cooler go all the way through the build mounting directly to the sled that the LEDs are on. Each layer has Artic Silver 5 between to transfer heat.



    Finished and mounted. Just a 1x1 aluminum L bridging 2 rubber flashlight mounts. Rubber to help minimized vibrations transmitted to the light.



    One more from the user's view.

    Great light for mowing. Put a medium ripple lens and a wide ripple lens in the holders. Throws far enough and wide enough to really mow in the dark and do a good job.

    Next up.... dual XML handlebar light for the bike.
    Fat guys need bikes too.

  6. #6
    mtbr member
    Reputation:
    Join Date
    May 2011
    Posts
    257
    Glad to see it worked out for you

Similar Threads

  1. can you help clarify which mount
    By yeti575nut in forum Brake Time
    Replies: 3
    Last Post: 01-12-2013, 10:23 PM
  2. I think I've been an idiot, please clarify.
    By Rustyy117 in forum Beginner's Corner
    Replies: 17
    Last Post: 06-23-2012, 10:47 AM
  3. Just want to clarify some details before hitting the upgrade button
    By rhysjenz in forum Drivetrain - shifters, derailleurs, cranks
    Replies: 11
    Last Post: 06-02-2012, 02:11 AM
  4. Replies: 3
    Last Post: 06-02-2011, 11:52 PM
  5. Replies: 52
    Last Post: 01-06-2011, 02:36 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •